Finally! After way too many hours, I have it resurrected! It's a long video but if you're interested you can go thru the whole process with me. It's very fascinating to see the naked Ebow exciting a string into vibrating all on its own and then sustaining it. 8)
This video is to correct an error I made on the schematic and to share some LTSpice simulations that seem to be pretty close to real world, based on the way my circuit is currently.
That's awesome Paul...
...that diode.
It's switched into the battery return path, right?
Can you post the schematic, I'm finding it hard to read in the videos.
Anyway, if the diode does switch into the battery return, there is a difference that may matter.
Quote from: anotherjim on December 29, 2022, 05:16:35 PM
...that diode.
It's switched into the battery return path, right?
Yes that is correct.
Quote from: anotherjim on December 29, 2022, 05:16:35 PM
Can you post the schematic, I'm finding it hard to read in the videos.
Anyway, if the diode does switch into the battery return, there is a difference that may matter.
I don't have a way to do that. Back in the day I would've just uploaded it to my website but I ain't got one no more.
If you don't mind losing ownership of the scheme, just drag the image file from its directory onto this page...
https://postimages.org/
300dpi is easy on the eye.
Copy the direct link and paste it into the image tags.
Quickly, the battery self-capacitance is an AC bypass and the diode will rectify AC currents using it.
Another great video Paul and an awesome effort as well.
I've gone through the video once but after three attempts I haven't done managed to do complete pass again A *whole lot* of things come to mind when I watched it the first time.
It's pretty darn close!
A couple of major questions were:
- When the unmodified ebow is in normal mode, it seems you never get string to produce the fundamental B, it's always twice the fundamental. Is that right?
- I noticed at the end of the video you say the new transmit coil was 96 ohms. So I guess you added more turns to the 79 ohm version. I wasn't sure at what point in the video it went to the 96 ohm coil?
About the 4u7 cap, after checking the diode direction thing which came up recently, I was sure you had the 4u7 cap right before.
Here's the pics of the tracks. When I drew out the pic I wasn't sure about the coil connection being the top right or not but now I am. If you think about the LED connection being to ground you can see how the diode and the cap are all connected together on the ground. You can see from the unedited top pic the PCB track along the top edge has been accidentally cut in a few places. That top track sure look like it goes the whole width of the board and that would connect the other side of the diode to the cap together. So the cap must be in parallel with the diode.
(https://i.postimg.cc/xN8vKzsQ/cap-connection-6-tracks.png) (https://postimg.cc/xN8vKzsQ)
4u7 is worth nearly 80R @ A440Hz.
A thread here long ago for a 386 mini amp had a series protection diode in the +9v and it sounded horrible even though the 386 had a 100uF local bypass. Removing the diode cured it. It would have been just as bad if the series diode was in the power negative feed.
It might be interesting in the sim to check the circuit 0v (diode anode) with reference to the battery negative terminal while in harmonic mode.
Quote from: Rob Strand on December 30, 2022, 05:17:25 AM
Another great video Paul and an awesome effort as well.
I've gone through the video once but after three attempts I haven't done managed to do complete pass again A *whole lot* of things come to mind when I watched it the first time.
It's pretty darn close!
Thanks. That was my objective, to get as close as possible with the big question marks that still remained.
Quote from: Rob Strand on December 30, 2022, 05:17:25 AM
A couple of major questions were:
- When the unmodified ebow is in normal mode, it seems you never get string to produce the fundamental B, it's always twice the fundamental. Is that right?
- I noticed at the end of the video you say the new transmit coil was 96 ohms. So I guess you added more turns to the 79 ohm version. I wasn't sure at what point in the video it went to the 96 ohm coil?
Yes, it is twice the fundamental... but that was sitting over an open string. I'm sure it must be mixing the fundamental in there too? I guess I need to sit down and REALLY listen to what it's doing when I use the Ebow while playing the guitar. I added more wire to the driver coil sometime after I said it became 79 ohms. I kinda lost track... I think I added more wire like three or maybe four times after the initial winding.
Quote from: Rob Strand on December 30, 2022, 05:17:25 AM
About the 4u7 cap, after checking the diode direction thing which came up recently, I was sure you had the 4u7 cap right before.
Here's the pics of the tracks. When I drew out the pic I wasn't sure about the coil connection being the top right or not but now I am. If you think about the LED connection being to ground you can see how the diode and the cap are all connected together on the ground. You can see from the unedited top pic the PCB track along the top edge has been accidentally cut in a few places. That top track sure look like it goes the whole width of the board and that would connect the other side of the diode to the cap together. So the cap must be in parallel with the diode.
(https://i.postimg.cc/xN8vKzsQ/cap-connection-6-tracks.png) (https://postimg.cc/xN8vKzsQ)
It's kind of confusing, isn't it? That's what I thought too initially, but having it that way in LTSpice, the simulations never made much sense. And I never got a phase shift at the 4.7uF cap, but I also wasn't looking for one. Could also be that I don't know how to correctly model it (I still am learning the program and I am not the sharpest tool in the shed). However, with that 4.7uF cap left where it is on the simulation in the video, I just re-simulated it with a guitar pickup equivalent circuit inductively coupled to the output coil and I think I have pretty much nailed it as far as the waveforms go! See very short video below. Failed to mention it, but I was looking at the output of the guitar pickup.
The switch on the Ebow is a center off 3-position slide switch. Reg mode switches the battery to ground thru the diode and the harmonic mode switches the battery directly to ground.
Quote from: anotherjim on December 30, 2022, 04:12:56 AM
If you don't mind losing ownership of the scheme, just drag the image file from its directory onto this page...
https://postimages.org/
300dpi is easy on the eye.
Copy the direct link and paste it into the image tags.
Quickly, the battery self-capacitance is an AC bypass and the diode will rectify AC currents using it.
Thanks I'll give that a try. It's currently at the end of the short video in my last response to Rob.
EDIT: It can be found here - https://postimg.cc/94VbQ7Tx
Ok, capacitors C2 and C4 are in series as drawn. The total capacitance in the output circuit is going to be just about 4.6uF. This would make the 220uF C2 cap pointless. I don't think the 4.7uF C4 can be where it is drawn.
If the diode did nothing more than drop a little voltage, it would have a negligible effect, overruled by whatever the battery voltage is. If C4 is across the diode, it will have a frequency-dependent effect on whatever the diode does. Increasing frequency makes the diode less relevant.
Quote from: anotherjim on December 30, 2022, 02:36:36 PM
Ok, capacitors C2 and C4 are in series as drawn. The total capacitance in the output circuit is going to be just about 4.6uF. This would make the 220uF C2 cap pointless. I don't think the 4.7uF C4 can be where it is drawn.
I tried simulating it with it the way I had it schematically before, and the regular mode looks a little closer to real world... but now the harmonic mode isn't right - lose the third harmonic after about 30mS and it becomes a triangle wave. I know from my working circuit that I was actually getting 3x the fundamental and it was maintaining indefinitely, but it could just be the program. The 220uF cap and 4.7uF cap do have the driver coil in between them. Seems to me that the 4.7uF cap is what is doing the phase shifting. I get none in LTSpice with the diode always in circuit. Also, in my testing when I had the diode reversing switch still attached, I found that in one mode reversing the diode did nothing and in the other mode it killed the power.
Quote from: anotherjim on December 30, 2022, 02:36:36 PM
If the diode did nothing more than drop a little voltage, it would have a negligible effect, overruled by whatever the battery voltage is. If C4 is across the diode, it will have a frequency-dependent effect on whatever the diode does. Increasing frequency makes the diode less relevant.
I measured supply voltage real quick. Harmonic mode was 8.83V and reg mode was 8.79V. I was expecting more of a drop but I think the capacitor messes with the voltage drop of the diode? In my mind the diode is used to drop the supply voltage just a little bit, which would shift the oscillator voltage down slightly and hence the oscillator frequency (2.4kHz vs 2.6kHz). I think that is what makes it do 2x the fundamental vs. 3x, in conjunction with the phase shifting. I only say that because I've built a few audio oscillators and I've noticed that they get a little off when the supply voltage is lowered. Seems like it just needs that little kick to switch from one mode to the other.
I don't honestly know what phase shift effect the caps will have on the coil's magnetic phase - such finery is above my grade. I would suspect that any phase shift is frequency dependent, no more than 90deg at any point and that of caps occurs opposite to inductors which shift 90degree in the opposite direction.
(https://i.postimg.cc/W4NbyyQC/Screenshot-2022-12-30-at-21-43-21-RLC-Low-pass-Filter-Design-Tool-Result.png)
Ignoring the 220uF as it's too large to affect. The above is the junction of an inductor (guessed at 2H) with 80R resistance and a 4.7uF cap. You can see the peak in response while about that frequency the phase shifts 180deg.
But, the capacitor and inductor are fixed values and this phase shift only occurs where the L & C shifts coincide.
I doubt the E-bow oscillation is particularly voltage dependent. It's set by L & C values and that really only drifts with temperature. If it was crucial here, plain direct battery power would not be good enough - the voltage can be whatever the battery makes in its condition and loading.
Forget reversing the diode, it's in series with the power supply so it won't do anything backwards as the diode will not pass current that way.
This is getting messy, and I don't want to fog things up, but have you tried as Rob suggested with the coil negative at ground and the 4.7uF across the harmonic diode?
QuoteYes, it is twice the fundamental... but that was sitting over an open string.
Very interesting.
QuoteThe switch on the Ebow is a center off 3-position slide switch. Reg mode switches the battery to ground thru the diode and the harmonic mode switches the battery directly to ground.
To me the waveforms don't appear the same. The shape looks similar but the details are *very* different. The left waveform has a sharp peak, the type of thing you get when feeding square waves (or clipped sine waves) in the circuits with inductors or capacitors. The right side looks like harmonics added to a base waveform, peaks but smooth peaks - that's the key difference.
QuoteThe above is the junction of an inductor (guessed at 2H) with 80R resistance and a 4.7uF cap.
I think the reason Paul sees an improvement with the 4.7uF cap is it is resonating with the output coil. If the inductance of the latest ("V2") output coil is about 12mH then the resonant frequency between the 4.7uF cap and 12mH is f = 1/(2*pi sqrt(LC)) = 670Hz. So basically it's helping boost the gain, that could compensate for the lack of sensitivity in the receive coil and/or the drive coil.
The spanner in the works is we don't know what the original coil was. For the wire diameter Paul had for the original output coil, approx 35AWG to 36AWG, an 8 ohm coil made a lot of sense. Once you pick the wire diameter and fill up a certain size bobbin it pretty much pins down the coil resistance. Being off one gauge has an effect but you can put some bounds on what you expect. Anyway, assuming that the coil was 8 ohm the inductance would be around 2mH and when we resonate that with 220uF it's down around 240Hz, so some peaking but in the low frequencies. In this case it's doing something a little different and the sensitivity of the coils don't *need* the gain boost from resonating.
For the new coil. I can't see how it's 22AWG (that's very thick). I can't get 79 or 96 ohm and fit in the winding space. I'm getting 40AWG or so.
Some worth noting about resonating mechanical systems like strings is they have quite a high Q. Quite a bit higher than we are used to seeing in electronics (except for crystals but they are mechanical). So what does that mean? Normally for a system to oscillate at the resonant we need close zero phase shift around the feedback loop. With a high Q circuit there the phase can shift from -90deg to 90deg over a very narrow band off frequencies. That means the amplifier doesn't need to be as fussy about maintaining the phase response at 0deg. (What happens is the oscillator detunes only a small amount to find the zero-phase frequency.)
On a system where the gain isn't quite enough we can add resonant circuit to boost the gain. Normally the phase shift from that would stuff up the oscillation but because of the narrow-bandness of the string it cuts us a lot of slack to boost the gain and not care about phase.
After looking at the video yesterday I was looking the frequency response, similar to your plot but I was looking at voltage in and current out. I realized the phase response wasn't so great. Then I started wondering about why the ebow oscillates the string at 2 fundamental instead of just the fundamental. It then occurred to me that the string is high Q and gives us a lot of slack for phase. There's something significant in all those points. It might even help explain harmonic mode.
That's the jist of what I was thinking about but I haven't had a chance to get back to it.
Quote from: anotherjim on December 30, 2022, 05:09:53 PM
I don't honestly know what phase shift effect the caps will have on the coil's magnetic phase - such finery is above my grade. I would suspect that any phase shift is frequency dependent, no more than 90deg at any point and that of caps occurs opposite to inductors which shift 90degree in the opposite direction.
(https://i.postimg.cc/W4NbyyQC/Screenshot-2022-12-30-at-21-43-21-RLC-Low-pass-Filter-Design-Tool-Result.png)
Ignoring the 220uF as it's too large to affect. The above is the junction of an inductor (guessed at 2H) with 80R resistance and a 4.7uF cap. You can see the peak in response while about that frequency the phase shifts 180deg.
But, the capacitor and inductor are fixed values and this phase shift only occurs where the L & C shifts coincide.
I doubt the E-bow oscillation is particularly voltage dependent. It's set by L & C values and that really only drifts with temperature. If it was crucial here, plain direct battery power would not be good enough - the voltage can be whatever the battery makes in its condition and loading.
Forget reversing the diode, it's in series with the power supply so it won't do anything backwards as the diode will not pass current that way.
This is getting messy, and I don't want to fog things up, but have you tried as Rob suggested with the coil negative at ground and the 4.7uF across the harmonic diode?
No worries, I (we) want to get it figured out as far as we can take it. I think we are just about there at this point. I probably jumped the gun too quick in my conclusions and I think y'all are correct about that the placement of that cap. The PCB has the diode and 4.7uF cap in parallel, no disputing that. I was thinking that I made a mistake... I apparently confused myself. :icon_lol: I have not encountered this sort of odd arrangement before, it's quite foreign to what I am used to seeing. LTSpice will produce similar results either way I do it. Anyway, after some tweaking it looks like it simulates better in LTSpice with cap in parallel with the diode. 0V is definitely more where I would expect to see it.
These images are comparing my rebuilt circuit in LTSpice to what I observed on the scope with an intact un-messed with Ebow.
https://postimg.cc/gallery/6yYp9Sj
Quotehttps://postimg.cc/gallery/6yYp9Sj
Those two shots definitely look similar.
Quote from: Rob Strand on December 30, 2022, 06:15:06 PM
I think the reason Paul sees an improvement with the 4.7uF cap is it is resonating with the output coil. If the inductance of the latest ("V2") output coil is about 12mH then the resonant frequency between the 4.7uF cap and 12mH is f = 1/(2*pi sqrt(LC)) = 670Hz. So basically it's helping boost the gain, that could compensate for the lack of sensitivity in the receive coil and/or the drive coil.
Seems reasonable. I think after a certain point I realized that the input coil didn't have enough windings on it and the circuit was as good as I was going to get it.
Quote from: Rob Strand on December 30, 2022, 06:15:06 PM
The spanner in the works is we don't know what the original coil was. For the wire diameter Paul had for the original output coil, approx 35AWG to 36AWG, an 8 ohm coil made a lot of sense. Once you pick the wire diameter and fill up a certain size bobbin it pretty much pins down the coil resistance. Being off one gauge has an effect but you can put some bounds on what you expect. Anyway, assuming that the coil was 8 ohm the inductance would be around 2mH and when we resonate that with 220uF it's down around 240Hz, so some peaking but in the low frequencies. In this case it's doing something a little different and the sensitivity of the coils don't *need* the gain boost from resonating.
I'll defer to your expertise here (and everywhere else :icon_wink:). I think that's probably why I was able to get as much out of it as I did. Seems to be some wiggle room here, but it can only be taken so far.
Quote from: Rob Strand on December 30, 2022, 06:15:06 PM
For the new coil. I can see how it's 22AWG (that's very thick). I can't get 79 or 96 ohm and fit in the winding space. I'm getting 40AWG or so.
Darn it! I measured 0.07mm with the digital calipers. I then looked at conversion chart for mm to AWG but I think I looked at 0.7mm instead of 0.07mm, which would be 40-41 AWG and not 22 AWG. Slaps self on forehead, again. :icon_redface:
Quote from: Rob Strand on December 30, 2022, 06:15:06 PM
Some worth noting about resonating mechanical systems like strings is they have quite a high Q. Quite a bit higher than we are used to seeing in electronics (except for crystals but they are mechanical). So what does that mean? Normally for a system to oscillate at the resonant we need close zero phase shift around the feedback loop. With a high Q circuit there the phase can shift from -90deg to 90deg over a very narrow band off frequencies. That means the amplifier doesn't need to be as fussy about maintaining the phase response at 0deg. (What happens is the oscillator detunes only a small amount to find the zero-phase frequency.)
On a system where the gain isn't quite enough we can add resonant circuit to boost the gain. Normally the phase shift from that would stuff up the oscillation but because of the narrow-bandness of the string it cuts us a lot of slack to boost the gain and not care about phase.
After looking at the video yesterday I was looking the frequency response, similar to your plot but I was looking at voltage in and current out. I realized the phase response wasn't so great. Then I started wondering about why the ebow oscillates the string at 2 fundamental instead of just the fundamental. I then occurred to me that the string is high Q and gives us a lot of slack for phase. There's something significant in all those points. It might even help explain harmonic mode.
That's the jist of what I was thinking about but I haven't had a chance to get back to it.
Interesting. I am still mystified by this harmonic mode, and 2x/3x the fundamental. I was thinking the mechanism for how this thing works was mostly in the circuit but it appears that also string physics may play a far larger role than I was initially thinking. For a simple circuit this thing sure is complex! All I wanted to do was mess around with the circuit in LTSpice and it's become this epic saga over the last 4-6 weeks! :icon_lol:
One interesting note is at times when I was experimenting, I would hear a small pixie dust twinkling kind of sound like the string was trying to get going but was not at the right frequency. I imagine that was some number of harmonics above the fundamental frequency. :icon_question:
Quote from: Rob Strand on December 30, 2022, 06:19:48 PM
Quotehttps://postimg.cc/gallery/6yYp9Sj
Those two shots definitely look similar.
Yes surprisingly so since I have made that circuit work again by brute force with a bunch of unknowns. :icon_mrgreen:
QuoteSeems reasonable.
I guess the main point is the 4.7uF with your latest drive coil does have observable effects.
QuoteDarn it! I measured 0.07mm with the digital calipers. I then looked at conversion chart for mm to AWG but I think I looked at 0.7mm instead of 0.07mm, which would be 40-41 AWG and not 22 AWG. Slaps self on forehead, again.
Thanks for checking. It all makes sense with that wire.
QuoteInteresting. I am still mystified by this harmonic mode, and 2x/3x the fundamental. I was thinking the mechanism for how this thing works was mostly in the circuit but it appears that also string physics may play a far larger role than I was initially thinking. For a simple circuit this thing sure is complex! All I wanted to do was mess around with the circuit in LTSpice and it's become this epic saga over the last 4-6 weeks!
The initial thinking was normal model has the coils in phase and produced the fundamental. Harmonic mode drove the coils out of phase, and the harmonic depends on the spacing of the ebow coils.
I believe there is an explanation to why the second harmonics is produced in normal mode! Look at the string motion of the harmonic and the second harmonic.
(https://s3mn.mnimgs.com/img/shared/discuss_editlive/2955358/2013_03_12_13_37_11/harmonics.JPG)
The pickups and ebow are on the right half of the string. When you place the ebow in this area *both* coils see the same polarity of the string for fundamental and second harmonic. In both case the string is up or down for both coils. Normally you would expect the fundamental to win but since the amplifier response isn't flat it might help to promote the string oscillation to be the second harmonic! You could argue that with more dramatic frequency shaping it might favor the third harmonic - hence the idea that a high-pass filter promotes harmonics mode.
If one of the coils has the phase flipped then the position of the ebow would need to be placed so that the drive coil was on he left of one of the string nodes and the receive coil was on the right of the same string node - sort of promoting rocking about that point. The fact it depends on position makes the results more variable.
Quotene interesting note is at times when I was experimenting, I would hear a small pixie dust twinkling kind of sound like the string was trying to get going but was not at the right frequency. I imagine that was some number of harmonics above the fundamental frequency.
It's quite possible.
QuoteYes surprisingly so since I have made that circuit work again by brute force with a bunch of unknowns.
I guess the other reason for similarities is they are both from the pickup and both from an ebow exciting the strings. The two waveforms have very similar origins. Even if we don't know what is happening we know they come from the same place. When you go to LTspice you can't only trust the waveforms unless you *know* you have captured all the physical and electrical effects in the LTspice. We also know that a simple spice simulation probably doesn't do that,
so we shouldn't expect them to be the same or be surprised if they aren't!
I had this idea to compare the sensitivity/gain and frequency response of the real ebow and the rebuilt ebow from outside the box.
(https://i.postimg.cc/MckhJmnr/Ebow-Sensitivity-Test-Jig-V1-0.png) (https://postimg.cc/MckhJmnr)
The set-up can only check the electrical phase of the coils. However you can adjust the results based on the magnetic phase using a compass. [I forgot to mention this is OK for A/B comparisons at a given frequency but to get a flat response output you need to feed the output into an integrator.]
For your video you place the compass to the side of the ebow however what you should check is compass needle direction normal to the coil faces (as shown in the pic). It should be evident which direction the magnets are strongest from the snap in the compass needle.
An idea to increase the sensitivity of the rebuilt unit was to place a small magnet *on the back* of the existing magnets. The way they stick together naturally is the correct orientation. However, if you have strong rare-earth magnets I first suggest adding a small spacer between the two magnet as those rare-earth magnets an completely changed the magnetization (and direction!) of other magnets when they touch them. You want to make sure you know exact what direction the existing magnets are before you start putting those rare-earth magnets near them!
It occurred to me later this set-up might be a pain in the butt for testing the Ebow. I have used it to test pickups in the past and it works fine. It's low inductance coil fed with a current and you correct the response with an integrator. The thing about the Ebow is we would like to inject a constant voltage into the LM386, ie. we want a constant voltage out of the receive coil. The low inductance coil won't do that unless you put the integrator before the test coil.
It seems our minds are all on the same page because I noticed you posted a pickup test idea in the lounge,
https://www.diystompboxes.com/smfforum/index.php?topic=129998.0
In your test set-up the coil is a high inductance coil fed with a voltage. That would be much more convenient to test the Ebow than the low inductance coil.
You might fine-tune magnets with pieces of the plastic magnetic tape/strip used for fridge badges etc. It won't be strong enough to alter the existing magnet a great deal but depending on its polarity can add or subtract to the field.
If you can go back to the one-string test bed, clip the scope probe neg on the battery neg (diode cathode) and see what signal is present on the diode anode. Whatever the diode//cap is doing should show as some kind of signal, although it might only be small mV amplitude.
Funnily enough the coil I settled on for my experimental sustainer had a DCR of 90. Mine is body-mounted like a pickup so had to be driven from a higher supply voltage because the distance from the string is variable when played. Also, to get harmonics seem to need a stronger drive anyway. At the moment the driving LM386 gets 11v.
It produces the fundamental.
Harmonic mode is polarity reversal but the interesting thing is that the harmonics repeat on either side of the 12th fret. That is the 3rd fret is the same harmonic as the 15th fret!
Here's a question. Does LTspice sim a battery fully? You added a series resistor for the battery internal resistance, but is the capacitance represented? What even would the capacitance of a battery be? Is there capacitance built into the simulated DC supply?
QuoteHere's a question. Does LTspice sim a battery fully? You added a series resistor for the battery internal resistance, but is the capacitance represented? What even would the capacitance of a battery be? Is there capacitance built into the simulated DC supply?
Other than a simple resistor it gets complicated. You can use circuit models which have a whole heap of networks with caps in parallel with resistors, not unlike pink noise filters. The idea is they approximate the Warburg Impedance which comes up in electrolyte impedance models. These days it comes up in advance battery monitoring.
I've been slowly piecing together the results of Paul's last video. I don't think there's much doubt that increasing the turns increased the drive. However, the way this is done in Paul's test was to take a *fixed* wire diameter and gradually add more turns. When we add more turns you would think it increases the field *BUT* you are also increasing the resistance. So if you have a fixed supply, or in this case a fixed AC voltage swing from the LM386, the increased resistance means the current drops and if the current drops the field drops. To a first order approximation the two effects can cancel out. You should not see an increase in the field *but* what you will see is less current being drawn to get the the same field! (To first approximation the maths shows you end up with the same field strength as well.)
So why do we see the string drive going up? It has to be something is sagging under load, either the battery or the LM386 output, that means the output voltage isn't constant as assumed.
So what else? adding more turns increases the inductance. If anything, that's going to increase the AC impedance at higher frequencies, decrease the current, and decrease the field. The fact Paul had an improvement with the 4.7uF output cap means the resonance between that cap and the inductance is undoing effect of the inductance.
Is that diode damaged? A high resistance diode would reduce the output. With an 8 ohm load it's likely to exceed the diode current.
As a side note, in Paul's experiment of increasing turns with same wire. As we add more turns the diameter of each turn becomes larger. So that means we would cause more decrease in current and less field. However this effect is offset by the fact the diameter of the coil is increasing and is could increase the field from that turn. The point is we are talking smaller effects here, they don't explain what we saw in Paul's test. The output sag theory is more likely. So more turns, less sag, and effectively more string drive.
Quote from: Rob Strand on December 30, 2022, 08:47:30 PM
I had this idea to compare the sensitivity/gain and frequency response of the real ebow and the rebuilt ebow from outside the box.
(https://i.postimg.cc/MckhJmnr/Ebow-Sensitivity-Test-Jig-V1-0.png) (https://postimg.cc/MckhJmnr)
The set-up can only check the electrical phase of the coils. However you can adjust the results based on the magnetic phase using a compass. [I forgot to mention this is OK for A/B comparisons at a given frequency but to get a flat response output you need to feed the output into an integrator.]
For your video you place the compass to the side of the ebow however what you should check is compass needle direction normal to the coil faces (as shown in the pic). It should be evident which direction the magnets are strongest from the snap in the compass needle.
That's an interesting test. I can try doing that sometime in the near future. I just did the compass test the way you suggest and if I'm holding the Ebow as I would normally, north is up for both coils.
Quote from: Rob Strand on December 30, 2022, 08:47:30 PM
An idea to increase the sensitivity of the rebuilt unit was to place a small magnet *on the back* of the existing magnets. The way they stick together naturally is the correct orientation. However, if you have strong rare-earth magnets I first suggest adding a small spacer between the two magnet as those rare-earth magnets an completely changed the magnetization (and direction!) of other magnets when they touch them. You want to make sure you know exact what direction the existing magnets are before you start putting those rare-earth magnets near them!
Noted. I didn't keep the rare earth magnets near the button magnets for very long.
Quote from: Rob Strand on December 30, 2022, 08:47:30 PM
It occurred to me later this set-up might be a pain in the butt for testing the Ebow. I have used it to test pickups in the past and it works fine. It's low inductance coil fed with a current and you correct the response with an integrator. The thing about the Ebow is we would like to inject a constant voltage into the LM386, ie. we want a constant voltage out of the receive coil. The low inductance coil won't do that unless you put the integrator before the test coil.
I'm sure I can figure out a way to set up a test jig. Would be interesting to do.
Quote from: Rob Strand on December 30, 2022, 08:47:30 PM
It seems our minds are all on the same page because I noticed you posted a pickup test idea in the lounge,
https://www.diystompboxes.com/smfforum/index.php?topic=129998.0
In your test set-up the coil is a high inductance coil fed with a voltage. That would be much more convenient to test the Ebow than the low inductance coil.
Thanks to you helping me fix my old Heathkit o'scope a year or two ago, which opened up a whole new world to me. I didn't even know anything about X-Y mode capabilities before that!
Quote from: Rob Strand on December 30, 2022, 07:43:46 PM
The initial thinking was normal model has the coils in phase and produced the fundamental. Harmonic mode drove the coils out of phase, and the harmonic depends on the spacing of the ebow coils.
I believe there is an explanation to why the second harmonics is produced in normal mode! Look at the string motion of the harmonic and the second harmonic.
(https://s3mn.mnimgs.com/img/shared/discuss_editlive/2955358/2013_03_12_13_37_11/harmonics.JPG)
The pickups and ebow are on the right half of the string. When you place the ebow in this area *both* coils see the same polarity of the string for fundamental and second harmonic. In both case the string is up or down for both coils. Normally you would expect the fundamental to win but since the amplifier response isn't flat it might help to promote the string oscillation to be the second harmonic! You could argue that with more dramatic frequency shaping it might favor the third harmonic - hence the idea that a high-pass filter promotes harmonics mode.
If one of the coils has the phase flipped then the position of the ebow would need to be placed so that the drive coil was on he left of one of the string nodes and the receive coil was on the right of the same string node - sort of promoting rocking about that point. The fact it depends on position makes the results more variable.
I think you're onto something there. I did some calculations for 25.5" scale guitar, and found that the area where the Ebow would normally be used is in the zone where the anti-node occurs for the 2nd and 3rd harmonic. The anti-nodes of 2nd and 3rd harmonics are 1.5" apart. The Ebow appears to be taking advantage of string wave characteristics. The change in waveforms at the output coil seems to promote one vs. the other. Those extra humps on the waveform when in harmonic mode I think would be all it needs to make it lock onto the 3rd harmonic. Also, when in harmonic mode there's kind of parallel paths to ground - one is battery return and the other one is coil going to ground thru the diode, as it is doing full time. The last big question in my mind is why the oscillator frequency of 2.4kHz & 2.6kHz?
Quote from: Rob Strand on December 30, 2022, 07:43:46 PM
I guess the other reason for similarities is they are both from the pickup and both from an ebow exciting the strings. The two waveforms have very similar origins. Even if we don't know what is happening we know they come from the same place. When you go to LTspice you can't only trust the waveforms unless you *know* you have captured all the physical and electrical effects in the LTspice. We also know that a simple spice simulation probably doesn't do that,
so we shouldn't expect them to be the same or be surprised if they aren't!
Yeah, I know that a program like LTSpice is only going to be an approximation of real world. I think it does a pretty good job in this case. It is hard to model something like this and even more so when you "know enough to be dangerous" :icon_biggrin:
Quote from: anotherjim on December 31, 2022, 05:38:55 AM
You might fine-tune magnets with pieces of the plastic magnetic tape/strip used for fridge badges etc. It won't be strong enough to alter the existing magnet a great deal but depending on its polarity can add or subtract to the field.
If you can go back to the one-string test bed, clip the scope probe neg on the battery neg (diode cathode) and see what signal is present on the diode anode. Whatever the diode//cap is doing should show as some kind of signal, although it might only be small mV amplitude.
That would be interesting to see. I can try doing that sometime in the near future.
Quote from: anotherjim on December 31, 2022, 05:38:55 AM
Funnily enough the coil I settled on for my experimental sustainer had a DCR of 90. Mine is body-mounted like a pickup so had to be driven from a higher supply voltage because the distance from the string is variable when played. Also, to get harmonics seem to need a stronger drive anyway. At the moment the driving LM386 gets 11v.
It produces the fundamental.
Harmonic mode is polarity reversal but the interesting thing is that the harmonics repeat on either side of the 12th fret. That is the 3rd fret is the same harmonic as the 15th fret!
That would be very trippy having the 3rd fret same as the 15th fret! :icon_eek:
Quote from: anotherjim on December 31, 2022, 11:09:24 AM
Here's a question. Does LTspice sim a battery fully? You added a series resistor for the battery internal resistance, but is the capacitance represented? What even would the capacitance of a battery be? Is there capacitance built into the simulated DC supply?
LTSpice has a box where you can enter a number for the battery internal resistance, but I have no idea what the program actually does with that info.
Well, you may know that some pedal designs put a diode in series with the +9v feed for polarity protection. This is an alternative to the reverse polarity diode placed between +9v and ground and the advantage is that the series diode will not cause a short circuit with a wrong polarity supply and all that happens is - nothing! The diode will not pass current in reverse. The design must add a supply bypass capacitor, large enough to have a low impedance at the lowest frequency the circuit works with. Incidentally, it makes no difference here if the series diode is in the + or - supply feed.
The series diode prevents the bypass capacitance of either a battery or DC power adapter from fully bypassing the AC currents produced in a circuit. It may increase the impedance of the bypass path in one polarity which can affect the operation of the circuit. Adding a supply bypass cap on the circuit side of the series diode solves this. The Ebow has no such supply bypass capacitor, there is only the battery, and that is on the other side of a series diode in harmonic mode.
Now, a battery cell has a lot in common with a capacitor (two plates with something separating them) and is often relied upon as the only path for circuit AC currents. Capacitance comes with the territory, it simply has to exhibit it. It's more important than the battery's internal resistance which is trivial when the battery is good but increases as the battery becomes discharged. So you can only really put a likely value to the resistance and move on.
So, an experiment in the sim might be to add a capacitor across the battery, maybe a few different values from 1uF to 100uF and see if there is any change in waveforms in harmonic mode.
In case the penny hasn't dropped, I'm trying to see what the diode is for. If it isn't there to make things asymmetric by rectifying AC currents and so increasing 2nd harmonic, then I don't know what!
Quote from: anotherjim on January 03, 2023, 01:59:45 PM
Well, you may know that some pedal designs put a diode in series with the +9v feed for polarity protection. This is an alternative to the reverse polarity diode placed between +9v and ground and the advantage is that the series diode will not cause a short circuit with a wrong polarity supply and all that happens is - nothing! The diode will not pass current in reverse. The design must add a supply bypass capacitor, large enough to have a low impedance at the lowest frequency the circuit works with. Incidentally, it makes no difference here if the series diode is in the + or - supply feed.
Yes, usually I see the diode across the power supply. Seems like generally speaking, people don't want the voltage drop of a series diode, so they put it across the power supply.
Quote from: anotherjim on January 03, 2023, 01:59:45 PM
The series diode prevents the bypass capacitance of either a battery or DC power adapter from fully bypassing the AC currents produced in a circuit. It may increase the impedance of the bypass path in one polarity which can affect the operation of the circuit. Adding a supply bypass cap on the circuit side of the series diode solves this. The Ebow has no such supply bypass capacitor, there is only the battery, and that is on the other side of a series diode in harmonic mode.
Now, a battery cell has a lot in common with a capacitor (two plates with something separating them) and is often relied upon as the only path for circuit AC currents. Capacitance comes with the territory, it simply has to exhibit it. It's more important than the battery's internal resistance which is trivial when the battery is good but increases as the battery becomes discharged. So you can only really put a likely value to the resistance and move on.
So, an experiment in the sim might be to add a capacitor across the battery, maybe a few different values from 1uF to 100uF and see if there is any change in waveforms in harmonic mode.
In case the penny hasn't dropped, I'm trying to see what the diode is for. If it isn't there to make things asymmetric by rectifying AC currents and so increasing 2nd harmonic, then I don't know what!
I don't know to model it in LTSpice. Rob mentioned Warburg Impedance, and I have no idea how to model that. I get the concept but doing the math on that is over my head. I tried doing the capacitor across the battery but that doesn't really affect the waveforms at all. I do see a difference in the current at the output coil between modes but voltage-wise looks pretty much the same no matter what value I plug in for capacitance. I think part of the problem is that the model for the LM386 may still be missing some items... as Rob had mentioned in one of these posts. There may be a protection diode or two in that chip somewhere that doesn't show up on the manufacturer data sheet (where they show the schematic of the IC chip). Currently the bottom half of the of the output waveform has a little hump in it when it should just be flat like a clipped sine wave. That kind of messes up one half of the output waveforms on these models. :icon_cry:
I don't know to what extent the modelling deals with DC supplies, but as many circuits can be lashed up without bothering with any power caps the voltage sources probably act as much like capacitors as they need to keep the voltage stable.
Is it possible to place a "differential" voltage probe across the harmonics diode? This so that the circuit ground doesn't ignore the fact that the battery ground will be a diode drop negative from the circuit ground. I mean, if the diode is doing anything, it must have a signal across it, but you won't see that if the normal voltage probe has its negative connection on circuit ground.
QuoteI just did the compass test the way you suggest and if I'm holding the Ebow as I would normally, north is up for both coils
Essentially opposite to the arrows on my drawing. What that should mean is you can treat the electrical phase as the overall phase. The round trip voltage gain should be a very good indication of the sensitivity. And it will show just much lower the gain of the rebuilt unit is. I was thinking if it's only a factor of 2 you could tweak the LM386 with some external components to double the gain.
QuoteNoted. I didn't keep the rare earth magnets near the button magnets for very long.
The change is a very fast process! What you can also do is place the existing magnets between two large rare-earth magnets to re-magnetize the existing magnets to maximum. The two rare earth magnets are orientated so they attract, effectively creating a field in between. (This set-up isn't quite as good as it could be as ideally the backs of the two rare-earth magnets should be linked with a magnetic path. I've see people using a vice as a permanent jig where the vice frame links the backs of the magnets. I wouldn't use a vice for a once-off since the process will magnetize the vice which would be extremely annoying. It's also difficult to demagnetize!)
QuoteI'm sure I can figure out a way to set up a test jig. Would be interesting to do.
I'm sure you will come up with something.
QuoteThanks to you helping me fix my old Heathkit o'scope a year or two ago, which opened up a whole new world to me. I didn't even know anything about X-Y mode capabilities before that!
No problem good to see it up and running. I used XY mode a lot on my analog scope. There was point in time where digital scopes didn't support XY mode and it made me move to different methods.
QuoteThe last big question in my mind is why the oscillator frequency of 2.4kHz & 2.6kHz?
In the case where the unit self oscillates the lower frequency it makes it easier to lock to the string. (I explained the details in the other thread).
In the case where the unit doesn't self-oscillate it does something as well. When the circuit is adjusted just below self-oscillation the frequency response has a large peak at 2.5kHz. This provides a gain boost. The gain will increase from low frequencies upto 2.5kHz. That increases sensitivity and emphasizes harmonics.
QuoteI don't know to model it in LTSpice. Rob mentioned Warburg Impedance, and I have no idea how to model that. I get the concept but doing the math on that is over my head.
There's nothing special about LTspice's internal resistance. It's just like adding a resistor in series with the voltage source. It defaults to 1 milliohm. (LTspice's Caps and Inductor also have a series resistance, which doesn't default to zero. FYI, if you don't set it to zero it screws up high Q circuits like my string model. However, my string model was rough so I didn't bother setting the series resistance.)
The problem with 9V batteries is the impedance is all over the map.
Heavy duty new: 25 ohm
Alkaline new: 1 to 5 ohm
When the battery runs down the resistance goes up quite a bit and the open circuit voltage actually drops a bit as well (which you can ignore an let the impedance do the voltage drop). Just how high the resistance goes up depends on the circuit you are powering more than the battery itself.
For example at heavy loads (1hr discharge) a 9V alkaline might reach the end of useful life at 10 ohm to 20 ohm. Notice that this is less than the resistance of a *new* heavy duty battery, meaning a heavy duty battery cannot supply such heavy currents.
When the alkaline battery in my DMM comes up with low battery and I leave the battery sitting around I've measured 10 to 20 ohms, even though the DMM is a low current device. (Simple low battery indicators use voltage, not impedance.)
In a light load device which can operate down to low voltages a 9V battery might reach end of life at 20 ohm upto 200 ohm; for either alkaline to heavy duty.
So up front, the range of impedance is enormous! I think 10 ohms would be a good starting point for the DC resistance. However it wouldn't take much to bump that out to say 33 ohm if you kept using a flat battery.
The AC impedance can be crudely approximated as follows:
(https://www.researchgate.net/publication/286486268/figure/fig11/AS:668708892585999@1536444032754/The-simplified-Li-ion-battery-AC-impedance-model.png)
- Choose what DC resistance you want to emulate, say 10 ohm
- ignore Ld
- ignore the LTspice source resistance, or set it to zero
- Add series resistor R0, value about 0.1 to 0.2 times the DC resistance
- Set Rct to 0.9 to 0.8 times the DC resistance. (you want R0 + Rct to add to the DC resistance.)
- Set the cap to Cd = 1 / Rct. This will be an enormous cap and sets a 1 second time constant
I just pulled cap value from the "7th planet", might need refining.
Since there is an average DC load you should still see a voltage drop determined by the DC resistance.
So arguably the circuit will show the same droop without the cap however, with amplifiers a large series
resistance can cause the amp to oscillate so the impedance model will help stop that happening to some degree.
Something else, imagine the AC output swinging +/-3V peak and the output load is 8 ohm. That's 3V/8 = 375mA which will cause a 10*0.375 = 3.75V drop across a 10 ohm battery impedance. So quite a bit of drop on a 9V rail. If you backed the load off to 16 ohm, 3V/16 = 188mA, drop across 10 ohm = 1.9V. As the battery impedance goes up drop the amount of swing of the amp will drop and there's a point where you won't get +/- 3V peak. If the load impedance is increased you will get less drop and more output swing. So for a *given* battery DC resistance there will be a load which gives the maximum amount of field. The thing is for a 10 ohm battery impedance the maximum field won't end-up with a 79 ohm coil, it will be a lower impedance.
Quote from: Rob Strand on January 04, 2023, 08:14:15 PM
[Essentially opposite to the arrows on my drawing. What that should mean is you can treat the electrical phase as the overall phase. The round trip voltage gain should be a very good indication of the sensitivity. And it will show just much lower the gain of the rebuilt unit is. I was thinking if it's only a factor of 2 you could tweak the LM386 with some external components to double the gain.
I see. That makes sense. With this new Ebow test that you propose, do I just get it to self-oscillate? Can't really get it close enough to a guitar string for it to work but a screwdriver ought to have enough mass to get it going, maybe. I would be using an unmolested one for this test.
Quote from: Rob Strand on January 04, 2023, 08:14:15 PM
In the case where the unit doesn't self-oscillate it does something as well. When the circuit is adjusted just below self-oscillation the frequency response has a large peak at 2.5kHz. This provides a gain boost. The gain will increase from low frequencies upto 2.5kHz. That increases sensitivity and emphasizes harmonics.
I went back and re-read those posts. So maybe they picked that frequency because of the gain boost and it also seems that this frequency range works best for exciting the string in the right frequency range (2nd & 3rd harmonics). Too high of an oscillator frequency and it just doesn't do anything. Not without tweaking the circuit I guess.
Quote from: Rob Strand on January 04, 2023, 08:14:15 PM
There's nothing special about LTspice's internal resistance. It's just like adding a resistor in series with the voltage source. It defaults to 1 milliohm. (LTspice's Caps and Inductor also have a series resistance, which doesn't default to zero. FYI, if you don't set it to zero it screws up high Q circuits like my string model. However, my string model was rough so I didn't bother setting the series resistance.)
The problem with 9V batteries is the impedance is all over the map.
Heavy duty new: 25 ohm
Alkaline new: 1 to 5 ohm
When the battery runs down the resistance goes up quite a bit and the open circuit voltage actually drops a bit as well (which you can ignore an let the impedance do the voltage drop). Just how high the resistance goes up depends on the circuit you are powering more than the battery itself.
For example at heavy loads (1hr discharge) a 9V alkaline might reach the end of useful life at 10 ohm to 20 ohm. Notice that this is less than the resistance of a *new* heavy duty battery, meaning a heavy duty battery cannot supply such heavy currents.
When the alkaline battery in my DMM comes up with low battery and I leave the battery sitting around I've measured 10 to 20 ohms, even though the DMM is a low current device. (Simple low battery indicators use voltage, not impedance.)
In a light load device which can operate down to low voltages a 9V battery might reach end of life at 20 ohm upto 200 ohm; for either alkaline to heavy duty.
So up front, the range of impedance is enormous! I think 10 ohms would be a good starting point for the DC resistance. However it wouldn't take much to bump that out to say 33 ohm if you kept using a flat battery.
The AC impedance can be crudely approximated as follows:
(https://www.researchgate.net/publication/286486268/figure/fig11/AS:668708892585999@1536444032754/The-simplified-Li-ion-battery-AC-impedance-model.png)
- Choose what DC resistance you want to emulate, say 10 ohm
- ignore Ld
- ignore the LTspice source resistance, or set it to zero
- Add series resistor R0, value about 0.1 to 0.2 times the DC resistance
- Set Rct to 0.9 to 0.8 times the DC resistance. (you want R0 + Rct to add to the DC resistance.)
- Set the cap to Cd = 1 / Rct. This will be an enormous cap and sets a 1 second time constant
I just pulled cap value from the "7th planet", might need refining.
Since there is an average DC load you should still see a voltage drop determined by the DC resistance.
So arguably the circuit will show the same droop without the cap however, with amplifiers a large series
resistance can cause the amp to oscillate so the impedance model will help stop that happening to some degree.
OK I used your simple battery impedance model (based around 10 ohms) with what are probably close to the actual values used in the Ebow, and the waveforms are getting closer still.
(https://i.postimg.cc/4nV3C7hx/Reg-Mode-ACTUAL-VALUES.jpg) (https://postimg.cc/4nV3C7hx) (https://i.postimg.cc/jwJSSCgv/Harm-Mode-ACTUAL-VALUES.jpg) (https://postimg.cc/jwJSSCgv)
Quote from: Rob Strand on January 04, 2023, 08:14:15 PM
Something else, imagine the AC output swinging +/-3V peak and the output load is 8 ohm. That's 3V/8 = 375mA which will cause a 10*0.375 = 3.75V drop across a 10 ohm battery impedance. So quite a bit of drop on a 9V rail. If you backed the load off to 16 ohm, 3V/16 = 188mA, drop across 10 ohm = 1.9V. As the battery impedance goes up drop the amount of swing of the amp will drop and there's a point where you won't get +/- 3V peak. If the load impedance is increased you will get less drop and more output swing. So for a *given* battery DC resistance there will be a load which gives the maximum amount of field. The thing is for a 10 ohm battery impedance the maximum field won't end-up with a 79 ohm coil, it will be a lower impedance.
I see. That makes sense. I remember early on I was seeing quite a drop in the supply voltage when I measured it. Maybe that would be my beacon for knowing when I am on the right track with the driver coil?
I'm wondering if there is a simple way to see what is actually happening at that diode. Sounds like I need a differential probe? Can I DIY one? If I follow what anotherjim was getting at, it seems like the diode might be rectifying AC current and the cap is there to smooth out the ripple? This is a new way of thinking for me... I am used to thinking that ground is always zero so to speak, but I guess that is not the case when it comes to current.
On the real circuit, you don't need a differential probe, you just clip the scope probe ground on the battery negative. If the rest of the circuit doesn't have the same ground as the scope, the probe tip on the diode cathode will give you a reading "across" the diode. A differential probe can give a better picture as it tends to cancel common-mode noise, just like a balance audio feed can. A 2-channel scope can be made to work differential with 2 probes by selecting "add & invert". Channel 2 is inverted and added with channel 1 and magically you have a differential probe. A Single channel scope will not have this function.
In the sim, you could maybe attach a different ground symbol to the battery negative and if the software allows it, select this battery ground for the probe ground. I don't know how LTspice works this, but I've a feeling it may be stuck using the circuit ground.
However, I don't see why you can't remove the extra ground points in the sim and just draw wires making all the ground connections going via the switch to a single ground symbol on the battery negative. Then it's more like the real circuit. Any signal source you feed in to replicate the string pickup must ground via a wire to the circuit ground, not a ground symbol. Now the sim probe will have to reference the battery negative as ground so a probe on the diode cathode will be across the diode.
There is an article called "Vibrating Wire Audio Filter and Oscillator" in the May 1968 issue of Radio-Electronics:
https://worldradiohistory.com/Archive-Radio-Electronics/60s/1968/Radio-Electronics-1968-05.pdf
starting on page 52. They didn't use any IC's, just 2N2953 and 2N2925 transistors but they had current running along the wire (guitar string) and a U-shaped magnet over the wire. This may be easier than using a second pickup as a driver. The vibration is in the direction of towards and away from the guitar body in the example they give, so you may need to rotate the magnet to get a better output.
QuoteOK I used your simple battery impedance model (based around 10 ohms) with what are probably close to the actual values used in the Ebow, and the waveforms are getting closer still.
Looks convincing to me.
QuoteI see. That makes sense. I remember early on I was seeing quite a drop in the supply voltage when I measured it. Maybe that would be my beacon for knowing when I am on the right track with the driver coil?
I'm not quite sure but early on I think you were using a Heavy duty battery? Later you switched to the Alkaline, and I think you had the Alkaline battery when you did the experiment of adding more turns - as per the video with the "version 2" output coil.
In your experiment add more turns of the *same* wire should give some more output since it reduces the battery loading. However, looking at your results you are getting a stronger effect than I would expect. So that makes me think there's more to the sag story than the basic battery impedance. Here were are talking electrical effects.
Another aspect of your experiment is the magnetic effect. When you add more turns is that helping the field drive the string. A larger coil tends to "project" the magnetic field further out. So that could show up as better string drive. By the same token, the effect of the small diameter iron core coil overshadow the coil shape aspect. However, both you 8 ohm coil and the later 96 ohm coil had more or less the same final coil OD so the projection theory doesn't hold up.
I don't know what's going on. My gut feeling is something is sagging more than expected, maybe the LM386 output.
As far as playing with the turns on the coil and the battery DC resistance, the following set-up looks OK to me. It does not factor in the possibility of the larger diameter coil having an effect.
(https://i.postimg.cc/NyrBhCGj/Ebow-Drive-Optimization-schematic.png) (https://postimg.cc/NyrBhCGj)
The way it is set-up is the Bout output tries to model the strength of the field from the coil. You can adjust the number of turns using the variable "N". At the moment the simulation steps through three different number of turns. The inductance and resistance are calculated from the turns. There two cases for the resistance. If you use "Rx" for the value of the output resistor the resistance is based on on filling-up the bobbin with the given number of turns. This would be considered an optimal coil. The second case is "Rx2" which is the case where you add turns of the same wire diameter - I've put that in there so so can see it produces a different result to "Rx". You can also see that adding turns in your experiment should increase the output, but the opposite is true for Rx!
I've also got the battery impedance in there to model the sag. It's set to quite a high value.
So here's the result when you change the turns but you also change the wire diameter keep the bobbin full.
[N increasing green -> blue -> red]
(https://i.postimg.cc/w7vMvp9W/Ebow-Drive-Optimization-RDC-50-ohm-bobbin-full.png) (https://postimg.cc/w7vMvp9W)
And here's your experiment of adding more turns of the same wire.
(https://i.postimg.cc/nMLpKGNr/Ebow-Drive-Optimization-RDC-50-ohm-same-wire-added.png) (https://postimg.cc/nMLpKGNr)
The traces overlap so here's a zoomed version.
(https://i.postimg.cc/fkgZ80hK/Ebow-Drive-Optimization-RDC-50-ohm-same-wire-added-zoomed.png) (https://postimg.cc/fkgZ80hK)
Conclusions:
- if the bobbin is kept full it seems even with a high battery impedance the output increases with a lower impedance coil (less turns).
(I'll check this again to see if we can increase the battery impedance further to see if there is a minimum load.)
- if the same wire is added to we get a small increase in output with more turns. The increase is somewhat smaller than the results in your experiment - that's a mystery to me!
There's a few other options on the schematic, for example the output cap can be automatically chosen to resonate with the output coil inductance at a specified frequency. ATM, the cap is fixed at 220uF.
Quote'm wondering if there is a simple way to see what is actually happening at that diode. Sounds like I need a differential probe? Can I DIY one? If I follow what anotherjim was getting at, it seems like the diode might be rectifying AC current and the cap is there to smooth out the ripple? This is a new way of thinking for me... I am used to thinking that ground is always zero so to speak, but I guess that is not the case when it comes to current.
Because the unit isn't grounded you can just wire the Oscilloscope across the diode. For LT spice you can plot the voltage across the diode directly.
As an aside, this old post would be something which would stress the output of the LM386 and show any output sag/drop under load. By running the circuit through LTspice we can see if the LM386 model works correctly when the LM386 output is stressed.
https://www.diystompboxes.com/smfforum/index.php?topic=124739.0
Here's the set-up for the simulation. The input source is a stiff power supply, not a battery.
(https://i.postimg.cc/7JrwT4xj/LM386-model-verification-LM386-charge-pump.png) (https://postimg.cc/7JrwT4xj)
Unfortunately, the original article put a spanner in the works because they talk about adding 1 ohm resistors in series with the IC outputs to measure current but they don't make it clear if those resistors were in place when they produced their results tables.
I can match their results if I place a 1.3 ohm resistor in series with the IC output.
The question is, did their results have the resistor or not? ahhh!
If not, then I would need to add 1 ohm emitter resistor to the output stage of the LM386 model. This is something I looked into earlier but I found no information if the LM386 has emitter resistors or not. I did find the LM380 schematics which show 0.5 ohm emitter resistors. Just to be clear, we can't just add the 1 ohm emitter resistors to my model the AREA parameter of the biasing diodes would probably need to be tweaked. For now, as a hack, we can just add 1 ohm in series with the output of the IC. Keep in mind at this point we don't know if we should add it or not.
It was a real pain in the rear but I managed to get some scope traces across that diode. Not sure they can be trusted 100% as it's not really a proper fully functional unit but it might give us some idea of what it is doing. :icon_question:
Regular mode, scope set to AC:
(https://i.postimg.cc/svVV818B/IMG-0389.jpg) (https://postimg.cc/svVV818B)
Harmonic mode, scope set to AC:
(https://i.postimg.cc/c67xXfDy/IMG-0387.jpg) (https://postimg.cc/c67xXfDy)
Regular mode, scope set to DC:
(https://i.postimg.cc/bs3zjpcm/IMG-0397.jpg) (https://postimg.cc/bs3zjpcm)
Harmonic mode, scope set to DC:
(https://i.postimg.cc/3kN38V3Y/IMG-0394.jpg) (https://postimg.cc/3kN38V3Y)
Not exactly sure what it's doing here, but it appears that in regular mode it would be suppressing harmonics?
I also made that snooping coil and did what I could with it. Couldn't use it as originally intended but I'm sure one of you brainiacs can find something useful in it. The main challenge was creating the Ebow Levitation Device so I could use the coil. That was a fun exercise making that appendage to attach to the testing jig. I didn't look at the guitar pickup output as I already have an idea of the output voltages I would be seeing there. Should I also look at that too? I still have the jig set up, so I could also do that if need be.
Specs on the coil are as follows: 20 turns of 0.40mm wire, coil O.D. 23mm, 0.77mH, 100 ohm series resistor on + connection of BNC cable, total DC resistance 104 ohms, wound CCW with starting point being + on the BNC cable. Coil was at bottom of Ebow, so about 2mm or so from the bottom of the coils (actually a little more than that if you count about 0.75mm-1mm for the thickness of the plastic housing). String was about 5mm above the pickup, which puts the snooping coil about 2.5-3mm above the pickup. Input/sensing coil always measured 2mV.
QuoteNot exactly sure what it's doing here, but it appears that in regular mode it would be suppressing harmonics?
It looks quite different to what I see on the simulation.
Regular mode has 20mV swing (due to the diode),
(https://i.postimg.cc/TybmgPZx/Regular-Mode.png) (https://postimg.cc/TybmgPZx)
Harmonic mode I'm seeing about about 6Vp-p.
(https://i.postimg.cc/bZ7SyhdB/Harmonic-Mode.png) (https://postimg.cc/bZ7SyhdB)
The above sims were with zero battery resistance but oddly enough adding a significant battery resistance didn't change things much. At 400Hz your "Version 2" output coil has an impedance of about 100 ohms so it will take quite a large battery resistance to produce a significant drop.
The 6mV swing on your normal mode waveform looks like the voltage could be across the switch itself (?), perhaps the CRO ground was on the battery -Ve terminal and not the circuit ground? The general shape of the waveform is OK but that originates from the clipped output of the LM386.
I'm seeing a larger swing in Regular mode. It's possible to see that if the actual LM386 output is dropping under load more than the simulation.
I haven't tried a zener in place of the diode.
I tried a zener and the waveform shape is wrong.
but ...
If the 4.7uF cap was 10uF it would fixed the voltage swing across the diode. ... and then I realized your harmonic mode is at 770Hz, which effectively does the same thing as making the cap larger. However, it's doesn't quite work out like that, since the voltage is still high,
(https://i.postimg.cc/CdD48S3S/Harmonic-Mode-770-Hz.png) (https://postimg.cc/CdD48S3S)
I'm puzzled. Should there be anything meaningful across the diode/cap in normal mode? I thought they were out of circuit then.
QuoteI'm puzzled. Should there be anything meaningful across the diode/cap in normal mode? I thought they were out of circuit then.
Yes, it's a bit confusing.
Up to now we have kind of put the diode + cap to the side.
Here's my take on what the Ebow is:
(https://i.postimg.cc/cv9TzzJH/Ebow-sch-V20-2023-01-09.png) (https://postimg.cc/cv9TzzJH)
If you go to reply #4 in this thread you can see the PCB.
The connector at the top left goes: GND, (+) VCC, VBN
At least that's how I think it all goes.
Paul's rebuilt unit has different coil specs.
(Just realized my schem shouldn't have "V2" coils, they are "original" coils).
Thanks, that is clearer. So either the whole circuit (except the output coil) returns via the diode or just the output coil does. Paul's waveforms make more sense to me now.
This is really hard to understand because although the 4.7uF bypasses the diode for AC, it is just small enough to have some impedance. The diode//cap arrangement suggests it is meant to represent an asymmetric impedance for AC, but the way it's switched over between modes, frankly makes my brain hurt!
Could it be, that in normal mode, the diode//cap has only a trivial effect and it was just convenient to switch it like that?
QuoteThis is really hard to understand because although the 4.7uF bypasses the diode for AC, it is just small enough to have some impedance. The diode//cap arrangement suggests it is meant to represent an asymmetric impedance for AC, but the way it's switched over between modes, frankly makes my brain hurt!
Could it be, that in normal mode, the diode//cap has only a trivial effect and it was just convenient to switch it like that?
I don't get it 100% either.
Since a long way back, the way I've been looking at it as:
- Normal mode is just the trivial diode drop. The thing that bugs me is why have a diode drop at all? You could easily arrange the switch to bypass the diode! Maybe a halfway reverse protection?
- Harmonic mode is some sort of high-pass filter with the aim to promote the string oscillation to occur at higher frequencies. (We know the high-Q string doesn't care so much about phase, so amplitude wins for oscillation.)
The way I saw the diode is a small cap alone looses too much signal so the half passing and half high-pass filter
is kind of "in between".
I'll admit upto this point I haven't put a lot of time trying to confirm those thoughts at all.
Is it possible that the 4.7uF cap is actually bad? Or is that how it's supposed to be with the very squashed sine wave at that diode in regular mode? I still suspect that 4.7uF cap, since it was damaged by the person that de-gooped the circuit board. It seems to test OK but I've had cases where electrolytic capacitors seemed to test OK but they really weren't. :icon_confused: I guess I could put a different 4.7uF cap in and see if I get different result. Also am wondering if that diode is perhaps not a 1N4148. I can't imagine what else it could be if not.
When I was initially figuring out the schematic, I saw it as regular mode battery return path as being forced thru the cap/diode and in harmonic mode there was a direct battery return. The driver coil also has to go thru the diode/cap to ground, but that is full time. For the battery return it's the path of least resistance. In AC design, the air always takes the path of least resistance. I'm thinking of this in more of a mechanical engineering mindset as that is what I do every day - I design HVAC for enormous 15,000-25,000+ sq ft houses in fancy gated communities in the middle of a very hot arid desert, which are centered around big green lawns where people try to whack a little white ball into a tiny hole from many yards away. No one wants to see any of my HVAC equipment, including the air devices, but I will for sure hear about it if something doesn't work as expected! :icon_lol:
(https://i.postimg.cc/zyryYWdN/EBOW-PCB.jpg) (https://postimg.cc/zyryYWdN)
That's a very clear view of the PCB.
If you do change the cap, how about checking what happens if it's out and there is only the diode?
QuoteIs it possible that the 4.7uF cap is actually bad? Or is that how it's supposed to be with the very squashed sine wave at that diode in regular mode? I still suspect that 4.7uF cap, since it was damaged by the person that de-gooped the circuit board. It seems to test OK but I've had cases where electrolytic capacitors seemed to test OK but they really weren't. :icon_confused: I guess I could put a different 4.7uF cap in and see if I get different result. Also am wondering if that diode is perhaps not a 1N4148. I can't imagine what else it could be if not.
QuoteIf you do change the cap, how about checking what happens if it's out and there is only the diode?
The cap is definitely doing something. There has to be some resonant effect, even if the diode makes it non-linear. Also I'm sure the cap there is doing something because the measured swing across the diode depends on it. I think the cap is working even though it doesn't match the sim. It's possible the cap isn't exactly 4.7u, that's no surprise for an electrolytic.
QuoteNo one wants to see any of my HVAC equipment, including the air devices, but I will for sure hear about it if something doesn't work as expected!
You can say the same for just about anything. There's plenty of $100,000 loudspeakers in such places. The owners have no idea of the enormous technical knowledge and 50 years of development history it took to produce it. Who thinks about why the steering wheel on your car is what it is? size, shape, profile, material ... someone has to make those design choices! It's easy to change something but only a few people in the world know what it takes to *improve* the design from one model to the next.
Quote from: anotherjim on January 09, 2023, 04:42:28 PM
That's a very clear view of the PCB.
If you do change the cap, how about checking what happens if it's out and there is only the diode?
That's the CAD drawing that I made of it. Been tweaking it over the last month or so. I typically do this for projects I work on/reverse engineer/clone, just for my own benefit, and sometimes for the benefit of others. :icon_wink:
I know that reversing that diode just killed the power in one of the modes, but I don't think I ever checked to see what happened if not in the circuit.
Quote from: Rob Strand on January 09, 2023, 05:02:59 PM
The cap is definitely doing something. There has to be some resonant effect, even if the diode makes it non-linear. Also I'm sure the cap there is doing something because the measured swing across the diode depends on it. I think the cap is working even though it doesn't match the sim. It's possible the cap isn't exactly 4.7u, that's no surprise for an electrolytic.
Yeah seems like it's doing something but not sure if it's working as intended. I have a feeling once that cap has to do some "hard work" it craps out. Could also be my driver coil is too big and loading down the LM386 output too much?
Quote from: Rob Strand on January 09, 2023, 05:02:59 PM
QuoteNo one wants to see any of my HVAC equipment, including the air devices, but I will for sure hear about it if something doesn't work as expected!
You can say the same for just about anything. There's plenty of $100,000 loudspeakers in such places. The owners have no idea of the enormous technical knowledge and 50 years of development history it took to produce it. Who thinks about why the steering wheel on your car is what it is? size, shape, profile, material ... someone has to make those design choices! It's easy to change something but only a few people in the world know what it takes to *improve* the design from one model to the next.
Yeah, good points. I have worked on some projects where I was told that the AV room equipment alone was $250,000. :icon_eek: I can't even imagine that. I live in a far different world than these people do.
QuoteCould also be my driver coil is too big and loading down the LM386 output too much?
Generally, a higher impedance load reduces current consumption.
The choice of 220uF for the 386 output capacitor "might" indicate an 8ohm load since the low-frequency point is 90Hz which suits guitar range, but 220uF could just be a safe bet "plenty big enough" value and the coil impedance is actually higher than 8 ohm.
It may be that working the 386 with a reduced load alters the effectiveness of the 4.7uF in the circuit as its impedance will be less significant. That could be re-scaled with a lower-value cap. If you take it as a simple RC filter, 8ohm and 4.7uF is the same as 80ohm and 0.47uF.
Quote from: anotherjim on January 10, 2023, 08:17:38 AM
QuoteCould also be my driver coil is too big and loading down the LM386 output too much?
Generally, a higher impedance load reduces current consumption.
The choice of 220uF for the 386 output capacitor "might" indicate an 8ohm load since the low-frequency point is 90Hz which suits guitar range, but 220uF could just be a safe bet "plenty big enough" value and the coil impedance is actually higher than 8 ohm.
It may be that working the 386 with a reduced load alters the effectiveness of the 4.7uF in the circuit as its impedance will be less significant. That could be re-scaled with a lower-value cap. If you take it as a simple RC filter, 8ohm and 4.7uF is the same as 80ohm and 0.47uF.
Right, but doesn't that also depend somewhat on the wire gage? I was thinking bigger wire = more current. I'm pretty sure that the wire I used was several gages larger than what's in the actual unit. In the DIY Ebow I made a week or so ago, using the internet schematic that is wrong, the LM386 seems to be very sensitive to what is happening at the input & output coils as far as the DC resistance and wire gage is concerned. I apparently killed one LM386 on my first attempt at trying to find an output coil that worked (ended up with 32 ohms, but I jumped from 8 ohms directly to 32). I also used wire gage that was too heavy, but my only goal was to see if it could actually work. I ended up with something which didn't really work - it got the string vibrating but it was not even close to being a usable device. I'm sure it could be refined somewhat but I still don't think I can get it to be a usable circuit.
I watched that YouTube video of the Russian(?) guy who made a working one with a 3D printed housing, but he was not using the internet schematic. Looks to me like he made the version of Ebow that pre-dates the Plus Ebow, and it only had an on/off switch. Looks like it had a feedback loop, otherwise it should have had one more resistor, two more capacitors and a diode. No idea what he used for the coils but he made it work by having them extend out of the bottom of the 3D printed housing. Probably had to do that because the coils weren't close enough to the strings otherwise.
On that diode, I was thinking last night that maybe it's intended to be a sort of limiter, or maybe to create asymmetry in the waveform? It must be there for some reason.
Diode is just one-way non-return valve. In the forward direction, the diode switches on and the cap across it is bypassed, in reverse, the only path is the cap since the diode will switch off.
This is definitely going to be asymmetric.
In normal mode, with the whole circuit DC current flowing in the forward direction back towards the battery negative, the diode never switches off and any AC current also gets a path thru the 4.7uF cap across the diode. You see a small waveform across the diode/cap because of the small DC volt drop of the diode junction and that the 4.7uF cap does not have zero impedance at the frequency. Rob mentioned earlier that the diode will offer some reverse polarity protection if someone tries to fit the battery the wrong way. What if the older E-bow without Harmonic mode also had this feature?
Switched over to the coil return only in harmonic mode, the negative half cycle of the output waveform has to go via the cap. For the positive half cycle, the diode switches on and bypasses the cap. This only happens like this in Harmonic mode.
When the waveform is negative going, the cap introduces an RC high pass filter (Actually RLC but ignoring the coil inductance for now). If the 386 is outputting square waves, a high pass filter will make it look more like a curved sawtooth with even harmonics taking over from the odd harmonics of the squarewave. However, the power in the coil will be reduced with the narrower waveform of a sawtooth so that may be the reason for treating only one half cycle of the wave.
Quote from: anotherjim on January 10, 2023, 01:58:59 PM
Diode is just one-way non-return valve. In the forward direction, the diode switches on and the cap across it is bypassed, in reverse, the only path is the cap since the diode will switch off.
This is definitely going to be asymmetric.
In normal mode, with the whole circuit DC current flowing in the forward direction back towards the battery negative, the diode never switches off and any AC current also gets a path thru the 4.7uF cap across the diode. You see a small waveform across the diode/cap because of the small DC volt drop of the diode junction and that the 4.7uF cap does not have zero impedance at the frequency. Rob mentioned earlier that the diode will offer some reverse polarity protection if someone tries to fit the battery the wrong way. What if the older E-bow without Harmonic mode also had this feature?
Switched over to the coil return only in harmonic mode, the negative half cycle of the output waveform has to go via the cap. For the positive half cycle, the diode switches on and bypasses the cap. This only happens like this in Harmonic mode.
When the waveform is negative going, the cap introduces an RC high pass filter (Actually RLC but ignoring the coil inductance for now). If the 386 is outputting square waves, a high pass filter will make it look more like a curved sawtooth with even harmonics taking over from the odd harmonics of the squarewave. However, the power in the coil will be reduced with the narrower waveform of a sawtooth so that may be the reason for treating only one half cycle of the wave.
I see, that makes sense. Thanks for explaining that! Your guess is as good as mine as far as whether or not the Ebow version before this one had that diode in it. I tend to think not, but maybe they had one for short circuit protection. Maybe they came up with this quirky arrangement simply because they have very little room to spare on that tiny PCB.
Here's some deeper dive analysis of Harmonic mode,
(https://i.postimg.cc/yDDYKpCW/ebow-harmonic-mode-sch-2023-01-11.png) (https://postimg.cc/yDDYKpCW)
(https://i.postimg.cc/TL6RrRLW/ebow-harmonic-mode-waveforms-2023-01-11.png) (https://postimg.cc/TL6RrRLW)
So after all that I ask myself if it's only resonating the inductance with smaller output cap to get more gain, why have the added complexity of the diode and DC shifting!
Quote from: Rob Strand on January 10, 2023, 05:13:32 PM
Here's some deeper dive analysis of Harmonic mode,
(https://i.postimg.cc/yDDYKpCW/ebow-harmonic-mode-sch-2023-01-11.png) (https://postimg.cc/yDDYKpCW)
(https://i.postimg.cc/TL6RrRLW/ebow-harmonic-mode-waveforms-2023-01-11.png) (https://postimg.cc/TL6RrRLW)
Very interesting stuff! Seems to me like the diode clamp is to create an asymmetrical waveform. What happens if that is not present? Will that still drive the string? Or will it drive the string and who knows what the result is? The "Ebow" that I recently made from the internet schematic doesn't come anywhere close to this circuit... it's a wild mess that makes little sense.
Quote from: Rob Strand on January 10, 2023, 05:13:32 PM
So after all that I ask myself if it's only resonating the inductance with smaller output cap to get more gain, why have the added complexity of the diode and DC shifting!
Is it enough to just change the frequency of the oscillator? Or does there need to be a corresponding change in the waveform to get the string to go one way or the other? The harmonic mode has those "extra" humps in the waveform... seems like that must be a factor. What keeps the string from just resonating at its normal frequency? Seems like that goes back once again to the place where the anti-nodes occur on the string. I think all these things work together. I did a crude model of just that diode/cap arrangement with the coils inductively coupled to guitar pickup equivalent circuits along with that simple battery impedance tweak, and I could easily get the harmonic mode waveform at the pickup but I couldn't achieve the regular mode waveform. It's quite likely that I just don't know how to really model that properly. Or maybe it needs to be far more complex so we can add the string wave characteristics? I don't want to really take if that far, that's really just a rhetorical question. That LTSpice is pretty amazing for a free program.
I think at this point we have an "Everything You Ever Wanted To Know About The Ebow But Were Afraid To Ask". Or maybe it's everything you didn't ask about. :icon_lol: Well, at least I've satisfied my own curiousity about it. Sure have learned a lot thru this whole process, and it was fun coming up with these testing jigs and whatnot. :icon_cool:
QuoteVery interesting stuff! Seems to me like the diode clamp is to create an asymmetrical waveform. What happens if that is not present?
Once the caps charge up there's no asymmetry, in fact the diode is essentially removed from the circuit. It serves only to shift the DC level. When the Ebow is first placed on the string it might take a few cycles for the cap to charge up, so have will have a short transient.
We expect the diode to do nothing, and pulling the diode sure shows that in the simulation. You can see the DC voltage across the out cap is now back to normal (Vcc/2).
(https://i.postimg.cc/mP8MZqHW/ebow-harmonic-mode-waveforms-no-diode-2023-01-11.png) (https://postimg.cc/mP8MZqHW)
Quote
Will that still drive the string? Or will it drive the string and who knows what the result is?
I'm pretty sure it will drive the string. The point about the 4u7 cap forming a resonance with the output coil inductance is not so much will it work, but will generate the higher harmonics as advertised. The idea behind it should help at least!
On your rebuilt "version 2" transmit coil the inductance is higher so you will need smaller cap to resonate at the same frequency. Since both the inductance and resistance of the coil are different you might find it can still resonate in a similar way.
QuoteThe "Ebow" that I recently made from the internet schematic doesn't come anywhere close to this circuit... it's a wild mess that makes little sense.
I guess the on-line schematic doesn't have all the frequency shaping tweaks. It doesn't support harmonic mode so it is less fussy as well. However, over all it's still a feedback system like the Ebow. I wonder if those DIY versions make the string vibrate at the fundamental, wheres as the normal mode for Ebow oscillates at the second harmonic (probably due to the frequency shaping).
Quote
Is it enough to just change the frequency of the oscillator? Or does there need to be a corresponding change in the waveform to get the string to go one way or the other? The harmonic mode has those "extra" humps in the waveform... seems like that must be a factor. What keeps the string from just resonating at its normal frequency?
Seems like that goes back once again to the place where the anti-nodes occur on the string. I think all these things work together.
The way I see it at the moment is the 4u7 cap resonating with the output coil is what promotes the string to vibrate at a higher frequency. From the earlier posts, the string is a high Q resonator so it doesn't case so much about phase. You just make the gain high at high frequencies and it will cause the string to vibrate at the higher frequency instead of the lower one (since the string has many possible modes of vibration).
QuoteI did a crude model of just that diode/cap arrangement with the coils inductively coupled to guitar pickup equivalent circuits along with that simple battery impedance tweak, and I could easily get the harmonic mode waveform at the pickup but I couldn't achieve the regular mode waveform.
The waveform shape is one thing but what frequency the string vibrates at comes from a completely different cause. The string is a resonator which can vibrate at many different frequencies. The spice model for the string was simplified in that it only modeled the fundamental mode. If you put a more complex and physically realistic string model in spice then it will have the capability to vibrate at different frequencies, and once you do that the changes you make in the circuit will show up as a physical effect. For example, boosting the HF gain will cause the string to vibrate at a harmonics.
If you wanted to use spice to simulate placing the Ebow at different *positions* on the string and exciting the string around node points then you would need to add a lot more to the string model. How does LTspice know what the spacing between the coils is and where you are placing the Ebow? You would need parameters for all that. It takes a lot of effort to get all that to happen.
QuoteIt's quite likely that I just don't know how to really model that properly. Or maybe it needs to be far more complex so we can add the string wave characteristics? I don't want to really take if that far, that's really just a rhetorical question.
It's not a simple matter to capture all aspects of the string. If it was easy, I would have tried it out already :icon_mrgreen: :icon_mrgreen: :icon_mrgreen:
QuoteThat LTSpice is pretty amazing for a free program.
Yes, really cool. It's like a big circuit calculator. I used to do all a lot of this stuff with algebra and hand-written programs. Circuit simulators have been around for a long time. They started off with only text output devices like terminals and printers. You could get free programs not long after PC's were around, first DOS text, then DOS with graphics, then the various Windows versions. Around the late 1990's to early 2000's there were heaps of free ones. Since LTspice arrived it's demotiviated a lot of that effort.
Here's a very crude model of a string with multiple resonances.
The loss resistance 10k ohm is the same for each resonance.
The anti-resonance at 250Hz looks a bit dodgy - I haven't thought about it enough :icon_mrgreen:.
The main point is it has multiple resonances.
(https://i.postimg.cc/7fxZPNrg/tline-string-model-V10.png) (https://postimg.cc/7fxZPNrg)
FYI, here's an LCR version with multiple resonances.
(https://i.postimg.cc/m1fWByM9/dual-resonance-string-model-V10.png) (https://postimg.cc/m1fWByM9)
I did a quick simulation with the following set-up:
- A string with three modes, frequencies f1, 2*f1, 3*f1.
- Ebow set to normal mode
- Ebow schematic, parts for original circuit as we see it so far.
- Feedback resistor set so no self oscillation
- Strength of string coupling set so unit starts string oscillation in about 0.5 second
I then increased the input filter cap (the 33nF cap) so it starts to filter the higher frequencies.
As the input filter cut-off becomes lower it promotes the string to oscillate at the lower
frequency modes. The aim is to show that modifying the response can cause different
string modes to occur (no intent to show more than that).
Result: it does just that.
Here's the schematic:
(https://i.postimg.cc/cK9Ly4S3/multimode-string-with-lpf-V10-schematic.png) (https://postimg.cc/cK9Ly4S3)
From the table you can see how large the filter cap needs to be before the string changes to the
next lower frequency mode.
Here's the waveforms when the input cap is 1, 3, 6 times the normal value. kc was chosen
to be a reliable value for each frequency based on the table on the schematic. Notice
that it doesn't take much to stop the 1200Hz mode, that means the input cap and
inductance values are just making it.
(https://i.postimg.cc/G8grNMvx/multimode-string-with-lpf-V10-waveforms.png) (https://postimg.cc/G8grNMvx)
The next step would be to get the string mode to change when switching from Normal mode to Harmonic mode.
As far as the Rebuilt Ebow goes, it should be clear that if the coils aren't the same as the original the resulting changes in the frequency response could screw-up Normal-mode or Harmonic-mode. We might be able to tune the input cap (33n) to help but switching between the two modes may or may not work with different coils.
Quote from: Rob Strand on January 11, 2023, 04:46:48 PM
I did a quick simulation with the following set-up:
- A string with three modes, frequencies f1, 2*f1, 3*f1.
- Ebow set to normal mode
- Ebow schematic, parts for original circuit as we see it so far.
- Feedback resistor set so no self oscillation
- Strength of string coupling set so unit starts string oscillation in about 0.5 second
I then increased the input filter cap (the 33nF cap) so it starts to filter the higher frequencies.
As the input filter cut-off becomes lower it promotes the string to oscillate at the lower
frequency modes. The aim is to show that modifying the response can cause different
string modes to occur (no intent to show more than that).
Result: it does just that.
Very interesting. So I guess the different waveforms are just kinda incidental and not necessarily intentional?
Quote from: Rob Strand on January 11, 2023, 04:46:48 PM
Here's the schematic:
(https://i.postimg.cc/cK9Ly4S3/multimode-string-with-lpf-V10-schematic.png) (https://postimg.cc/cK9Ly4S3)
From the table you can see how large the filter cap needs to be before the string changes to the
next lower frequency mode.
Here's the waveforms when the input cap is 1, 3, 6 times the normal value. kc was chosen
to be a reliable value for each frequency based on the table on the schematic. Notice
that it doesn't take much to stop the 1200Hz mode, that means the input cap and
inductance values are just making it.
(https://i.postimg.cc/G8grNMvx/multimode-string-with-lpf-V10-waveforms.png) (https://postimg.cc/G8grNMvx)
The next step would be to get the string mode to change when switching from Normal mode to Harmonic mode.
The capacitor values & corresponding frequencies is useful info. If I ever feel like revisiting this project, or maybe try building a proper one from scratch this info will be handy. I'm wondering what the Russian(?) dude with the DIY Ebow used for his coils. Those worked but they probably needed to be a little bit stronger so he didn't have to make them extend thru the bottom of his 3D printed housing.
Quote from: Rob Strand on January 11, 2023, 04:46:48 PM
As far as the Rebuilt Ebow goes, it should be clear that if the coils aren't the same as the original the resulting changes in the frequency response could screw-up Normal-mode or Harmonic-mode. We might be able to tune the input cap (33n) to help but switching between the two modes may or may not work with different coils.
Yeah I think that is my situation with the re-built one. It has trouble with one mode but not with the other. I had socketed that fb cap and had the 20-turn pot in place of the fb resistor and I just messed with it until I found the best compromise. Was far from perfect, but my only real goal was to get it working again on some level. Did better than I expected I would.
QuoteVery interesting. So I guess the different waveforms are just kinda incidental and not necessarily intentional?
I'd say incidental to a point. Many oscillators use clipping to control the gain and it's amazing how crappy the waveforms can be. The resonant/filtering network, smooths it all out. In the case of the Ebow the smoothing would be done by the string. However, the thing about the Ebow is when you place the drive coil over the pickup the crappy output waveform is getting injected directly into the pickup so you will hear some of it.
QuoteThe capacitor values & corresponding frequencies is useful info.
There's quite a few caveats:
- Those cap *values* only apply to the assumed 150mH coil. If you change the inductance of the coil you need different cap values to do the same thing, C_new = C_old * L_old / L_new; where L_old = 150mH and C_old are the values from the table.
- A second thing is my string is 400Hz (with modes 800Hz and 1200Hz). The different strings and fretted notes cover a wider range.
- If you hold the ebow close to the string (ie. increasing the coupling kf) you might get slightly different results.
I guess the main point is you can get the string to change what mode it resonates at but how to do it is buried in a zillion specific details - that's where the engineering of the Ebow has already been done.
QuoteI'm wondering what the Russian(?) dude with the DIY Ebow used for his coils. Those worked but they probably needed to be a little bit stronger so he didn't have to make them extend thru the bottom of his 3D printed housing
The Russian guys I saw were using those old buzzer/speakers which used to be inside the cases of PC's. You can buy these. They are electromagnetic devices, not the piezos types which look identical from outside. I posted some stuff in the "other" thread.
QuoteYeah I think that is my situation with the re-built one. It has trouble with one mode but not with the other. I had socketed that fb cap and had the 20-turn pot in place of the fb resistor and I just messed with it until I found the best compromise. Was far from perfect, but my only real goal was to get it working again on some level. Did better than I expected I would.
I think overall a tight enough box has been put around the problem to get the jist of what needs to be looked at. We have some idea of causes and effect.
QuoteI did a quick simulation with the following set-up:
- A string with three modes, frequencies f1, 2*f1, 3*f1.
- Ebow set to normal mode
- Ebow schematic, parts for original circuit as we see it so far.
- Feedback resistor set so no self oscillation
- Strength of string coupling set so unit starts string oscillation in about 0.5 second
FYI: While the results look convincing, there's a bit more to the why it works than I expected,
particularly for the case with kc=3. I'll put it on the back-burner for now.
This is very fascinating. I have a a couple Sustainiac sustainers. if you're not familiar, it's like the EBow but is a pickup that installs in the neck position. You can't I've the sustainer and the signal from the bridge pickup drives the sustainer which drives the stings like the EBow.
It has a couple modes, like the EBow, a fundamental mode, harmonic mode, and a mix mode that mixes the two. I could post some pictures if that would be interesting?
QuoteThis is very fascinating. I have a a couple Sustainiac sustainers. if you're not familiar, it's like the EBow but is a pickup that installs in the neck position. You can't I've the sustainer and the signal from the bridge pickup drives the sustainer which drives the stings like the EBow.
I only know if from my early digging around on this Ebow stuff. I have a PCB pic but it has heatshrink over it.
QuoteIt has a couple modes, like the EBow, a fundamental mode, harmonic mode, and a mix mode that mixes the two. I could post some pictures if that would be interesting?
Be interesting to see the structure they use and if they use the same tricks. One thing's for sure, the devils in the details with these things: coil inductances etc.
Do you know if normal mode on these devices vibrates the string at the fundamental or the second harmonic (like the Ebow)?
Quote from: Rob Strand on January 12, 2023, 05:00:21 PM
QuoteIt has a couple modes, like the EBow, a fundamental mode, harmonic mode, and a mix mode that mixes the two. I could post some pictures if that would be interesting?
Be interesting to see the structure they use and if they use the same tricks. One thing's for sure, the devils in the details with these things: coil inductances etc.
Do you know if normal mode on these devices vibrates the string at the fundamental or the second harmonic (like the Ebow)?
I have a Fernandes Sustainer System on one of my guitars. Was broken, given to me by a dude at Guitar Center in 2009 I think it was. I fixed it and put it into my old hot rodded '92 Ibanez EX370-FM. It's kinda like the Sustainiac pickup but without the mix mode. The guitar has to be rewired so that all of the pickups go thru their circuit board. When you activate the sustainer, it bypasses all of the pickups and uses only the bridge pickup. I like it because I can get those Phil Keaggy volume pedal with an Ebow kind of sounds on any combination of strings and play chords too. Sometimes I just leave it on while doing riffs or whatever, kinda fattens up everything and it's like being on the verge of feedback all the time.
Anyway I never thought to look at the output of it before. I just looked at the guitar output with the scope. I let the open B string sustain and in the "regular" mode it appears to be tripling the fundamental, and in "harmonic" mode it appears to be around 1.5kHz... so that's about 6x the fundamental? I didn't think it was that high! Both of the waveforms are basically a sine wave. Quite different than the Ebow. I wonder how the Sustainiac compares. I found the patent for it some time ago, it's got quite a few pages and my head melted when I started reading through it. :icon_lol: This was around the time when there was an enormous DIY sustainer thread going on at ProjectGuitar dot com forum, ca. 2007. That's how I obtained that destroyed Ebow PCB, from someone over there. That thread apparently doesn't exist anymore. :icon_confused:
QuoteAnyway I never thought to look at the output of it before. I just looked at the guitar output with the scope. I let the open B string sustain and in the "regular" mode it appears to be tripling the fundamental, and in "harmonic" mode it appears to be around 1.5kHz... so that's about 6x the fundamental?
Interesting. Getting it to consistently pull out the 6th harmonic with a basic feedback loop is going to be tricky. Perhaps the drive-coil and pickup positions need to be factored in. From what I can see the drive-coil is always towards the neck and the output coils are bridge and/or middle.
My naivety regarding guitar sustainers is starting to show because I didn't realize there were so many alternative products to Ebow.
QuoteI found the patent for it some time ago, it's got quite a few pages and my head melted when I started reading through it.
There's a very long patent on sustainers. The idea looked a lot like the DIY Ebows, perhaps with a phase switch. I need to read it again to see if it mentions any of the things we have learnt from the Ebow.
Here's one more interesting tidbit. This morning I was curious to see what kind of waveforms that the previous version of the Ebow produced (the black one with white logo and red LED). No big surprises but I think it explains a bit about the Plus Ebow which I have been focused on. I'm curious to know if it also has that cap & diode arrangement. I wish I had an X-ray machine just about now. :icon_biggrin:
You can see the results in this short video
Here are some pictures of my Sustainiac. I'm searching for the wiring diagram. Besides driving the sustainer the board also handles switching. When the sustainer is activated the bridge pickup is on - regardless of the switch position - and the guitar will work even when the battery is dead.
(https://i.postimg.cc/jD433V4m/0354-E787-311-A-4-A8-D-8-F69-DAE2-A68-D480-E.jpg) (https://postimg.cc/jD433V4m)
(https://i.postimg.cc/F1YqrBhk/217-EC726-48-FD-4-C34-AE93-9483-D8-A48-BF1.jpg) (https://postimg.cc/F1YqrBhk)
(https://i.postimg.cc/zHHPxVxF/2-C8163-B5-5093-4-B53-B5-D1-10-D34-B96-A2-FC.jpg) (https://postimg.cc/zHHPxVxF)
(https://i.postimg.cc/bd45s9vC/528648-CE-8-A47-4-D15-9724-193-B5-F706-C2-E.jpg) (https://postimg.cc/bd45s9vC)
(https://i.postimg.cc/SXZTqW5G/60-E25-F3-B-B208-4192-B33-F-8-D6-E497104-E2.jpg) (https://postimg.cc/SXZTqW5G)
(https://i.postimg.cc/68X1h6nC/B6-A206-DA-EBB8-4-D0-F-B80-B-C6-B8-C8716-AE8.jpg) (https://postimg.cc/68X1h6nC)
(https://i.postimg.cc/gwcSfMMJ/DD938-AEA-A902-45-AD-9-AFF-A8-ADC90444-A7.jpg) (https://postimg.cc/gwcSfMMJ)
(https://i.postimg.cc/1f6j6Vn9/E7040-AF2-5929-4-ED2-B4-AA-D3296-A9-FC691.jpg) (https://postimg.cc/1f6j6Vn9)
soggybag : https://www.sustainiac.com/install.htm
lots of different wiring types..depending on guitar pups etc..... 8)
QuoteHere are some pictures of my Sustainiac.
It's going to take a bit of effort going through that one, SMD, lot's of external connections.
QuoteI'm searching for the wiring diagram.
I had a wiring diagram from part 2 of this blog. Superficially it seems to match the PCB.
https://settechitarre.wordpress.com/tag/diy/
but deadastronaut's link is more comprehensive.
The Sustainiac patents give some info on the circuits and the thinking behind them. They are very long. I can't even remember which one I found earlier on, maybe the 2000 version.
US4941388 - Sustainaic, Hoover 1990
US5932827 - Sustainaic, Osborne & Hoover 1995
US6034316 - Sustainiac, Hoover 2000
The IC's on the PCB all look fairly common. The NDS8958 is a Fairchild P-channel + N-channel MOSFET - probably used to drive the coil. Plenty of transistors and diodes I haven't looked up.
Quote from: deadastronaut on January 14, 2023, 05:33:26 AM
soggybag : https://www.sustainiac.com/install.htm
lots of different wiring types..depending on guitar pups etc..... 8)
This Sustainiac is from early 2000s. The new ones have a different connector. I have on a piece of paper somewhere.
There are a few ways to wire it. I used a push-pull pot to turn the sustainer on. Another push-pull pot for the mix mode.
It works well. Mine has an issue where there is a little "grit" in the clean sound when the sustainer is off. I asked about and got a couple conflicting answers, no solutions. I got the impression it wasn't "fixable". Otherwise it works.
QuoteIt works well. Mine has an issue where there is a little "grit" in the clean sound when the sustainer is off. I asked about and got a couple conflicting answers, no solutions. I got the impression it wasn't "fixable". Otherwise it works.
If you probe the outputs of the comparator and the opamp you might find one of those is still enabled and clipping. The patents show muting circuits but perhaps the muting circuit is too far up the signal chain and the stuff before is clipping. Just how the grit gets into the clean circuits is up for grabs: power, Vref, or across one of the JFET switches. For that matter it could be the JFET switch itself leaking through because of a strong signal feeding it.
It's an amazing product. I think what I have the Sustainiac stealth. The newer stealth pro is improved.
Look carefully near the two smaller light blue electrolytic caps and you'll see a little pile of surface mount resistors. Looks like there was an error and the needed to jump something or make a voltage divider?
Quote from: soggybag on January 15, 2023, 10:17:55 PM
Look carefully near the two smaller light blue electrolytic caps and you'll see a little pile of surface mount resistors. Looks like there was an error and the needed to jump something or make a voltage divider?
Yes, I noticed the mod. I can't quite work out what it is doing. I meant to come back to it but I wasn't able to extract any more info.
The circuit is like:
cap --> resistor 1 (10k) --> mod resistor 2 (10k) --> resistor 3 (10k) --> pin 9 (input of 40106)
The 40106 is a CMOS hex Schmitt inverter, a digital part.
There's a tap between resistors 1 and 2 1 which goes to an unloaded SMD part, cap or resistor who knows. The other side of the unloaded part connects to the first cap in the above chain of parts. There must be some other tracks from that chain of parts.
As a wild guess it looks like some sort of signal detection circuit and the R's and C set a time constant of filter. It could equally be inverters used for the signal switching! The purpose of adding mod resistor 2 is still a mystery.
calling mr marossy... 8)
hi paul, do you have the dimensions of the ebow?
i have a cheapo 3d printer and would like to knock up an ebow...
not a direct copy. just a crude version ,but i just need the base plate dimensions really....
the battery box will be at an angle, for a more ergonomic design, instead of hooking your hand to be parallel with the strings. etc...which bugged me when i owned one....
cheers in advance.... 8)
Quote from: deadastronaut on March 19, 2023, 06:11:58 AM
calling mr marossy... 8)
hi paul, do you have the dimensions of the ebow?
i have a cheapo 3d printer and would like to knock up an ebow...
not a direct copy. just a crude version ,but i just need the base plate dimensions really....
the battery box will be at an angle, for a more ergonomic design, instead of hooking your hand to be parallel with the strings. etc...which bugged me when i owned one....
cheers in advance.... 8)
I was thinking not long ago that a DIY Ebow is so much more in the realm of possible these days with 3D printers being so ubiquitous these days.
I have a link to PDF file derived from my AutoCAD DWG that is actual size at my website. You can get it here: http://www.diyguitarist.net/PDF_Files/EBOW%20Data.pdf
brilliant, cheers man your a star.....
yeah im new to 3d printing, but its definitely got a lot of possibilities....
i,ll crack on. nice one. 8)
(https://i.postimg.cc/ns66FLTT/Basic.png) (https://postimg.cc/ns66FLTT)
This is how i am reading the circuit from the PCB board
which i think when in the REG stage will change things, that what is in the Spice Diagrames from the Videos (also notice a 33nF and 68nF towards the end so summing to be 100nF)
https://3dwarehouse.sketchup.com/model/5bbeefa3-b968-4762-859d-7b0321f576f7/ebow-simil?hl=en (https://3dwarehouse.sketchup.com/model/5bbeefa3-b968-4762-859d-7b0321f576f7/ebow-simil?hl=en)
if anyone can cut it so that only the one with the lid detached is being printed would be dam cool
well i managed to split it down but the BASE (bit that runs on the strings) is
2.57 (Length) x 1.26 (width) x 0.97 (Hight)
guy who measured it has it down as
48.36mm (Length) x 34.85mm (Width)
So length is a ratio of 18.81712:1 and Width is 27.65873:1
If it was in INCHS so that 2.57 (65.278mm) and 1.26 (30.004mm)
So Length is a Ratio of 0.74083152:1 and 0.92183475536:1
There is nothing new under the sun. There were more types of programmable instruments than a player piano - this one combines piano and violin and was made in the 1912 time frame. It was installed in bars as a precursor to the Seebeck jukeboxes that were seen in diners in the 1950's and 1960's. You insert a nickel in the coin slot and the programmed roll determines what notes will be hit.
It had the original sustain: there is a polished motor housing to the right of the violin that has a long shaft going to the left where it ends in a little white fabric roller above the strings where the bow would be placed. Additional solenoids select the string. The rotating part is brought down into contact with the string for a sustain that continues at the same amplitude as long as the note is played. The big polished housing behind and above the violin neck has solenoids that push down on the strings at the appropriate position to sound the correct note.
(https://i.postimg.cc/sXwK16LQ/DSCF0030.jpg)
There were lots of competing players of this type. This one has a xylophone, a triangle and a drum:
(https://i.postimg.cc/Fsp4xNJw/DSCF0039.jpg)
My choice for a sustain would be the vibrating wire oscillator shown in the article starting on page 51 here:
https://worldradiohistory.com/Archive-Radio-Electronics/60s/1968/Radio-Electronics-1968-05.pdf