Ebow Exposé Part IV - RESURRECTED!

[Paul Marossy]

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. 

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.

[anotherjim]

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.

[Paul Marossy]

...that diode.
It's switched into the battery return path, right?

Yes that is correct.

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.

[anotherjim]

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.

[Rob Strand]

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. 

[anotherjim]

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.

[Paul Marossy]

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.

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.

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. 

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.

[Paul Marossy]

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

[anotherjim]

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.

[Paul Marossy]

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.

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.

[anotherjim]

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.

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?

[Rob Strand]

Yes, it is twice the fundamental... but that was sitting over an open string.

Very interesting.

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.

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.

The 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.

[Paul Marossy]

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.

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.  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

[Rob Strand]

https://postimg.cc/gallery/6yYp9Sj

Those two shots definitely look similar.

[Paul Marossy]

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.

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 ). 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.

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. 

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! 

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. 

https://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. 

[Rob Strand]

Seems reasonable.

I guess the main point is the 4.7uF with your latest drive coil does have observable effects.

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.

Thanks for checking.  It all makes sense with that wire.

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!

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.


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.

ne 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.

Yes 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!

[Rob Strand]

I had this idea to compare the sensitivity/gain and frequency response of the real ebow and the rebuilt ebow from outside the box.



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.

[anotherjim]

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!

[anotherjim]

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?

[Rob Strand]

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?

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.