EM3207 (v1.1) - MN3207 based EHX Electric Mistress (9V) clone

[armdnrdy]

Thank you!

[GodSaveMetal]

THANKS MAN!!! GREAT!!!!!!!!!!!!

[lrgaraujo]

Thanks a lot! That's a nice sounding one!

[Thomeeque]

..and the pin 2 of the 1M pot is not connected in this layout, i see other schematic with this pin connected to the first pin

Yeah, Build instructions, Build Note #7

Hey guys, nice to see this thread still alive and kicking, cool stuff, thanks for pics and samples - still enjoying them (and special thanks to Dave for pointing people to the right places )

Cheers, T.

[DrAlx]

I've been looking through the earlier part of this thread, in particular the behaviour of the VCO when trying to achieve high clock rates. Thomeeque gave these measurements showing that for small CVs and delays, the linearity between CV and BBD delay is lost.  This can be seen as a flattening of the straight lines at the far left.  I think I can explain what causes this.



I set the CV to a few volts, and used a scope to measure how the clock capacitor voltage changes while charging.  The graph of  capacitor voltage versus time sort of has two sections. A short section at the very start of the charge process where the cap is being charged at high rate, and a much longer section for the rest of the charge process where it is being charged at a slower rate. 
Low CV thresholds correspond to stopping the charge process in the first section where the charge rate is fast, and this gives the flattened CV to delay characteristic on the left of the above picture. Higher CV thresholds correspond to stopping the charge process in the second section where the charge rate is lower and governed purely by the transistor current source.  So what causes the high initial charge rate?

I believe the rapid initial charging is due to the reverse current through the discharge diode D2 as it switches itself off.  As D2 goes into reverse bias, it takes time for the diode to remove charge carriers and create the depletion region and the associated reverse current through D2 charges up the clock cap.  Simulation seems to back this up.  There are a few ways of mitigating the effect of this reverse diode current:

1) Increase the supply voltage. Scaling up voltages and currents everywhere makes the effect of diode reverse current less noticeable overall.  Decreasing the supply voltage makes the non-linearity more noticeable.

2) Increase the value of the pullup resistor R34 at the comparator output.  This makes the diode dump its charge onto the capacitor over a longer time period.  The  gradients of the two charge sections mentioned above then become more closely matched, but the "knee" between them moves to a higher voltage. Its just a guess, but maybe this explains why the original 9V EM uses a larger pullup resistor than the 15V Deluxe EM (10k rather than 3k3). Unfortunately, increasing R34 makes it harder to drive the 4013 quickly.

3) Use a bigger clock capacitor.  This makes the unwanted charge from D2 produces a smaller voltage increase on the cap.  This is evident in the graphs above.  The curves flatten out at lower CVs as the clock cap increases in value.  Note that changing the charge current using the trim pot RT3 only changes the gradient of the large linear portion of the graph.  There is little effect on the location of the "knee" i.e. the voltage at which the curve flattens out.

4) Keep the CV away from the non-linear region. In other words, work out the limitations of the VCO and design an LFO section to match it.


It's interesting that the type of measures taken to speed up the VCO cause the non-linearity to get worse. e.g. lowering the pullup resistor or using a smaller clock cap.

Although the EM3207 schematic shows a 22pF timing cap, Thomeeque later recommended 47pF on this thread and I would agree based on the above graphs and explanation.  It is better to use 47pF and higher charging current (by adjusting clock trimmer) than to use 22pF and lower charging current.  Funnily enough, the Madbean Current Lover and Hartmann Flanger (both clones of the 9V EM) use a 22pF timing cap rather than 47pF.  So you can probably just about get away with 22pF so long as the CV doesn't reach the non-linear bit of the charging curve.

So why not use an even bigger cap and charging current? I think there is a limit as to how big you can make them.  If you scale both up by a factor of 10, then the capacitor will have to discharge 10 times as much charge, and this becomes harder to do because the comparator realises the cap voltage has dropped and shuts off the diode before the capacitor can "fully" discharge.  (Note that the capacitor never fully discharges anyway because of the voltage drop across D2).


I see there is a design earlier in this thread for a faster VCO which claims to match the aim of doubling the Deluxe EM clock rate while keeping a linear characteristic. Has anyone (other than the designer) built and tested the VCO in the following link ... ?

http://www.diystompboxes.com/smfforum/index.php?topic=91981.msg803653#msg803653

It uses 22pF and a low value pull-up resistor.  So the nonlinearity in the CV to delay characteristic should be worse, but the CV appears to be kept deliberately high (1.6V to 6V) so I am thinking it stays away from the non-linear part of the characteristic. 
I'm asking 'cos I'm toying with the idea of a Deluxe EM clone using the 3207 but running on 9V instead of 15V.

[Thomeeque]

 Hey Dr. Alx, that is impressive piece of serious R&D, thanks a lot for sharing! I did not continue on any deeper research on expanding clock range later. I was partly discouraged by gain loss in MN3207 at higher clocks and partly by the fact, that I have liked effect at default (limited) clock range, I am Gilmour type of the flanger user (setting it to get just subtle effect ). Plus later I have installed my EM3207 prototype into friend's multi-fx unit and that was it. But recently I have bought one LM319 IC to at least try the Ralf's clock.. Did you get somewhere meanwhile?

[Thomeeque]

.

[DrAlx]

Hey Dr. Alx, that is impressive piece of serious R&D, thanks a lot for sharing! I did not continue on any deeper research on expanding clock range later. I was partly discouraged by gain loss in MN3207 at higher clocks and partly by the fact, that I have liked effect at default (limited) clock range, I am Gilmour type of the flanger user (setting it to get just subtle effect ). Plus later I have installed my EM3207 prototype into friend's multi-fx unit and that was it. But recently I have bought one LM319 IC to at least try the Ralf's clock.. Did you get somewhere meanwhile?

Thanks Thomeeque.  I ordered a LM319 last week for the same reason.  That's as far as I've got.

[elflicho87]

Hi everybody,

First of all, a HUGE thanks to Tomas for sharing his great work here, and for giving support to other builders all over the forum.

I've just finished building my EM3207, and I'm glad to say that it works. I'll upload some pics later when calibration process is over (and when all the parts I ordered for the project arrive home grrrrr...)

Just one quick question for you diyers: I sadly do not own an oscilloscope, so I'd like to know if there are some advices about the clock trimmer calibration. I've managed to set the bias (with the formula and by ear) and the feedback (before oscillation starts) trimmers. Now on the clock trimmer: any suggestion? The pedal already sounds good but I'd like to get the best out of it, you know...

Thank you!
nic

[DrAlx]

Hi everybody,

First of all, a HUGE thanks to Tomas for sharing his great work here, and for giving support to other builders all over the forum.

I've just finished building my EM3207, and I'm glad to say that it works. I'll upload some pics later when calibration process is over (and when all the parts I ordered for the project arrive home grrrrr...)

Just one quick question for you diyers: I sadly do not own an oscilloscope, so I'd like to know if there are some advices about the clock trimmer calibration. I've managed to set the bias (with the formula and by ear) and the feedback (before oscillation starts) trimmers. Now on the clock trimmer: any suggestion? The pedal already sounds good but I'd like to get the best out of it, you know...

Thank you!
nic

You don't need a scope. Just ears and an in-tune guitar.  Here is how I always do it. Use maximum color (regen) and maximum range in filter matrix mode. When clock trim is correctly calibtated that should give a delay of about 8ms. A delay of 8 ms corresponds to a frequency of 1000/8 = 125 Hz.  That is approximately the low B on the guitar's A string. So what you do is smack the strings over the pickups and listen to the metallic sound coming from the effect.  It should sound a bit like "twanging" a ruler on a desk. Adjusting the clock trimmer will change the pitch of that sound, and you try to make the pitch the same as the B (i.e. A string second fret).

[DrAlx]

Here's what I was trying to describe above.

https://soundcloud.com/alex-lawrow/bbd-clock-trimming

Actually according the Electric Mistress Mystery page, the max range delay is more like 7 ms, which if 1000/7 = 142 Hz.
So the note would be between C# and D.  So instead of tuning to the "B" on the 2nd fret, tune a couple of frets further up.

[elflicho87]

Thank you very much Alex

I've just finished the process as you described it. Boxed it up and played a bit.
Sound good to me!

Again thanks to Tomas for all the work here and Alex for the reply and the link to the audio file!

cheers
nic

[PRR]

I will not re-read all 20+ pages.

Are we looking at this fragment?



I make it out as about 30uA of current in Q2, for an oscillator which goes over 1MHz.

That seems very thin to me.

Likewise, 22 even 47pFd sounds a lot like stray parasitic capacitance.

For Mhz operation I would dart-board mA of current. This gets C up near 500pFd-1000pFd, a good step up from the universal parasitics.

Actually getting 30X the current just means 30X reduction in R31 R32 R33 RT3. Hmmm, than makes RT3 733 ohms, an odd value. You show control voltage as high as 9V but also <4V. We could let the V across RT3 be maybe 2V or 4V (instead of 0.45V?) and get a more practical RT3 value. Details.

The next question is: can we re-set a mA of charge current fast enough? You would think a LM311 and a 1N4148 would dump current faster than a MHz. But I recall from synthesizer design that reset times can be disappointing. And LM311 has about 20 Ohms to-ground resistance for low output voltages. I recall using 2-transistor switches instead of a diode, to get really-swift reset.

[DrAlx]

Yes it's that circuit fragment (and the 4013 that it then drives).

The circuit fragment (with 47pF clock cap) is from the original 9V Electric mistress (which used a 512 stage BBD).  This build takes that circuit but uses a 1024 stage BBD as a substitute.
So the requirement was to make the circuit fragment shown take the same control voltage range as the original pedal, but produce double the clock rate.  The original thinking was to use 22pF rather than 47pF but that stopped the CV -> Delay characteristic from being linear for small delays.  To cut a long story short, it turned out to be better to keep the original 47pF and just adjust RT3 to scale up the charging current by a factor of about 2. That achieved what was desired to get this 9V Electric Mitress clone working "to spec" against CV.  (Lowest delay when configured is not much less than 1ms.)


Now it so happens that the same circuit fragment is used in the 15V Deluxe Electric Mistress which has a wider range of CVs (due to larger supply) and that pedal can get much lower delays.  So if you want to clone a Deluxe EM but use a 1024 stage BBD instead of 512, the circuit fragment needs to run much quicker than was necessary for 9V build.  So the question was how quick could things be pushed while still making sure that the full range of delays were achievable with no loss of linearity in CV -> delay characteristic.  (Probably a pointless exercise due to BBD signal loss when overclocked but kind of interesting anyway).

The posts on the earlier part of the thread looked at a few things like trying quicker comparators, lower pull-up resistor on the comparator output to better drive the following 4013.
What I saw posted as a solution (which apparently worked but I never got round to doing a test build) used a quicker comparator and a shift in the range of CV voltages (to avoid the non-linearity that occurs at low CVs).  So I am guessing it could work but would involve messing around with the CV stage of the original DEM circuit too.

What you say about dumping charge more quickly using two transistor switches instead of a diode is interesting.  Any chance you could post a pic of what you mean Paul?

[Thomeeque]

Again thanks to Tomas for all the work here..

Always pleasure, congratulations to the successful build!

What you say about dumping charge more quickly using two transistor switches instead of a diode is interesting.  Any chance you could post a pic of what you mean Paul?

Yes, please!

Cheers, T.

[RickL]

I've just finished building one of these and it works... for about 4 seconds. Then the delayed sound starts to distort and eventually fades away to nothing. If I short the battery terminals or remove the battery for a couple of seconds the delayed sound fades back in again for a few seconds, then fades out again. I'm getting signal into the input of the 3207 but nothing out of Out 1 or Out 2 when the signal fades.

I'm measuring about 9.4 V at Vdd (pin 5), about 8 V at Vgg (pin 4) and 0 V at ground (pin 1) of the 3207. These voltages stay constant. When I short the supply Out 1 and Out 2 slowly climb from 0 V to about 8 V then stay there. I'm getting clock signals at CP1 and CP2 that change speed with the Rate pot.

I've tried replacing the 3207 with the same results.

Suggestions? I get similar symptoms from an old EHX EchoFlanger and if I can fix this I may be able to fix the EchoFlanger.

[Scruffie]

First off, similar issue doesn't mean the same problem, but i'm pretty familiar with the echoflanger circuit if you want some help with that.

Second, sounds like it's either the BBD bias or the output pulldown, check the resistor to ground from pins 7 & 8 of the 3207 is properly connected and check the v.ref electrolytic orientation and soldering.

Which exact schematic are you using?

[RickL]

I'm using the schematic and layout here: http://thmq.mysteria.cz/em3207/build/EM3207_v1.1_Build_Instructions.pdf

R14 isn't connected directly from ground to pins 7 & 8, but I assume it is the pulldown resistor. I'll check it. The bias is Vgg isn't it (pin 4). I'm getting a solid 8 volts there that doesn't change as the voltage climbs on pins 7 & 8.

[Scruffie]

No the bias is pin 3 of the BBD which is fed from IC1B and set through RT2, the electrolytic I referred to is C12.

[Lothric]

Hello everyone, I know it might be stupid question, but I have some issues getting my hands on tantal caps. Can I use standard electrolytic caps instead? (missing 33uF, 1uf...) the closest values I have are  22uF 2,2uf tantals.