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

[DrAlx]

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.

Electrolytics are fine.

[Lothric]

Big Thanks to Thomas for this project. Finally I have build this thing! still have to debug few things tomorrow because I have a ticking noise from clock, will change c17 to 47pF and use filter caps on the IC's see how it goes... once again big thanks to everyone involved into this project

[Lothric]

found problem, ground loop !!!, extra caps didnt help at all so at the end I didn't put them at all, used shielded wires on the input and output (grounded everything to the output jack which is connected to enclosure the input jack is isolated from the enclosure). very very nice flanger. Thanx again to everyone involved here specially Tomas 

[shoggoth2]

Just built one of these, it sounds fantastic.  I used a "bare box" style enclosure, which is very tight, so there's a daughterboard hidden underneath with most of the clock circuit on it.  I had the PCB's made up at Gold Phoenix (I did a big batch of different PCB's in a multipanel project, so overall a very good value compared to just getting a single board made up).

It's mostly the original schematic Thomeeque put together, plus some extra filtering here and there for reasons of paranoia, 47pf for the clock cap at C17, and the "sweep ratio" trimpot mod that DrAlx came up with.

Only problem I have is some popping on activation, I'm thinking it's a result of the "true bypass" 3pdt switch.  Pounding it on and off several times before turning the amp on helps.

Here's the guts:



Here's the enclosure - I screwed up the top waterslide decal, a combination of letting it sit in the water too long so it didn't have enough glue, and reassembling it so many times that I marred the surface pretty badly.  I'll do the next one better.

[D.C.]

Only problem I have is some popping on activation, I'm thinking it's a result of the "true bypass" 3pdt switch.  Pounding it on and off several times before turning the amp on helps.

I've heard that that's often a function of the choice of output capacitor. Did you change any component values near the output?

I wonder if anyone else has had popping issues. 22 pages of posts is a lot to go through, though...

[DrAlx]

Only problem I have is some popping on activation, I'm thinking it's a result of the "true bypass" 3pdt switch.

Have you wired the switch to turn the power to the board on and off? If so then change things so that the board is powered the whole time, and let the switch only handle the audio connections.

[DrAlx]

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.

So one year later I finally come back to this.  Paul's right as usual    I tried the following set of component changes

62k ==> 3k3
1M2 ==> 22k
3k9 ==> 2k7 (and no clock trimmer)
47p ==> 470p
10k pull-up resistor on the comparator ==> 1k

This was just plugging things into breadboard with messy wires everywhere.

The CV range that managed to trigger the CD4013 OK was from 0.995V to 6.94V.
This gave really good CV to delay characteristic (as far as my handheld scope could measure) with corresponding delay times from 0.32 ms to 9.73 ms.  Graph here...

https://1drv.ms/x/s!AvrH61utWEtEhSZ-54HeAD0DV-PZ

And when I looked at the capacitor waveform as it charged it was properly triangular.  The initial fast charging due to diode reverse current wasn't evident as it was with 47pF clock cap.

[DrAlx]

I've built an original 18V EM with SAD1024 using the schematic on the Electric Mistress Mystery page.  Even allowing for the volume drop, there is a noticeable difference in sound compared to the EM3207.  I know the EM3207 is a clone of the 9V EM and that there are differences in the audio path compared to the 18V EM.  I like the sound of the EM3207 but I prefer the sound of 18V EM so I wanted to see if I could modify the EM3207 to achieve that sound.  I think I now understand why the original EM sounds like it does, and will describe how the EM3207 can be modified to make it sound much closer to both the original 9V and 18V versions.

Here is how the relevant section of the EM3207 compares to the 18V version.  I've highlighted the key differences in component values.





The 18V EM uses smaller cap values for C8 and C10 and this reduces frequency content in the lower midrange (a couple of hundred Hz).  That's higher than the frequency of the low open strings on the guitar, and those strings are indeed quieter than the high strings on the 18V version. 

So I changed C8 and C10 to 47 nF.  The side effect of reducing the low-mids like this is that there is a noticeable volume drop overall.  This reduction of low mids is one of the main contributions to the volume drop in the 18V EM.  So the next thing I did was increase R9 in the pre-emphasis section from 5k6 to 6k8 (as in the 18V EM).  This increases the overall gain and recovers most of the volume drop lost to reduced low-mids.  Increasing R9 didn't give me any problems with headroom or clipping.

In the end, I ended up changing all cap and resistor values in the audio path to match the 18V EM values (i.e. all the values in red on the above diagram apart from R14 and C7 on the BBD output).  These changes made the EM3207 sound more like the 18V EM but I could tell that something still did not sound the same.   This was when I decided to take measurements given that I had already made the necessary component value changes to make the two circuits "match".  I only have a handheld scope that is not lab quality but it is still better than making subjective comparisons based on my ears.

I started by measuring what I'll call "BBD gain" for the two circuits.  To be more precise, for each circuit I measured 20log10(Vout/Vin) where Vin is the pk-pk voltage measured at the BBD input, and Vout is the pk-pk voltage measured at the smoothing cap C7.  I took measurements in filter matrix mode with no feedback, using 7 different clock rates covering the whole sweep range (using doubled clock rates for EM3207 of course), and using 21 audio frequencies from 100 Hz to 11 kHz.  The following graph shows the measured BBD gain.



The EM3207 shows a mainly flat BBD gain of roughly 2 dB, in agreement with the MN3207 datasheet.
The 18V EM shows a significant roll-off in BBD gain in the audio band.

If we subtract one curve from the other ("18V EM BBD Gain" minus "EM3207 BBD Gain") we get the following curve that gives the relative BBD gain between the two circuits.  The point of doing this is to use the EM3207 BBD gain as a baseline for comparison, effectively subtracting out any drop in BBD gain that is common to both circuits.



This last graph shows that the BBD gain in the 18V EM has suffered a sort of "shelving" effect. In other words, there is a roll-off in gain that flattens out at low and high audio frequencies.  The reason for this is that the 18V EM does not just have a 10k resistor on its BBD output.  It has the rest of the mixing circuit hanging off it too.  This means that for high audio frequencies there is not 10k on the BBD output but a much lower impedance instead.  Lower impedance on the BBD output means lower BBD gain.
 
In the EM3207 circuit, Q1 shields the BBD output from the rest of the mixing circuit and so prevents the roll-off in BBD gain.  This makes the EM3207 a more "pure" flanger but on the other hand it stops it emulating a key part of the original circuit's tone.  (Note that the original 9V EM by EHX will also have a roll-off in BBD gain but it will be "shelved" differently to the 18V EM because it has "trim resistors" in series with the drain-source resistance of the BBD output.)

I believe Thomeeque included Q1 because the outputs of the MN3207 have much higher drain-source impedance than the SAD1024.  Something like 4k5 compared to 1k for the SAD1024.  So the MN3207 chip needs 47k for R14 in order to give similar gain to the SAD1024 with 10k.  Including Q1 allows all the other caps and resistors in the mixing/feedback circuitry to use the values of the original EM circuit.  The other option would have been to leave out Q1 entirely and instead rework the mixing/feedback circuitry by scaling up resistor values by a factor of 4.7 and scaling down cap values by the same factor (just as Thomeeque did for the components on the BBD output). I am not sure why that approach was not taken.  I could be missing something    The overall output impedance would go up by doing that, but that could be corrected with an output buffer.

Anyway I was too lazy to change out any more components, and wanted to see if there was a simple modification that would give the EM3207 a roll-off in BBD gain matching that of the 18V EM.  I worked out I could get a good approximation to the shelving effect by putting (15k + 6n8) across the 47k resistor at the BBD output of the EM3207 as follows.



Note: A similar sort of trick could be used to mimic the BBD gain roll-off seen in the original 9V EM, although I didn't calculate the values needed for that as I don't have a working reference circuit of the 9V EM from which to take measurements.

I took another set of measurements after my "shelving" modification, and the following graph shows what happened to the relative BBD gain.



The relative BBD gain between the two circuits is now pretty flat across the audio bandwidth and varies by less than +/-0.2 dB. Overall the EM3207 still has a BBD gain that is about 0.5 dB higher than the 18V EM.  That could be fixed in several ways (e.g. by reworking the R and C values of the things hanging off the BBD output pins) but I didn't want to change any more values and think I will live with the 0.5 dB difference in BBD gain in my build.
 
Summary of overall result with the above modifications ...

1) The circuit now sounds much more like an original 18V EM.  Maybe I'm cork-sniffing but I think the difference is significant.  You would think that throwing away high frequencies in the wet path would make things sound less clear but I found the opposite to be true.  My EM3207 now has the same sort of watery clarity as the original 18V EM. That could be related to point 3 below though.

2) The EM3207 ends up with a small volume drop. It is not as bad as the original 18V EM though because of the boost at the input.

3) Noise is noticeably lower after the mods   Connecting/disconnecting the "shelving" mod at the BBD output makes this obvious.  Maybe that explains the improvement in clarity.  For the same overall volume, there's less noise present, especially in the high frequencies because those get to the output more through the less noisy "dry" path than through the noisy "wet" one.  I'd say the noise reduction alone makes the shelving mod worthwhile.

3) Feedback/Color pot is less harsh after the "shelving" mod.


What I've learnt from all of this is that the original EM (probably my favorite pedal) sounds the way it does mainly because of all the signal that it throws away.  It reduces low-mids during mixing, and it reduces highs in the wet path due to the "shelving" in BBD gain.  (The sweep giving linearly increasing delay is also important).

Based on what I've learnt above, my plan is to do a clone of the 18V EM but using MN3207 chip running at 9V.   My aim is to make it a  "sound-alike" and "control-alike" rather a straight copy of the circuit.  So basically I am going to rip out the LFO/VCO used in the 18V EM (which is horribly noisy) and replace it with something along the lines of the LFO/VCO of the 9V EM but with better VCO values and different filter matrix switching and range pot behaviour.
I think I can reduce the part count a bit too.
One other thing that I will do is redo the BBD gain measurements for the EM3207 but without the clock buffers.  I know the circuit will work without the buffers but getting a quantitative measurement of how much they affect the BBD gain (if at all) will be useful.

[Cozybuilder]

Alex- Very impressive amount of work you've done on this, great write-up, can't wait to try it out.

[12Bass]

Nice work, DrAlx!

It seems that a large part of what makes the EM unique is the coloration to the wet path.  Some audio samples would be great. 

With my SAD1024 A/DA build I took things in another direction - increasing the bandwidth of the delay path - which makes it more "hi-fi" sounding, especially when flanging high frequency content.  This increased noise somewhat, but I prefer the overall tone.  I also altered the gain staging slightly to try to maximize the SNR through the wet path.  Raising the HPF on the feedback path also helps to clean things up as regeneration increases.

[DrAlx]

I got back from work today and had another listen.  The change in tone is all from the C8 and  C10 change which makes sense since that affects the midrange.
The shelving mod doesn't change the tone but does make it less noisy and hence clearer. 
(I was listening with headphones so could here the change easily).
I was playing one handed and making/breaking a connection to the shelving mod with the other hand I could hear some of the hiss disappearing.  The actual guitar sounded the same though. I was just playing clean. No distortion.
The other effect from the shelving mod is to do with how it affects color/feedback. You can feedback more before the thing oscillates.

[DrAlx]

While taking measurements to investigate the effect of clock buffers (something I'll post on another thread) I found I was
getting a different shelving effect on my original 18V EM compared to the measurements I had taken before.
I figured out that I had changed the setting of the feedback trimpot RT1 on the original EM and this had altered things.
So the hack I mention above (of adding 6n8 + 15k) only approximates one particular setting of RT1.
I had a think about dropping Q1 and scaling resistor and cap values instead, but decided against it to avoid degrading noise performance.

I now think a better way to clone the shelving behaviour of the original circuit is to give the smoothing cap C7 a value of 3n3 as in the original 18V EM, but move the cap to the other side of Q1 and separate it from Q1 with a resistor.  The value of this resistor should be chosen to equal 10k in parallel with the drain-source impedance of a BBD output on a SAD1024.   i.e. the impedance seen by the C7 to the left should be the same as in the original circuit.



This makes the shelving effect the same as the original 18V EM for all settings of RT1.
The other advantage of this approach is that the BBD gain can be independently trimmed to match an SAD1024 by tweaking R14, and no other cap or resistor values would need to be changed.

[G.esp]

I'm planning to build this one. I read that there are some fixes to improve sound, someone has a new schematic with all these fixes?

[DrAlx]

Shared here.
https://1drv.ms/i/s!AvrH61utWEtEiAbzPDPv5UK-geVK
The "Rds || 10k" comes to a value of about 1.1k to 1.2k based on my measurements on an actual SAD1024.

[daz061]

so does anybody sell a prefabbed board for this project

[AS74]

Has anybody made the effect with DrAlx last changes?  Opinions will be appreciated as I am currently populating the pcb.

Thank you.

[NFX]

I would also like to know, so have you etched your own board or had 1 made for you AS74?

[AS74]

Etched my own board.

Thanks to Tomas.

[Tompski]

Hello
Sorry to ressurect an old thread,

I have a version of this that uses the Madbean Current Lover circuit using MN3007, I have an original V2 Electric Mistress, and the modern pedal seems to be much thinner and occupy a higher frequency range in the top end, before I start swapping out capacitors has anyone got a list of possible mods that make the pedal sound more like an original V2? I think the originals sound warmer, thicker and more organic.

I can follow instructions and have good tidy soldering skills but a lot of the technical talk on here goes over my head.

Any help in getting this circuit to sound more like an original would be greatly appreciated.

thanks

Tompski

[Govmnt_Lacky]

Hello
Sorry to ressurect an old thread,

I have a version of this that uses the Madbean Current Lover circuit using MN3007, I have an original V2 Electric Mistress, and the modern pedal seems to be much thinner and occupy a higher frequency range in the top end, before I start swapping out capacitors has anyone got a list of possible mods that make the pedal sound more like an original V2? I think the originals sound warmer, thicker and more organic.

I can follow instructions and have good tidy soldering skills but a lot of the technical talk on here goes over my head.

Any help in getting this circuit to sound more like an original would be greatly appreciated.

thanks

Tompski

The only mod you could do to make them sound identical (both sound like the V2) would be to rebuild the Current Lover to use the SAD1024 chip 

Sorry to say but, you will never get the same sound from it with an MN-series BBD as you would with a Reticon SAD-series.