Isolate EHX Micro Synth square wave as pedal?

[Drewmeyer]

Hey there, looking to build a fuzz pedal for the bassist in my band. One of the hottest fuzz bass tones I've heard is the EHX micro synth's square wave distortion, and I'm wondering if anyone knows a way to isolate that section of the circuit as its own individual pedal. Here's a schematic I found, if it helps:
http://files.effectsdatabase.com/docs/schematics/eh_microsynth.pdf
Thanks a lot, I appreciate any help you can give!

[Keppy]

It depends on how close you want it to be.

1) You need the power supply. The original used a transformer to get +9v and -10v. You can probably get close enough with a charge pump inverter making -9v to go with your +9v supply. Something like Madbean's Road Rage would do it.

2) To get the square wave tone with no dynamics, you need everything up to test point 3 except R5 & R6, plus A6A and the surrounding components, plus R34 & C15, plus make R35 a pot, then move C14 to the pot wiper as an output cap. That should do it, although you might be missing some additional distortion from the CA3094 OTA.

3) The EH MicroSynth is unusual in that it used an envelope detector to impose dynamics onto the square wave after the distortion. If you want that sound, it gets complicated. You need everything up to A3A, plus both sections of the "SQUARE WAVE SHAPER," the "PEAK FOLLOWER," and the "SQUARE WAVE MODULATOR." The "SQUARE WAVE MODULATOR" needs a 3080 or LM13600/13700 to replace the unobtanium CA3094. Leave out the modifications from step 2, put a 1-10uF output cap between A8 and R36, and reduce R37 to 1k. Even then, the sound probably won't be absolutely identical. It appears that the entire output of the Micro Synth passes through the filter circuitry, regardless of whether that circuitry is set to do anything. You may notice a difference.

This is complicated, and results may vary. If you're up for it, I suggest breadboarding the circuit first to make sure you get what your bassist wants. Otherwise, there are tons of less complicated bass fuzzes in the world.

[StephenGiles]

Wait a minute......did you say 'peak follower' in that tangle of control circuitry? Of course, must look at circuit when I get up.....should keep a copy in my bedside stack!

[facon]

How about a Roctave Divider without the flip flop section?

[Keppy]

Wait a minute......did you say 'peak follower' in that tangle of control circuitry? Of course, must look at circuit when I get up.....should keep a copy in my bedside stack!

Yes. A5B, set up as a half-wave rectifier, which feeds the square wave modulator, which adjusts the bias of the 3094 OTA to impose an envelope on the square wave.

[the3secondrule]

Bumping this, as i'm attempting something similar. Did actually this happen?

I'm going to attempt to build a pedal with the square/octave up/guitar controls. maybe a passive tone control to tame things a little. any suggestions for an out put/mixer stage?

Cheers

[the3secondrule]

sanity check for my schem please?

should this work?

[Scruffie]

IC2_A has pins 2 & 3 swapped and you've completely missed the OTA out?

Edit: Ah I see you're following Keppy's suggestion.

[the3secondrule]

IC2_A has pins 2 & 3 swapped and you've completely missed the OTA out?

Edit: Ah I see you're following Keppy's suggestion.

Yep - fixed pins 2&3 now, and I'm going as per Keppy advice. I'll try the simple version first to see if it gets me in the ballpark, and if not I'll try the dynamic version. anything else obvious I've missed? I'll probably start laying this out tonight...

[the3secondrule]

 

[StephenGiles]

IC2_A has pins 2 & 3 swapped and you've completely missed the OTA out?

Edit: Ah I see you're following Keppy's suggestion.

Yep - fixed pins 2&3 now, and I'm going as per Keppy advice. I'll try the simple version first to see if it gets me in the ballpark, and if not I'll try the dynamic version. anything else obvious I've missed? I'll probably start laying this out tonight...

Has anybody sussed a "dynamic version" of the Microsynth envelope generator?

[the3secondrule]

Schem updated. have added the OTA and Octave.

not sure if I can just omit the peak follower and square wave modulator, or if pin 5 on the OTA needs to see current / voltage to work?

Edit>> Schem now updated to include the PF and square wave modulator.

[the3secondrule]

Have built and verified this

The schem above needs one fix (squelch  trimmer should go to v-, rather than ground) I'll update it tonight

[Keppy]

Nice, how's it sound? Any chance of a demo?

[the3secondrule]

Nice, how's it sound? Any chance of a demo?

It sounds much like i remember my old microsynth sounded, the octave up sound is much more raw and fuzzy, and could benefit from some filtering ( or maybe it's just because I went with near enough is good enough for the component values I didn't have ).

I'll try and whip up a quick and dirty iPhone demo over the  weekend...

[nocentelli]

No input cap? Is that because vref is ground?

[the3secondrule]

No input cap? Is that because vref is ground?

Honestly, I just went with the schem from the microsynth. But as it's a bipolar power supply (I used +/-9v), I believe the input cap isn't necessary

[Keppy]

No input cap? Is that because vref is ground?

Honestly, I just went with the schem from the microsynth. But as it's a bipolar power supply (I used +/-9v), I believe the input cap isn't necessary

Normally, even with a bipolar supply you'll see a coupling cap to block any stray DC at the input. Any stray DC voltage there (say, from another pedal with a leaky output cap) will be amplified by the input stage, and since most of the following stages are DC coupled the problem only gets worse from there. .1uF will allow bass down to ~20 Hz, giving you safety and predictability without altering the frequency response.

[the3secondrule]

OK, schem updated above - how's that?

[Keppy]

The input cap should be on the input before R1 & R2, not after. You'll likely also want a 1M+ pulldown resistor to ground on the input side of the cap. Sorry, I should've put that part in my last post. Thanks for making this!