EHX microsynth squarewave fuzz (or suggest another squarewave fuzz)

[Scruffie]

Schem updated. used a 3080 as I don't have a part for 3094 in eagle. I intend to build it with a 3094 though, as they are still available from smallbear.

Just use a LM13600, it'll drop in, save those CA3094 for repairs (they're very rare) and save you about $8 or something.

It's exactly the same part internally for the most part, you only need a CA3094 if you need access to the darlington buffer and for some special functions.

[the3secondrule]

Just use a LM13600, it'll drop in, save those CA3094 for repairs (they're very rare) and save you about $8 or something.
It's exactly the same part internally for the most part, you only need a CA3094 if you need access to the darlington buffer and for some special functions.

Noted, and schem updated.

If I understand rightly (which I probably don't) Pin 1 (amp Bias) correlates to pin 5 on the 3094, and controls the gain.
I'm not sure what to do with this information though

[Scruffie]

Pin 1 or 5 depending on chip controls the transconductance.

I think (don't hold me to this at all) that pin 5 is being used to basically gate the square wave off depending on your playing dynamics.

[kothoma]

I'm not sure how the OTA is doing its thing. But I'd say you'll at least need to include the parts marked FULL WAVE RECTIFIER, PEAK FOLLOWER, and SQUARE WAVE MODULATOR, as you did. But maybe more from that corner of page one?

The lowpass filter tries to make the input signal more sine-wavy. And that is doubled in frequency.
This octave-up signal goes into the peak follower. That looks like creating two peaks per original wave cycle.
But what does the square wave modulator opamp+transistor do? Bias offset?

It looks like that signal makes the OTA reduce gain at the upper and lower peaks of the input signal and so distorting/waveshaping the input signal into something square-wavy?

Why is (originally) more of the envelope follower supplied into the square wave modulator opamp+transistor?

[the3secondrule]

yeah,

I've updated my schem above to include the full wave recto, peak follower and square wave modulator.
Theres some discussion on DIYSB, suggesting that the peak follower / square wave modulator are imposing the dynamics of the original signal on the square wave? http://www.diystompboxes.com/smfforum/index.php?topic=107049.0

This is all above my pay grade, I just want to built a crazy fuzz

[kothoma]

I see your initial design followed 2) of Keppy's post. That would imply that all waveshaping/distortion is done with opamp A6A. But the gain of A6A seems too low for this.

In 3) Keppy indeed claims that the OTA just (or mainly) reestablishes the original envelope.

He's certainly right that the original dynamics is restored. But I still have the feeling that also essential distortion is introduced here. But maybe I'm wrong.

[the3secondrule]

yup - consensus seems to be that the OTA is responsible for the squarewaving of the signal, and i'm not gonna argue 

at the end of the day, I like the fuzz tone enough to want to replicate it as closely as I can, even if I don't necessarily understand the workings...

[kothoma]

I think your current design with the A5B/A6B/Q1 path into the OTA could do the trick.

Looking a bit more at the rest of page one I still can't make up my mind if this is needed for the square.
Maybe this has some noise gate function?

[Scruffie]

I think your current design with the A5B/A6B/Q1 path into the OTA could do the trick.

Looking a bit more at the rest of page one I still can't make up my mind if this is needed for the square.
Maybe this has some noise gate function?

As I mentioned above it looked like it's there to gate off the square wave.

Having pulled up the factory schematic for the reissue, the OTA is named 'Sustain Modulator' so yes I think it is a gate probably to stop the square from going out of control or glitching too much.

However while it may not strictly be needed I imagine it does impart some qualities on the sound.

[kothoma]

As I mentioned above it looked like it's there to gate off the square wave.

Right, that escaped my attention... Nice to see my guesswork supported.

However while it may not strictly be needed I imagine it does impart some qualities on the sound.

Yes, hard to tell from the drawing. At least it doesn't look like a gate used to chop up the signal into a square wave.

[the3secondrule]

Also, any suggestions for fun stuff i can do with the unused half of the 13600?

[kothoma]

Yep. Add the slow attack VCA of page two

OK, then you'll need all stuff from page 1 (only no A7B), but you can use the OTA as mixer stage and save one opamp of your current design.

[the3secondrule]

Actually, if I can use the OTA as a mixer, then I'll probably use the final OPamp for a tone stack. I assume the squarewave will need some filtering, since it would otherwise pass through the filter section of the microsynth

[the3secondrule]

Kothoma -got any examples of how I'd use the OTA as a mixer?

[kothoma]

Now you make me think, but wouldn't you just feed in the different signals via additional C29/R86s similar to a regular opamp?