My multi-folding wavefolder schematic

[Krakenpine]

I designed a wavefolder that has multiple folds. I got inspiration from Buchla wavefolders, which creatively (ab)use op-amps by driving them with lower voltages to get them clipping and then sum signals from their negative inputs, which in normal operation with negative feedback would always be zero volts. They need lots of different resistors with quite exact values, and it’s a hassle to use many op-amps and have different voltages for them, so I tried to achieve the same thing with diodes and using op-amps only for summing.
The result may not be the most mathematically accurate wavefolder, but I think it’s definitely better sounding than digital wavefolders I’ve used.

The level of input signal directly affects how much it is folded, so in the beginning, after the first buffer, is just a linear pot as a volume control. So, modulating the amount of folding needs a VCA before this. One could be integrated in this, actually a vactrol would be quite good as we don’t want the signal level to go to zero. Maximum folding is achieved when the signal is 10 volts peak-to-peak, and folding starts when level exceeds about 0,6 volts (or a threshold voltage of a diode). Over 5 volts starts to get clipped, but still folding.

The FUND-pot controls the amount of the original signal in the output. When it is 0 ohm, the fundamental is at maximum and at 10k ohm the fundamental is mostly kinda completely gone. Output level when the pot is at 0 is 10 V p-to-p, but when the pot is at maximum, it’s about half of that. If you want to compensate that, you can add a 100k pot in series with the feedback-loop resistor of the last op-amp. If you want to get really clever, you can use a 100k dual-pot whose first half controls the Fundamental and second controls the gain of the last op-amp. That requires also that you changes resistors 15, 16 and 17 to 100k. I only simulated that, but it should work. But the changing of output level isn’t that big of a deal, so I didn’t bother.

If you want more folding, you could use germanium or schottky diodes, so you can have more of them and still have the sum of their threshold voltages to be 5 volts. Or just boost the signal and use normal diodes, but more of them.

Here’s the schematic:

I built mine on a proto board, as I still can’t figure out how strip boards work. But here’s a layout I used as a guide:

And I also made a video about this. It has some sounds at the end, after my rambling. It would probably been a good idea to first write this, and then records myself talking, but well…
https://youtu.be/VrNYqfbT7Bg

[Sonosus]

Thanks for this, this is great. I might make a PCB module if that’s ok with you.

[Krakenpine]

Sure, just mention “Krakenpine” and my email “krakenpine@kapsi.fi” on things related to it.
I really should make a website for these, with proper licensing, which would probably be CC BY-NC-SA, “This license lets others remix, adapt, and build upon your work non-commercially, as long as they credit you and license their new creations under the identical terms.”.
Or depends what you mean by PCB in this? Fabricating physical PCBs and even selling them for mandatory costs and postages is ok too, if you aren’t trying to make a real profit.

[Krakenpine]

I started thinking more about this while biking to work. Actually a vactrol-based VCA wouldn’t be that good for this, as the VCA should be as fast and snappy as possible to have a FM-modulation-like attack on sounds. Also, you could add a straight dc-voltage to the incoming signal to bias it and have the wavefolding be asymmetrical.
Which makes me think that as usually VCA-chips have atleast two in them, a stereo version of this would be quite nice as you could modulate the amount of folding and biasing on different channels separately with different modulation sources, but if same main signal is in their inputs, it would still be in exactly the same phase in output of the both channels (as there is no capacitance or filtering).

[Sonosus]

Yes, just fabbing my own for personal use, possibly selling the excess ones (less than 5) for price/postage. If I do this I’ll let you know beforehand and probably put aside a PCB for you if you want. Thanks for clearing that up

[Krakenpine]

Ha! I alreadly made my own PCB. A bit adjusted design, this has two inputs and a distortion. And in my hubris I put “v1.0”, and immediately noticed that I wired the distortion pot backwards. And 1M ohm is a bit too large. And that Fundamental Adjust pot should be 10k, but that’s just a mistake in building.
LFO into the second input controls the asymmetry of the folding, and with distortion it actually can change a sinewave into a pulse width modulated square wave. Something that I didn’t expect, but “happy little accidents” like Bob Ross said.

[Krakenpine]

Some person also made a better version of this and sent me an eurorack module he made as a gift. Awesome I say.

[Jos]

The two pictures make the schematic look like it was folded. Which nicely underlines that we are dealing with a wave folder circuit here.

[hitachii]

Is there a final schematic for this one? I would like to build it but I don’t know how to implement the symmetry or the CV based on the original provided schematic alone.

Otherwise, if anyone has any ideas or suggestions I’d love to hear 'em!

[Krakenpine]

I started over-engineering things again. Symmetry is easy, it’s just CV added to the signal before the folding stage. CV for folding is a basic VCA-circuit in the main signal, as attenuating it makes it fold less. CV to the Fundamental gets more complicated as there has to be VCA for the “fundamental” signal and for the whole wavefolded signal to keep the signal level steady, and they need to be synced. That was actually solved by the guy who sent me a physical module.
But then I also wanted the distortion in the end and for it to be voltage controlled. And it not to be hard clipping distortion, but a guitar-pedal tube screamer -style overdrive, meaning a diodes in a op-amps feedback loop. Just lowering the signal level before the op-amp doesn’t sound good, as one could imagine output, when input is sinewave, a sinewave riding on top of a square wave. If the input level gets lower, the “sine” on top of the square wave gets lower and output starts to look just hard clipped wave. Which means, that “lowering” the distortion doesn’t actually decrease the amount of distortion linearly, but first changes it from soft clipping to hard clipping and then starts to decrease it. Anyway, I just tested a circuit where I can adjust the gain of the op-amp with control voltage, and it seems to work. Actually, that kind of voltage controlled overdrive could be its own module, as I don’t think I’ve seen schematics for those.
On a personal level, I’ve changed jobs two times after starting this thread, no wonder I’ve had a slight scheduling problems in finishing projects.