Quad Wavefolder is a very nice addition to the system - relatively easy to make, and opens up a whole new sound palette for your synth, especially with sinewaves and triangles. It doesn't sound as most 'old school' (and hence widely known and appreciated) wavefolders do, but it definitely adds harmonics to your sounds, or can even be used to mangle CVs, since there is no decoupling in the signal path - or acting as a very freaky submix for sounds thanks to the dual signal input. One unit consists of two identical folding stages, the only difference being that the second one has a voltage control over how deeply it affects the signal. It can go from no effect to full effect, so the whole unit can be CV'd between one fold or dual fold. Four such units allow cascading up 8 wavefolders for some extreme folding, or using them separately for different sounds. So far, i've been using it as a pair of twin folding stages, but there can be many more variations.
Whole thing inspired by the tiny repeated 'differential clipper op amp' part of Ken Stone's wave multiplier schematic.
Schematic
Here's the schematic for the Quad Wavefolder. On the picture is just one unit - the module consists of four such units. Of course you can build as many as you want - maybe even just one - but it's a lot of fun to use it as a mangling mixer or stacked wavefolder, so i recommend making four. What is not shown on the picture is the normalization of signals, or what i did with the switches. If you have high quality jacks (like Thonkiconn), then you can simply can normalize the output of one unit to the second input of the next unit. In my case (only had crappy jacks which can't be trusted about normalization), or in case you use bananas or any other connectros without signal normalization, you can use an SPDT swtich - hook up the output of one unit and the input of the next unit to sides, and the middle would go to the second in of the next unit, thus letting you decide if you want to use the second input, or cascade audio from the previous unit.
The feedback resistors around the op-amps could either be 22k or 47k; the first choice makes the thing less likely to take the signal into overdrive, but also decreases the out amplitude, making the whole thing sounds puny when stacked. So if you plan on cascading them or using as a weird mixer, use 47K - it will bring up the signal to higher amplitude. I had much better experiences with the second options.
The vactrols could either be the fancy ones, or - the way i do it - the hardcore DIY vactrols made out of a GL5528 LDR and an orange or red LED. The choice is yours, but i'd say DIY vactrols infuse soul and a personal quirk to your device.
Since the device uses a diode clipper to ground at its core, it is very touchy to input signal amplitude and can go from slight wavefolding to complete mangling depending on it - the smaller the input signal, the more visible changes it will get. You can build an x1-x8 amplifier to pair with it for volume recovery.
Media
Extreme folding on 4-unit cascade
Loud triangle through 1, 2 and 3 wavefolder units
A very basic krell with an overdriven triangle, controlled by a serge SSG
