I’m on a DIY streak! This is Monotropa v3 from Reverse Landfill. It is a 4-band fixed filter designed for distortion and feedback. There’s a knob for the strength of each band, a feedback knob and an output level knob. Three direct-coupled inputs are mixed, with one being attenuated by the feedback pot. The three outputs are AC-coupled, with one having a normal connection to the feedback pot.

The module has no voltage control, but… The three DC-coupled inputs allow mixing in of offsets, LFOs, and a second audio source, which can add animation. Offsetting a processed signal sounds a bit like the offset feature on wave folders. Monotropa can self-oscillate, too. And the feedback input and third output can easily patch another module into the feedback loop.

Construction

I bought a factory kit from Thonk. All the assembly information including BOM, build guide, schematic, and a handy patch guide are downloadable from the link at the top of this post. This is a very easy single PC board, through-hole component project. The parts are all standard. The full kit is handy, because there is a variety of different resistor and capacitor values needed for the four filter bands.

I added washers under the pots and jacks and used different knobs than the ones in the kit. The pots are Alpha vertical t18 9mm with the split knurled shaft. I happened to have some nice black knobs and used those.

A Little Demonstration

I set up a repeating sound, made with the Athrú wave folder processing a sine wave and hit with an envelope. This sound goes through the Monotropa. The first part of the recording is the dry, though rich, sound. Then, in succession, I turn up and then down each of the four bands from low to high, with no feedback. After that I start bringing up the feedback and playing around with the band knobs.