Here is a DIY Mattson Mini Modular VCF in a MOTM panel. George Mattson’s description:
4-Pole, 24dB/Octave State Variable Filter jointly designed by Synth DIY guru Jim Patchell and George Mattson.
The filter design is based off the now-obsolete SSM 2040 filter and has simultaneous Low pass, Band pass and High pass outputs.
The PC board I got from George is a prototype for the surface-mount version of the VCF that you can get from him for DIY, as shown in his online store.
I designed a Front Panel Express panel for it, based on the standard MOTM 1U filter format, similar to the MOTM-485 and MOTM-490 filters.
The DIY build was about as simple as it gets. I made a mounting bracket from thin sheet aluminum. All connections to the panel use MTA headers, requiring no soldering to the assembled-and-tested PC board from Mattson. I’ve started making my own cables, using the MTA press fit tool, 59803-1. You could more easily use the premade wiring kits from Mattson.
For the potentiometers I selected 100K linear ceramic metal Bourns 91A1A-B24-A20L, which are reasonably priced high quality pots available from Mouser electronics.
This is a very clean sounding VCF. It tracks 1V/octave pretty closely over several octaves after tweaking the onboard trim pot. It also self-oscillates at high resonance setting and makes a clean sine wave from the low pass output, ideal for use as an auxiliary VCO.
Here’s a photo of the assembly.