Hi LMNC people
I recently built an adaptation of the Kassutronics MS-20 filter (ditched the HPF/LPF blend feature in exchange for a two potentiometer-controlled inputs mixer + AUX input, which also corrected the typical phase inversion issue of the MS-20 design, created and etched my own PCB with DIY Layout Creator) and am quite happy with it.
So of course I needed a HPF counterpart. Did not want to build another MS-20 just for the HPF (which incidentally is just 6dB per octave) so I grabbed my breadboard and came up with a vactrol controlled 12dB HPF filter based on a Sallen Key topology, with a non linear resonance path borrowing the René Schmitz/Kassutronics LEDs trick.
It works quite nicely on my breadboard, even allowing self-oscillation. If this is not wanted, you can lower the value of the 15K resistor to taste.
It accepts CV signals, but only positive values will impact on the vactrols (homemade, of course!) which in turn control the filter cutoff. This however can be tamed by using the Cutoff potentiometer as a biasing control.
Be sure to match (to the best of your skills) the 3 bright white LEDs (vactrols + pilot) and the 2 LDRs for the vactrols.
This is a crappy video of how it sounds https://youtu.be/pXHRSzNXQyY.
Schemo:
Please excuse my very poor schematics drawing skills. Feel free to breadboard/build and let me know your thoughts on it!