Resonant Low-pass Gate based on standard Sallen-Key filter topology, using hand-rolled vactrols and op amps.
Features:
- Gnarly resonant feedback
- Overdrive
- Intermediary to west and east coast sounds
- Organic vactrol response
For the vactrols, I used GL5506 photoresistors and standard yellow LEDs. I couldn't find my heat shrink tubing so I just used very snugly-fitting tubing from a lamp cord and hot glue to hold it together.
There can be a lot of variation due to component tolerances, so you may want to experiment with different capacitor values. Specifically the 104 (0.1uF), you could perhaps go a little lower in value with that if you wanted a less squelchy sound.
So this is based on a normal Sallen-Key low pass circuit, straight off wikipedia. However I've also added a feedback path and some biasing with a virtual ground (you can share the same unbuffered virtual ground between at least two of the filters), which creates a super harsh resonant feedback and overdrive. With the feedback knob turned fully CCW, it has a fairly soft, west coast sound; but as you turn it CW, it turns from a tender thunk to an acidic growl.
Pictured above is layout I used on veroboard (stripboard works too). The potentiometers are mounted like in the photo at the top. You want the resistance to decrease as you turn the knob CCW. The colored traces are jumpers.