Voltage-controlled filter (4HP)

Inspired by:
KS-20 Filter by Kassutronics (among others).

interactive version

A classic synth VCF, this design has been implemented by many, many different people. I originally started with Look Mum No Computer's design, and settled on something very similar to Kassutronics' version.

Build notes

There's solder bridges for the CV and audio inputs that set the function of the bottom attenuator: it can either be a Drive for the input level, or an attenuator for the CV input (but obviously not both at once). Note that you'll have to manually bridge the other connection between the pads if you want it to work, which is especially important when using the attenuator for CV (since the audio input will otherwise be disconnected).