State variable VCF
This is a state variable VCF. It's a two pole 12dB/oct. filter originally designed by Bernie Hutchkins and then adapted by Ray Wilson.
I changed the opamp from TL084 to a TL074, and the LM13700 to a NE5517. Further more I have substituted the 2N3904 with a BC550C and the two 2N3906 with BC560C. Also, I have decreased the value of R8 from 9.1K ohm to 6.8K ohm and decreased R9 from 270k to 150k. The change of R8 makes the filter just tip over the resonance level creating (maybe) to much howling resonance... Changing R9 might make the filter saturate, however, I like the extra timbre this distortion adds. Finally I added a 100k potentiometer panning between HP and LP creating a notch output.
Unfortunately there are no building instructions for this module, but at least you'll find the schematic, some photos, and some comments.
The finished module. The Notch-function was added after the panel was drilled why the potentiometer and the output jacks are a bit stuffed together.
That's it! Have fun and make nice filter sweeps!