Mosha Eurorack Modules #6: VCF

The VCF is a 6 HP module that has an LM13700 based filter using a mixture of designs by Look Mum No Computer, Réne Schmitz and Moritz Klein. It is nearly 1V/oct, but not quite, but the voltage control circuit is simpler than most. There is a -12V to 12V offset that can be added manually to the CV input, and a separate knob for resonance. There are two inputs. The entire VCF part is using the gold colors, the purple colors are a separate filter that isn't voltage controlled, but uses a two-colored LED for a more gritty response in the feedback loop. It also uses 10nF instead of 1nF capacitors.

A full circuit diagram is included in this blog post, however, this may not exactly match the built version and there may be errors in it. Although multiple circuits suggest to use the buffers internal to the LM13700 there is some trouble making sure that they are offset correctly, and it is therefore easier to use the TL074. This also provides a cleaner sound.