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. 

Mosha Eurorack Modules #4: Double VCA

 

The double VCA is a 6 HP module that has two VCA circuits and a mixing circuit. The two VCA (gold and purple) are identical and organized vertically. The VCA level is only controlled by CV, the knobs are part of the mixer that mixes the VCA with a third input.

The circuit is based on the vintage VCA of electric druid. It uses the "more practical current source" as described there, since no mixing of current sources is needed.

The mixer is a simplified version of the Doepfer A-100 do-it-yourself page. It doesn't have the part that inverts it back to normal and it doesn't have an offset voltage. In fact, it doesn't even have a knob to control the volume of the third input.