well, for this one, it's alternative filter (low pass) if you can not get a resonance with the first one... that, what it says on the in the text... > There is another VCF (ru_synth_diy - 7.jpg), a Sallen-Key low pass, > either with "switched resistors". I don't know what it is for (haven't read > the text yet). Maybe it serves as a VCA.