Schematics / Four Pole Lowpass Filter
Four pole Lowpass Filter by René Schmitz , this is René's version of Moog Type VCF ,make sure you visit his page for more details , circuitry and tips on OTA's , tempco and temperature compensation http://www.uni-bonn.de/~uzs159/index.html . From his notes on this filter :
Although I have built various filters, I'd like to present my version of the famous Minimoog filter here. I have made some alterations to the circuit diagram that already floats around in the net. Some people have had difficulties when implementing it from that image. I know my version is not exactly like in the mini, but it works. I've found out that the matching of the transistors in the diffamp section is critical for the self oscillation. So Q7, Q5 and Q8, Q6 should be matched pairs. No need for all to have the same Vbe. I wonder why this isn't mentioned anywhere?!