Four pole Lowpass 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?!