Classic 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. The reason is that the mini used some oddball voltages, which made some changes necessary. I know my version is not exactly like in the mini, but it works. I've now incorporated some further alterations, that were pointed out by Frank Springer. The input resistors raised, and changes in the output amplifier, so that there is unity gain overall. With the old values there was some net gain, and some clipping in the output amp. I've never bothered, because I frequently use a summing mixer ahead of the filter, and would just lower the input accordingly. You might stick with the 10k input resistors, to allow to overdrive the filter, if you have some means to attenuate the input ahead. Also R70 was increased, because otherwise it could not be properly scaled.