EFM VCF6c Diode Ladder Filter

"The Electronics For Music EFM VCF6c is a very distinct sounding filter. It will self oscillate nicely at higher resonace settings. Changing C9 to 100 uF (or more) will extend the lower range.

It's a good idea to match all of the diodes for same forward voltage drop. I also matched Q1 and Q2 for hfe. In both cases take pains to make sure they are all at the same temperature! Let them sit in a thermally stable environment (away from drafts, windows, heat/AC registers, etc) for a while beforehand, and only handle them with tweezers or pliers. Don't hold the tweezers in your hand for long, either."

You can find more info including the schematics here.