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.

The VCF6C has no adjustment for CV scaling, but this is easily corrected in the same manner as for the ARP 4072 filter. A 2k trimmer is added in series with R18.

Only one leg of R18 was mounted through the hole in the circuit board. The wiper lug of the trimpot was bent out and inserted in the other hole (after enlarging the hole a bit). The free end of R18 was then soldered to an end lug of the trimpot. See the photos below.

A nice touch would be to add an op-amp summer instead of connecting R14 -> R17 directly to the base of Q3, giving a nice high-impedance CV input. As it is, this filter can "pull down" the scaling of other modules if it is directly connected to the same control voltage source.

Artwork for panel silkscreen