There was a discussion thread on synth-diy about the lowest parts count multimode VCF that doesn't use a CEM type chip (does this remind us that some of us need to get a life?) Anyway, here's my low parts count multimode VCF that sounds good and fat.
It's a pretty standard cookbook design with Vactrol 5C2's substituted for the resistors that control cutoff frequency.
UPDATE! I have layed out an updated version (REV3) of this board. First revision in over 15 years....Tested working.
FILES AVAILABLE FOR THIS:
NOTES ON BUILDING THE VACTROL VCF:
This is a simple board, very straightforward stuff, but for me, normal 5V P/P signal inputs may cause the output to distort, especially at high-Q settings. Adjust the voltage divider at input to accommodate your setup. You may need to R11 can be anywhere between 10K and 7K or so, experiment with that to set the initial cutoff frequency. Also for modulars: you probably want to add a 1K resistor in series with buffered outputs--otherwise if an audio output goes to a dead short you will heat up the op amps and maybe fry them. Alsok consider gaining up the outputs regardless--I figure would be to just add op amps to each output stage with an appox 6db gain.
About using this VCF: it doesn't sound that good at every setting.
Some of the Q=1 sort of settings sound a bit thin. It won't oscillate, but you can mess around with the resistors in the feedback network (R10; R8) and change the amount of Q for the circuit and make it squeal if you really want. As far as mods, I parallelled another 10K on top of R10 and put a switch on it to take the new value in and out of the circuit, which added a bit of sharpness to the Q--a sort of "turbo-Q" switch. To further muck with the Q, experiment with the value pot that goes between "Q" and "Q Ret". I used 100K linear for the Q pot, see the wiring diagram. To tune this to your setup, you can adjust the input levels with R12/R13, and control the overall cutoff freq. modulation amounts with R18, R5, and R1....the 5C2's give it a sloppy, slow, phat sort of feel, which is good for certain audio applications and not for others. Other vactrols should work fine, but you may need to adjust R8 and R10. I found that 5C2's are not all created equal, so you may need to experiment with parts values C1 and C2 as well to get the sound you want or try different 5C2's in the circuit until you get a sound you like.
For those new to the synth world (or perhaps born after 1990?) the Korg MS20 filter is well known for its distorted "Peak". This means, when you crank the resonance, you get some nice dirty sounds. It's also both high pass and low pass depending on how your wire it
I wanted a couple of MS20 filters in my setup so I made a PCB of Rene Schmitz's MS20 design; his stuff works on +/- 15V and from what I've built so far, always sounds good. This is an exact copy from Rene's site, no mods or changes other than the CV input resistor values. I've built 2 of these now, and the 8-28-19 design works, but there is one pretty big mistake in the 12-28-18 schematic that means you have to do a couple of pretty simple trace cuts and wire kludges to the PCB but otherwise everything works and sounds good I think. And then you get that nasty peak distortion. Yeh! If you want to know more, blog post about building this VCF is here.
The mistake can be seen in the schematic above. R12 is supposed to connect to the right side of R11 not the left! That means a couple of pretty simple trace cuts in the corresponding PCB and some wires added to fix (yes, a pain, but so would strip boarding this entire design then trying to troubleshoot if it doesn't work). If you end up fabbing the board, see the trace fix details below.
Here are some files to download: