There was a discussion thread on synth-diy where everyone was arguing 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.
Eagle files of REV3 are here (schematic) and here. for the board layout
PDF file for schematic of REV3 is here and pdf for board layout is here here.here is the old layout....I have built it a few times and it works
BUILDING THE VCF:You can get the parts layout in pdf format here and a parts list in Microsoft Windows "doc" format here. I also have a ready to transfer PCB mask, and a pdf of the above schematic here. Eagle fans can get sch and erc files here and here. Note that this rev of the filter was created using an older version of Eagle (many years ago). You will need to clean up the text files a bit before you can create gerbers.
About the layout: Not much to say, this is very straightforward stuff, but, For those of you who love to did dle there is a small sandbox area on the PCB for adding your favorite whatever the heck it is.
About using this VCF: Well 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, and 100K linear for the "CNTRSND" which will control the default center frequency. 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. Note that the 5C2's give it a sloppy, slow, phat sort of feel, which is good for certain audio applications and not for others. 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. This circuit in general is simple enough that you should be able to go completely crazy with it.