I am on a quest not sure why to find a VCF with the lowest parts count. I could get one of those one chip solutions but that's cheating?
I created a long time ago a Vactrol VCF that's pretty low but I may have beaten it here....
I found this on the web. 8 parts!! It's from an old PAIA stompbox design.
I can sort of understand how it works I think, we are robbing current from the Op amp's feedback path, which changes the cutoff frequency.....but I was curious how it sounded, so I perf'd it.
I tried different cap values--anything from .01 to about .2 is worth a listen. The NPN can be whatever I figure, I used a 3904. I used a 741 for the opamp since that seems appropriate for the era.
The 1MB values must be to not suck too much current out of whatever is feeding this filter I am guessing. I didn't mess with those values, but overall, parts values are not critical; from messing with this micro circuit the caps can be different values for instance and the VCF will still work. I mocked it up in everycircuit and it appears to be a lowpass of some stripe.
So how does it sound? Well, strange. With a triangle wave going in it sounds to me like some of the filters you hear in really old japanese drum machines. It's worth taking 15 minutes and perfing I think.
I will incorporate it into a Lunetta VCO sound maker I am building and then post some samples somewhere.
Here is the schematic:
Here it is on perf: