Friday, July 6, 2018

QUICK ONE: Super Microvcf--8 components?

Quick one--

I am on a quest not sure why to find a VCF with the lowest parts count.  I could get one of those one Curtis chip clone 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 change those values, overall, part values are not critical for this circuit; from messing with this micro circuit the caps can be different values for instance and the VCF will still work. 

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 old japanese drum machines.  It's worth taking 15 minutes and perfing I think.

I may 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:

More the meantime Don't breathe the fumes!

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