tube VCF

[Ken Stone]

I remember a lot of people commented on the tube VCF designed by René
Schmitz. I'm wondering if anybody else has built one too. 

I finished the tube portion of mine last night, and no doubt it has some
peculiarities due to the different tubes and transformer I used. I used some
small mains transformer with a 1k primary, and who-knows-what ratio for the
feedback. What is interesting is that I am getting way too much
feedback/resonance, and while this could be easilly fixed, I find it
worthwhile to leave as is.

With the resonance low, it behaves much like one would expect a filter to
behave. As resonance is increased, the filter begins to oscillate at higher
frequencies though not lower frequencies, due to the high pass nature of the
feedback, as outlined in René's article. The higher the resonance, the
greater the range of the self oscillation. Okay, so nothing unexpected so
far. Self oscillation over the full range of the (pot controlled) frequency
sweep occurs around half way around the resonance pot rotation.

Push the resonance pot further around, and run an oscillator into the
filter, and it is possible to get some chaotic responces, period doubling,
chirping and other peculiar behavior.

So far I have only been using front panel controls to adjust it, though feel
the behavior is worth exploiting musically.

Ken.
_______________________________________________________________________
Ken Stone   sasami at blaze.net.au  
Catgirl Paradise <http://www.anime.net/~kens/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>