[sdiy] SSM2164 state variable filter

[Ian Fritz]

Hmm ... starting to sound a bit familiar.  :-)  I used carefully balanced 
(+) and (-) feedback in my low distortion multiphase VCOs.  For example:
http://www.electro-music.com/forum/topic-19841.html
I didn't set these up for filter operation because they are dedicated to 
multiwave mixing.

Ian


At 12:27 AM 7/17/2010, David G. Dixon wrote:
>I think that Tom W and I may have found a simple solution to the SVF
>oscillation problem which will allow zeners across the BP integrator cap to
>generate robust oscillation at low frequencies, and I'm interested in Neil
>J's take on it:
>
>Simply take some of the BP output directly back to the input summer through
>a fairly large resistor (to give the pendulum a bit of a push, as it were)
>-- essentially, negative damping -- but cancel this out at low resonance by
>decreasing the damping feedback input resistor to the summer accordingly.
>When the resonance gain VCA is cranked down, the damping will disappear, but
>the "push" will remain.
>
>In my circuit, all the input resistors are 30k.  Hence, I'm thinking to feed
>the BP output back to the summer through a 330k resistor, but to decrease
>the input resistor from the damping circuit from 30k to 27k.  According to
>simulation, this should work very well, giving virtually the same response
>at low resonance, but pushing the BP output up against the zener limit at
>oscillation.
>
>Now we just have to confirm that it works in reality...