[sdiy] SSM2164 state variable filter

[David G. Dixon]

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...


> David G. Dixon wrote:
> > Zeners across the first integrator cap work perfectly and are very
> > simple.
> > As far as the designer in me is concerned, this case is closed.
> 
> Trouble with that approach is that at low frequencies the current
> output from the 2164 gain cell is pretty small.  At this level the
> leakage current through the back-to-back zeners becomes important,
> and in effect you have a leaky integrator.
> That's why in my filter design the limiting zeners are not across any
> of the integrator caps, but sit on the LPF feedback path.