[sdiy] SSM2164 state variable filter

[David G. Dixon]

Tom: sorry I didn't put a clipping network on the SVF schematic in my little
analysis document.  It would have saved you a bunch of grief.  I was too
anxious to get on to the Korg MS-20 emulation.  Anyway, better late than
never; see below...

> I'm playing with a SSM2164 SVF circuit again. I'm still not happy with it,
> or perhaps still not happy with my understanding of it.
> 
> There are several things that I've been told/learned which I have no
> particular reason to doubt:
> 
> 1) The circuit is essentially an oscillator, given the feedback from the
> LP output.

Without damping, the circuit is an oscillator -- a very crappy one which
won't give anything even resembling a sine wave, since the oscillations
aren't filtered, as they would be in a 4P cascade filter.  The oscillations
will look more like a square wave with slanty sides.

> 2) The "resonance" path from the BP output actually controls the damping
> by cancelling the oscillation. This is why the control works "back to
> front".

Yes.  And this is why 2164 makes such a nice control element here, since it
naturally gives a "reverse exponential" response and a gentle approach to
the oscillation point.

> 3) Some type of limiting or clipping is required for the oscillation to be
> stable and low distortion.

Absolutely.

> 4) The best place for such a network would be in the resonance/damping BP
> feedback path.

No.  The limiting network (back-to-back zeners) must be placed across the
first integrator cap (which generates the BP output).  This will give very
nice sine waves during oscillation, and still allow nice resonance during
filtering.  Placing the zeners across the second integrator cap (which
generates the LP output) would still give sine waves during oscillation, but
the filter won't work properly since the resonance will clip very close to
the peak signal level.  Putting the zeners across the input summer's
feedback resistor doesn't limit the oscillations.

For a signal input of +/-5V, I would recommend two 1N4736 6.8V zeners.
These will give sine waves with only about 0.01% THD from the BP output, and
virtually perfect sine waves from the HP and LP outputs, and will limit the
degree of noticeable clipping on the BP output during resonant filtering.
Of course, when the input signal and the filter are tuned to the same
frequency, the two will reinforce each other and then the BP output will
clip fairly severely, but otherwise not.  No clipping whatsoever should be
apparent on the HP or LP outputs.  Lower-voltage zeners could be used, but
then clipping of the BP signal becomes more noticeable (but perhaps this
adds a bit of desirable "fuzz" to the sound...?).


That's my take on the situation, based on simulation.  Others may wish to
chime in with differing viewpoints...?