[sdiy] SSM2164 state variable filter

[David G. Dixon]

> As the filter sweeps through the harmonics of the input waveform,
> you get a characteristic 'sound'.  Lower harmonics have higher
> amplitudes than higher ones, and the higher harmonics (of course) are
> closer together.
> 
> If your input signal amplitude is near the zener voltage, this will
> effectively 'compress' the amplitude of lower harmonics selectively.
> 
> This might sound 'good' ot 'weird' depending on your preference.
> 
> Of course if you are most interested in an oscillator, the zener should be
> chosen for the amplitude you'd like.
> 
> Its a BIG trade-off. Higher zener voltages sound more like a classic SVF,
> lower ones like an SVF followed by a compressor.
> 
> The idea of pre and post filter amplitude is really worth consideration.
> The sounds are drastically different...

I always put input attenuators on all my signal processing modules (and I
almost always put 3-channel input mixers on all of my VCFs), so this is less
of an issue.  If I wanted high Q without distortion, I'd simply attenuate
the input and then amplify the filter output at the VCA or the mixing board.

> even more so if its ~not~ an SVF, but an MS-20 type filter :^)

Funny you should mention that, because in my version of the late-model MS-20
filter, based on 2164 VCAs and integrators, no zeners are required
whatsoever.  At self-oscillation in both filter stages (HP resonance gain of
2, LP resonance gain of 1), the circuit naturally limits the sine waves to
1/2 of the opamp rails, with 0.01% THD on the HP output, and 0.001% THD on
the LP output.  In this case, it is easy to use zeners across the resonance
gain amp feedback loops for further limiting, and to dial them up or down as
desired.  I've used 1N4728 3.3V zeners, with one end connected to the -
input of the gain amp, and the other end connected to the wiper of a 100k
pot with one leg connected to the gain amp output and the other connected
through a 33k resistor to ground.  This gives me linear control of the sine
wave amplitude from about 6.7V down to about 1.6V over the full range of the
pot.  I'm putting this pot on the panel, which will allow me to dial in any
amount of "MS-20-style" clipping distortion that I desire, which people tell
me is such an important aspect of the original MS-20 filter's character.