[sdiy] SSM2164 state variable filter

[David G. Dixon]

> Hanging the zeners off the LP output down to ground (as Neil does in his
> 3P SVF) gives an extremely flat oscillation level across the whole range
> (a very nice result, actually) but unfortunately it achieves this by
> apparently driving the oscillation *much* harder which results in a
> heavily distorted sinewave. I can see clear spikes of 3rd and 5th harmonic
> on my scope's FFT display, and I didn't need the display to hear that they
> were there.
> 
> I'd like to experiment using different zener values, but unfortunately I
> don't have any to hand. Maybe next week. The thought is that if I reduce
> the zener's limit, I can make sure that the oscillation is always limited
> by the zeners and not by some other limit of the circuit. At the moment, I
> don't think that the zeners have any effect at the lowest freequencies. I
> guess I should test that.

In my simulations, putting any zeners on the LP feedback, no matter how
small, sends the LP and BP oscillations to the opamp rails and makes them
look like trapezoids, while the HP looks like a truncated triangle wave.
According to simulation, this simply will not work.

I'm wondering why the simulations are so far off from the breadboard.
They've never been before, unless it is some behaviour of the 2164 which is
not being captured by the extremely simply model I'm using for it.