[sdiy] SSM2164 state variable filter

[Stewart Pye]

Is there any reason to not use the SEM method of "deadband" type diode 
network in the feedback network from the bandpass output, so that even 
when the resonance control is set for no damping once the bandpass 
output reaches a certain threshold the signal will be fed back to the 
filter input,  thus dampening the response?

Regards,
Stewart.


David G. Dixon wrote:
> 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...?
>
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