[sdiy] SSM2164 state variable filter

[cheater cheater]

Gergo and Harry,
I'm having a problem picturing how this would look on a schematic. Is
one of you up to making a quick sketch of the sallen-key topology (or
any other well known topology) with that?

Thanks
D.

On Thu, Jul 15, 2010 at 17:41, Gergo Palatinszky
<Gergo_Palatinszky at epam.com> wrote:
> Hi,
>
> If you would like to move further link a pot(~50k...100k) in series the middle zener and you got the ultimate feedback regulator circuit :O)
>
> Br,
>        Pala
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Harry Bissell
> Sent: 15 July 2010 17:33
> To: cheater cheater
> Cc: sdiy DIY
> Subject: Re: [sdiy] SSM2164 state variable filter
>
> OK I'll get into this one :^)
>
> In most cases the clipping does not matter, as the filter response still
> predominates. I've used anti-parallel LEDS, silicon diodes, germanium
> diodes, shottky diodes, and a large variety of zener diodes.
>
> The symmetry isn't too much of an issue either, with feedback in the filter
> most asymmetry gets corrected out...
>
> If you are really concerned, use a bridge of silicon diodes (matched if possible)
> with a single zener in the middle. The silicon diodes would contribute only a small
> amount of the total clipping, so in general its much better than two back to back
> zeners.
>
> I have not gone so far as active clipping, which can be as precise as you like
>
> H^) harry
>
>
>
> ----- Original Message -----
> From: cheater cheater <cheater00 at gmail.com>
> To: David G. Dixon <dixon at interchange.ubc.ca>
> Cc: sdiy DIY <synth-diy at dropmix.xs4all.nl>
> Sent: Wed, 14 Jul 2010 20:54:55 -0400 (EDT)
> Subject: Re: [sdiy] SSM2164 state variable filter
>
> One thing that's interesting to me is how the matching of the diodes
> matters. What if you took the two diodes, and one of them had say a
> 20% higher voltage, creating some form of asymmetric clipper? Could
> sound nice, couldn't it?
>
> What about different types of diodes - both types being the same
> voltage but everything else different (for some definition of
> 'everything else' that makes for interesting results) - how could the
> sound change when using one type of diode vs the other?
>
> What about transistor clipping? Tube clipping? OTA clipping? Germanium
> vs Silicon? :)
>
> Cheers,
> D.
>
> On Wed, Jul 14, 2010 at 20:50, David G. Dixon <dixon at interchange.ubc.ca> 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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> --
> Harry Bissell & Nora Abdullah 4eva
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