[sdiy] SSM2164 state variable filter

Thu Jul 15 19:06:05 CEST 2010

erm, of course that was supposed to be about SVF, not sallen-key..
ignore the mixup ;-) thanks Richard for pointing it out

Cheers,
D.

On Thu, Jul 15, 2010 at 18:27, cheater cheater <cheater00 at gmail.com> wrote:
> 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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