[sdiy] SSM2164 state-variable filter

[Tom Wiltshire]

On 15 Aug 2008, at 16:05, Ian Fritz wrote:

> At 08:27 AM 8/15/2008, Tom Wiltshire wrote:
>> Hi all,
>>
>> I've done a bit more work towards trying to eliminate the frequency
>> effect from the resonance on the SSM2164 SVF filter I'm playing with
>> (http://www.sowa.synth.net/modular/m_vcf.gif). I'm just looking at a
>> single filter section, without any of the CV mixer stuff.
>>
>> I drew a graph of the resonance response:
>>
>>         http://www.electricdruid.com/ResonanceGraph.gif
>>
>> Note the linear scale. It probably doesn't look as bad on a log  
>> scale!
>
> Ah!  It wasn't clear before what you were talking about.  You  
> problem looks to me more like a low frequency problem than a high  
> frequency one.  Opamp phase shifts, slew rates, etc. aren't  
> important at just a few kHz.  Something else is going on.

Perhaps it is. Roman Sowa says on his page about his filter that  
it'll oscillate right down to 0.3Hz, which mine doesn't. It won't  
oscillate without a signal input below about 1.5KHz.

Above that point, the oscillation level increases markedly towards  
the top of the range. Roman Sowa says his filter oscillates with high  
distortion above 10KHz, which suggests the level goes up on his too.

T.