[sdiy] SSM2164 state-variable filter

[harrybissell at wowway.com]

I'd look at if the capacitors are high quality and well matched... and to see
that the resonant feedback VCA is working correctly. If there is some
residual leakage, that will limit the possibility of oscillation.

The SVF needs ~no~ feedback to oscillate. Try breaking the connection from
U1B pin 7 and R8 (33K). Ground that end of R8 to elimiate all feedback.

I'd use onny polystyrene caps for the integrators... and I'd get the tightest
match I could find (maybe 1% if possible, at least 2%).

Maybe some control range limitation in the current coing to U2C pin 11 is
keeping that VCA partly on ???

H^) harry



On Fri, 15 Aug 2008 16:18:34 +0100, Tom Wiltshire wrote
> 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.
> 
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Harry Bissell & Nora Abdullah 4eva