[sdiy] SSM2164 state-variable filter

[harrybissell at wowway.com]

What is the signal level at the input to the resonance feedback
VCA ?   If it is high enough to clip a diode or zener, I'd add a
symmetrical clipper at the junction of R8 and R9.  This should make the
oscillation level more stable (I think).  A self resonant filter will not
have stable amplitude unless something clips, somewhere...


H^) harry

On Fri, 15 Aug 2008 15:27:27 +0100, 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!
> 
> Neil J also suggested that the effect might be because of limited  
> slew rates of the op-amps. I haven't got a high-speed op-amp handy,  
> but figuring that if that was the case, a worse slew rate should 
> make  the effect worse, I tried a LF444. The slew rate of TL074 is 
> 13V/uS, against 1V/uS for LF444. There was  no change at all.
> 
> I've also experimented adding various capacitors over the 30K input  
> capacitors. This just seems to limit the resonance overall, without  
> much effect. I haven't yet started trying compensation on the first  
> mixer stage.
> 
> If anyone has any further ideas, I'd appreciate it. I'm running a 
> bit  short myself.
> 
> Thanks,
> Tom
> 
> On 10 Aug 2008, at 18:41, Neil Johnson wrote:
> 
> > Hi Tom,
> >
> > As I'm just finishing up the design of an SSM2164-based 3-pole  
> > state variable filter I think I can offer some thoughts...
> >
> >> Firstly, the resonance varies with cutoff frequency, being more  
> >> pronounced at higher frequencies (in fact, it'll oscillate at high  
> >> frequencies when it won't when you tun the cutoff down. Is this  
> >> normal? Do all SVF designs do this?
> >
> > I think in theory resonant fequency *should* be independent of Q.   
> > However my suspicion is that various non-linearities and other  
> > frequency-dependent effects (op-amp slew rate, limited GBW, etc)  
> > change the behaviour sufficiently that there is a connection  
> > between the two.
> >
> > This is one aspect of my design that I am currently experimenting  
> > with.
> > An early draft of the schematic is here:
> >    http://www.njohnson.co.uk/pdf/MSM/msm-vcf.pdf
> >
> > [Note: there are some errors in that version - a revised version  
> > will be appearing sometime soonish once I'm happy with the core  
> > design.]
> >
> >> Secondly, the resonance increases as the resonance CV increases.  
> >> Whilst this is logical enough, the CV serves to increase the  
> >> attenuation of the VCA, so increasing the CV actually decreases  
> >> the output from the resonance VCA. Does this mean this VCA is  
> >> being used to cancel out the effect of the other feedback loop?  
> >> (through R6 or R53 on Roman's schematic)
> >
> > Yes - in reality what you are changing is the 'damping'.  At 0V the  
> > gain cell has 0dB attenuation, so you are at maximum damping  
> > (lowest Q).  As the control voltage increases so does the  
> > attenuation (at 33mV/dB), so there is less damping (higher Q) and  
> > the filter starts to ring - eventually going into self-oscillation.
> >
> >> Thirdly, what is the function of the 5V6 zener diodes on the final  
> >> integrator? I haven't put them in yet, and it seems to work.
> >
> > They limit the loop gain when in self-oscillation, otherwise you  
> > can end up slamming against the rails.
> >
> > Cheers,
> > Neil
> > --
> > http://www.njohnson.co.uk
> >
> >
> >
> >
> 
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Harry Bissell & Nora Abdullah 4eva