[sdiy] SSM2164 state-variable filter

[Tom Wiltshire]

Hi All,

I've currently got one of Roman Sowa's SSM2164 12dB/Oct state- 
variable filters on my breadboard (http://www.sowa.synth.net/modular/ 
vcf.html). After having some problems getting it running (which  
eventually turned out to be a bad regulator in the power supply and  
nothing to do with the filter) I've now got sound going through it,  
and all the outputs working. It sounds good, although I haven't built  
a 12dB filter before, so it sounds quite "mild" to my 24dB/Oct- 
accustomed ears. Good work, Roman, and thanks for sharing.

However, I've got a few questions for people who know more about  
filters/filter theory than I do.

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?

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)

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.

Any help appreciated. Thanks!

Regards,
Tom