[sdiy] Synthancon VC-Resonance

[Ryan Williams]

hi nathan,

I might suggest ditching that amplifier. take a look at:
http://yusynth.net/Modular/EN/STEINERVCF/index.html

that's Yves Usson' site. he shows a few variations of the filter you 
posted at ken's site. also, at the very bottom is a link to nyle 
steiner's paper on the filter. Incase you haven't done an analysis of 
it, this should give a good idea how the filter really works. I have 
considered putting in an OTA based amplifier to replace the bjt amp in 
the normal version. One important thing to remember is that the input 
impedance to the amp needs to be fairly high.

your question about the resistor scaling. those resistors set the the 
feedback on that bjt amplifier. You could change them around a little 
bit so long as you kept the same gain as it has now. I think you'll find 
that making them large will ruin the amp, but it's been a while since I 
looked at such a circuit.

I hope you get a good circuit going.

-Ryan Williams

nathan settembrini wrote:
> Hi,
> I am doing a project for the Theory of Music Synthesizers class at 
> Georgia Tech.  We have put together the Synthacon VCF filter 
> (http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html 
> <http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html>) and are 
> trying to add voltage controlled resonance to the circuit.  We have 
> tried using a JFET as a voltage controlled resistor to replace the pot 
> but think we were overdriving the amp and getting a lot of noise.  The 
> other thing we tried was using a Vactrol to replace the pot, but can't 
> figure out a way to get the resistance low enough (without supplying 
> rediculous amounts of current) to replace the 1k pot.  Does anyone have 
> any ideas?  Can the 2k2, 1k, and 1k pot all possibly be scaled to larger 
> values and the circuit still function properly?  Thanks
>  
> Kyle