[sdiy] Moog-type highpass filter

[Antti Huovilainen]

On Thu, 22 Apr 2010, cheater cheater wrote:

> yes, but both pre-gain and post-gain are in a single control for ease of 
> use (I know it won't be exactly unity gain, it was just a shorthand).

Should be fairly trivial to do with half of SSM2164 for example, or some 
other linear VCA.

>> Which I have on one of my diy synths... there are separate pots for each
>> VCO and a master volume after the filter. It was intended for a theremin
>> so there is ~no~ VCA at all.
> Correct me if I'm wrong, but wouldn't that just overdrive the input
> stage? That means for example IC3b on this schematic:
> http://yusynth.net/Modular/Commun/MOOGVCF/Moogfilter-sch.jpg

No. The input stage _and every following stage_ are driven equally as 
hard. It's the same old equation for each stage (incl. input):

dV/dt*C = Ictrl * (tanh(Vin/2Vt) - tanh(Vc/2Vt))

> I wasn't really even talking about overdrive - just about having
> enough AC peak-to-peak current so that it can modulate the
> transistors' resistance in a significant way. I understand that

They are the same thing. I find it better to ignore the whole "variable 
emitter resistance" as the differential pair analysis explains it much 
better. Just pick the Vin that gives you whatever amount of distortion you 
like.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova