[sdiy] Another SSM2164 question

[Neil Johnson]

Hi all,

> The quick and dirty way to compensate the resonance is simply to  
> feed (some of) the Freq CV to the resonance VCA. As Neil said, the  
> VCA can be considered to control  "damping", so as the frequency  
> climbs, the damping increases.

I tried this.  Its a nasty hack and I didn't like the result.

> I'd like to remove the effect at source if possible, but I'm pretty  
> sure this would deal with it.

One thing I noticed on my filter (may not apply to all filter  
designs) is that it looks like the limiting factor is the slew rate  
of the op-amps.  There are two pieces of evidence for this:

1) High input levels also decrease Fc (hinting at an amplitude- 
related problem);
2) Watching the trace on a scope, once the Q feedback level reaches a  
certain point the rise/fall slope does not change, only the  
increasing duration of a cycle.

One thing to try would op-amps with faster slew rates (I'm using  
TL07x family).

In the meantime, I've opted to limit the feedback amplitude to  
minimize this effect, while still allowing self-oscillation, using a  
scheme very similar to that used in the Oberheim OBx.

Neil
--
http://www.njohnson.co.uk