AW: Re[2]: AW: Re: MIDI -> CV and state var VCF resonance cont

Haible Juergen Juergen.Haible at
Tue Mar 11 14:00:00 CET 1997

>      The 3080 works fine as a VC resonance control in both state-variable
>      filters as well as 4-pole designs. Just watch out for little DC
>      offsets that may be added by it in state-variable filters - any
>      offsets may mess up the loop. There's a circuit in EN#37 that shows
>      this.

Sounds interesting! I always wondered why the SEM filter had the
loop AC-coupled. And the OB-X filter even has a series resistor at the
ota output, before it is mixed with the diode-resistor-network.
I did always suspect that it has to do with stability, but never fully
checked it out.

>      Also with state-variable VCQ circuits, you can add an audio input to
>      the VCF at the Q OTA. When you use this input as the signal input to
>      the VCF, the signal will be attenuated as the Q is increased, so that
>      the VCF will be less prone to be nudged into runaway oscillation by
>      transients at the input. This is covered in the EN#37 circuit too.

And would this not be the answer to your recent parametric EQ question
as well ?


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