[sdiy] Resonance control taper ?

[Neil Johnson]

Hi Roman,

> Neil Johnson via Synth-diy pisze:
> > I *think* he means that with an SVF you can dial in a very high Q and
> > the filter will not oscillate, whereas other filter structures, such as
> > ladder, at high Q can randomly burst into oscillation when you least
> > expect it, and so you either limit the maximum Q or you need a very fine
> > Q control as you get close to that point.

Roman Sowa wrote:
> Maybe I was doing something wrong all those years, but every SVF I made
> was oscillating without much effort.

Ah, what I meant by "high Q" is a very high Q but where the filter is
still a filter, just ringing for a long time (possibly seconds).  I
agree that the SVF topology is very easy to make an oscillator, but to
make a good oscillator needs careful attention to the AGC loop to
minimise injection of modulation artifacts.  The oscillator in the
Audio Precision System One, for example, is an SVF and has quite a
complicated AGC to (a) get the filter level to the right amount
quickly, and (b) minimise distortion introduced by rapid amplitude
changes.  The "leveler" circuit is an entire schematic page all to
itself.

Cheers
Neil