[sdiy] Pole Dancing
rburnett at richieburnett.co.uk
Tue Sep 18 23:45:10 CEST 2018
> You bring up an interesting point. What's the effect of the Moog poles
> being mistuned a bit? The answer is, not much. Worst case, you may need
> a little more feedback to get the job done.
Exactly. The effect is much more pronounced in the TB-303 filter though.
Here there is no buffering between stages in the ladder and the open-loop
poles are spread over more than a decade in frequency! That's why the
TB-303 filter requires so much gain in the feedback path to get it anywhere
near to self-oscillation.
> Even if the tuning is substantially off, the two right poles will pair up,
> and the two left poles will pair up, and they'll split off into a very
> similar X pattern.
Many people have pondered the effect of the bottom capacitor in the TB-303
ladder filter being a different value from the others. As you say, it has
little consequence other than that you need a bit more feedback gain to make
> Also note, a Moog Ladder pole pattern is nothing like a Butterworth
> filter, where all the poles contribute roughly equally.
Yes, from a technical point of view once closed-loop feedback is applied the
response doesn't resemble any of the classic textbook filter responses.
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