[sdiy] Ladder filters and gain drop, that old chestnut

Richie Burnett rburnett at richieburnett.co.uk
Thu Aug 27 17:52:28 CEST 2015

Yeah, that's right. Positive feedback around something that has a negative gain in the forward path, is exactly the same as negative feedback around the same thing with a positive gain in the forward path, except that the polarity of the overall output is reversed with respect to the input to the input.

The magnitude of the feedback gain required to reach self oscillation is the same, and the drop in passband gain is identical in both instances. The overall output is just inverted.


Sent from my Xperia SP on O2

---- mskala at ansuz.sooke.bc.ca wrote ----

>On Thu, 27 Aug 2015, Tom Wiltshire wrote:
>> What happens if you stick an extra unity-gain inverter at the end of the
>> filter?
>> Then you'd be able to use positive feedback for the resonance, and
>> adding the input signal into the resonance VCA (or pot) wouldn't change
>> the tone so much (since it'd be a simple sum, rather than the difference
>> of the two signals). Does this make sense, or do I need more tea?
>More tea for you, I think.  Inverting and then adding is not really
>different from subtracting.
>Matthew Skala
>mskala at ansuz.sooke.bc.ca                 People before principles.
>Synth-diy mailing list
>Synth-diy at dropmix.xs4all.nl

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