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

Gordonjcp gordonjcp at gjcp.net
Wed Aug 26 23:11:46 CEST 2015

I've been playing around with a diode ladder filter a bit recently, and it really does seem to lose a hell of a lot of gain at moderate resonance settings.  Now the low output level isn't a hassle since I've got the potential for enough gain in the following VCA stage.  It would be nice, though, if the output didn't go so nuts at *low* resonance settings - I estimate there's a good 10dB between self-oscillation and "no feedback".

So I was wondering, what influences this?  I noticed that the TB303 doesn't seem to suffer from it so badly, among others.

If I set the feedback level to be constant and quite high, would controlling the resonance by reducing the gain in the differential amp give a decent swing of resonance without the output increasing to silly amounts?

Gordonjcp MM0YEQ

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