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

Richie Burnett rburnett at richieburnett.co.uk
Thu Aug 27 18:31:39 CEST 2015

> When it comes to the gain drop, shouldn't it be possible to add more
> poles & phase shifts somewhere else in the frequency range in order to
> avoid some of the cancellations?

If you cascade more than four poles in the forward path, you get to your 180 degrees of phase lag required for resonance quicker, and with *less attenuation*. Hence you need less gain in the feedback path and therefore suffer less attenuation in the passband at the onset of self-oscillation. But of course a higher order filter will have a much steeper rolloff slope in the stopband too.

Conversely a 3-pole filter like the Tb-303 requires a *LOT* more gain to get close to self-oscillation. And the drop in the passband gain at high resonance settings is even more pronounced than the moog ladder! You get about 26dB drop without any measures used to compensate it.

Anyone who has used a real Tb-303 will know that the measures taken to compensate for the volume drop at high resonance are marginally effective at best. The volume all goes through the roof when you turn the resonance down to zero!


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