A guy on AH asked the list where he could get an affordable variable-width
bandpass filter and I replied as follows. Seemed maybe worth cross-posting:

Re: [AH] variable bandwidth bandpass filters...

... I whipped up a quickee example mp3:

http://66.155.197.104/vogelscheiss/misc/doepfer_varbandpass.mp3

The setup is this:

AUDIO: a 4-note chord of VCOs feeds into the audio input of a Doepfer A-121
Multimode. Lowpass output of that filter runs into the audio input of a
second A-121. The highpass output of this second filter runs out to the
mixer.

CONTROL: LFO square wave gates an ADSR. Postive ADSR output feeds CV1 on
the highpass, inverted ADSR output feeds CV1 on the lowpass, so every time
the highpass Fc goes up the lowpass Fc goes down, widening the passband.
The sine output of the same LFO feeds CV2 on both filters, so that you're
getting some additional unison movement. The Fc knobs on the filters are
set to taste, but at a certain point in the ADSR cycle they crisscross and
cut off all sound. Resonance on both filters is set to 7.

I added a little MoFX echo to make myself happy.

Voila, variable-width bandpass. Don't modulars rock