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

From unknown freak
Sent Thu, Oct 30th 2003, 06:31

Further to my previous message, I whipped up a quickee example 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

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?

----- Original Message -----
From: "unknown freak" <>
To: "James R. Coplin" <>; <>
Sent: Wednesday, October 29, 2003 11:15 PM
Subject: Re: [AH] variable bandwidth bandpass filters...

> Another option would be to buy a lowpass and highpass from a brand like
> Doepfer and use them in serial.  Voltage control of bandwidth could be
> accomplished by using a single CV for both CV inputs but inverted for one
> them.  That signal could be an LFO, ADSR, or anything.  The other CV
> could take a single multiplied voltage as from an ADSR or whatever.  Given
> the price of Doepfer filters, we're not talking about such an expensive
> proposition.  $92 for an A-120 lowpass, $111 for an A-123 highpass (but
> probably better to get two A-121 multimodes at $123 each -- -12dB slopes
> lots more options).  An A-175 dual voltage inverter is only $49.
> you're talking around $300.
> At any rate there's no such thing as a $450 anything from Serge per se, is
> there?  You'd need to find about $2000 worth of buddy modules to fill out
> the panel.
> ----- Original Message -----
> From: "James R. Coplin" <>
> To: <>
> Sent: Wednesday, October 29, 2003 11:06 PM
> Subject: [AH] variable bandwidth bandpass filters...
> > Does anyone know if anyone besides serge makes a variable bandwidth
> > bandpass modular filter? It needs to be resonant and have vc over
> > bandwidth. If nobody does, somebody should, I'd probably by 3-4 right
> > off the bat. With Serge prices these days I don't think I want to shell
> > out $450 a filter plus the misc mod modules if there is another option.
> >
> > James R. Coplin
> > ***************
> > If anyone asks of my whereabouts,
> > simply tell them i've gone out the window
> > for a spot of tea and am not
> > expected back any time soon.
> > ***************
> >
> >