keyboard triggered filter

Sean Costello costello at
Mon Nov 8 22:29:26 CET 1999

Hi Matthew:

What you are describing sounds like a state-variable filter, with the cutoff
frequency controlled by a keyboard. This will definitely work for feedback
oscillator applications - I was doing this the other day (with a Moog filter) to
control the pitch of feedback echoes. There are plenty of filter kits available
out there - Paia ( has an inexpensive dual state-variable
filter, Blacet ( has a filter module available, and MOTM
( has a Korg MS-20 filter that can be switched
between highpass and lowpass mode. There are plenty of other people on this list
that offer circuit boards for Moog filters, state-variable filters, etc.

The only problem with the typical DIY solution is that none of these come with a
keyboard - you will have to construct a 1V/Octave keyboard, or a MIDI interface
that outputs these values, in order to control the filter in the way you want
to. If you want a really quick solution to your problem, you might want to just
look for an analog synth with a filter input that is fairly cheap, as these will
have the keyboard built in. Some options might be:

Moog Rogue (my personal favorite for fairly cheap synths)
Moog Micromoog or Multimoog (not very cheap nowadays, but has GREAT processing
capabilities for external inputs)
Arp Axxe
Sequential Circuits Pro-1
Yamaha CS-5 or CS-15 (not sure if filter tracks keyboard)
Octave Cat or Kitten (the Cat is a great synth that usually sells for $300 or so
- a ridiculous bargain, in my opinion)

In addition, all sorts of other monophonic analog synths can be modified to have
an audio input. This is great for the sort of feedback work you are doing, as
you can bring up the oscillator levels to have a drone in conjunction with your
feedback effects, creating a VERY complex chaotic interaction. This works best
with a filter that has a fair amount of internal distortion, as nice
intermodulation frequencies will be generated. Moog filters are great for this,
and I have heard that the MS-20 type filters are also excellent for this

By adjusting the resonance (or emphasis, or Q) of the filter, you can control
how quickly the feedback pitch approaches a relatively pure sine wave - assuming
that the feedback network allows for this. I have found that certain effects in
a feedback loop, like a reverberator, do not necessarily produce a feedback
pitch that is identical to the cutoff of the filter; I presume that this is
because there are certain resonances in the reverberator itself that are
stronger than the filter resonance.

Hope this helps,

Sean Costello

Matthew Helt wrote:
> hello,
>   Im wondering if anyone knows about any such device available in a kit
> or schematic form somewhere instead of me having to start from scratch
> on this one (though i wouldnt mind much)
>   for live performances and such it would be great to have a keyboard
> that had an input and an output, and when you pressed a key the box
> would tune the filter to that note (or chunk of audio spectra, im not
> really into the idea of it filter all but that note) and then trigger
> the filter. a switch to low pass for everything below that note and high
> pass for above, along with a width respot for how much spectra in
> bandpass mode would be ideal...
>   i use a 4 track wired into itself with a bunch of effects/eq's for a
> feedback oscillator live, it would be neat to have even greater control
> on what exactly the output is since the on board/osciallator eq's only
> serve to shape the sound and having to run yet another eq would not be
> feasable.
>   if nobody has leads on something like this, what kind of chips would I
> start with? i want to get datasheets to start looking into this.
> thanks to everyone for your help, this list is the best source of
> information i have found yet,
>  -matt helt

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