Any suggestions on how to add a resonant filter to a keyboard that does not have one?
I have a Alesis Quadrasynth (QS4) and want to do midi-triggered filter sweeps in real-time.
there is a free vst called north pole that can follow the envelope of the audio through it
I have heard wonderful things about this box: http://www.sherman.be/index.php/products/filterbank
If you are comfortable with circuits (or learning) it wouldn't be especially hard to build your own midi-controllable filter with an Arduino. The schematics of a moog ladder filter are widely available.
You can try looking at a Guitar FX pedal as well.