variable slope resonant HPF
2002-01-17 by jhaible
Spent some time on a new filter idea. Not finished yet, but tested enough to know that it will work: I wanted a HPF for my "Moog-style" JH-5. I thought a 4pole HPF with built-in filter coupler would be right after the 904A-style filter. But variable slope and resonance should be added. In the end I had both features combined: One Slope / Resonance pot: ccw end is 6dB/Oct, 12 o'clock position is 24dB/Oct, with practically linear action in between. Turning the knob further clockwise will keep the 24dB/Oct slope, but also adds resonance. There are two additional "correction paths" built in: Variable slope would change the cutoff point, too. This is corrected with a CV sidechain. With a trimmer I can choose a certain attenuation (like -6dB or -3dB) to be at an almost constant frequency when the slope is changed. 2nd, pass band gain is corrected to approx. unity for increasing resonance. Slope / Resonance is voltage controlled, with (-6dB/Oct)/Volt sensitivity in the slope range. Not finished yet, but it looks promising. JH.