So - first of all, this is my first post! I'm not an electrical engineer (most often, I'm on the software side of things), but, well, it's been an interest, and I couldn't put off registering on this great forum any more!
Anyway - I am working on some <analog> audio effects for music, and while twiddling knobs is fun, voltage control is even funner! So, I have been working on a voltage controlled -12db/octave Sallen/Key lowpass filter, which uses JFETs as voltage controlled resistors in the RC loop.
Initially, I just passed a straight DC control voltage into the JFET gate, but quickly found out that this caused
CRAZY distortion! Obviously, the impedance of a JFET in ohmic mode is controlled by the gate to source voltage, not just the "absolute" gate voltage!
So - I figured that the best way to get past this problem was to couple the input signal (audio) onto the DC control voltage, so I did this. The result is much better, but it isn't really optimal. The coupling capacitor (C1/C4 in the drawing) shouldn't be too big or the filter will react too slowly to control voltage changes - and if it's too small, it doesn't do the job properly either (same distortion effect as with straight DC control voltage, i.e. without input coupling)
The basic circuit layout is the following -
Am I going off in a totally wrong direction on this VCF design? Or are there some neat tricks to make sure that the gate/source voltage stays constant thus allowing for actual "proper" filter cutoff (and resonance) control?
Thanks in advance!!!!!
Best regards from David!