Hi all,
This is my first post here so let me know whether I am posting in the wrong section, or anything!
I was wondering whether anyone had success building Tim Escobedo's Q&D VCF, or a variant of it (I understand that the topology is an active bridge-T filter, so variants might be published elsewhere).
I put the second version on a breadboard with my guitar at the input and a digital cab simulation pedal at the output and all I am getting is a nasty metallic distortion. The 100K potentiometer does some filtering, but the resonance potentiometer does not seem to change anything and the distortion does not go away no matter how I set up the 1M trimmer or the guitar volume knob. Curiously enough, I also get a glitch sound (as if one is tuning a VCO or an AM radio) whenever I connect the battery to the breadboard.
I also tried a modified version suggested elsewhere where the LED is replaced by two 1N4848 diodes with the 100K resistors replaced by 10K to increase the sweep of the filter and I am getting the same distortion. Also, when I use a white LED I get some sort of weird noise and modulation going as if the filter is connected to an LFO.
I am sure there is a logical explanation to all this as I am probably doing something (or a few things!) wrong, so I would appreciate any insight.