Tim Escobedo's Q&D VCF, distortion issue

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Joined: 29 Jan 2023, 22:07

Post by kgemenis@mail.com »

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).
simplevcf_252_119.gif (10.06 KiB) Viewed 60 times
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.

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