Envelope Filter based on a Lovetone Meatball / Mu-Tron III kind of state variable filter circuit.
Added envelope controlled LFO
EHX Y-triggered style sweep direction control
recorded with a custom made Jazzmaster and Yamaha THR10
Qi-Gon is an envelope filter/auto-wah with envelope controlled LFO speed/tremolo
As you can see it is based on the Lovetone Meatball, with modified envelope follower. I borrowed biasing idea from the Dr Quack, which allows using any kind of opamp.
It has a continuous sweep direction pot, similar to the Y-triggered Filter. I have no idea if it works the same way technically but the results are the same. I know that Mark Hammer is quite knowledgeable about the Y-Triggered and may be able to tell. I possess one but have never managed to understand that crazy circuit …
Turn to the left, the sweep goes up, then as you turn to the middle position the sweep gets weaker until there is no sweep at noon. Then the sweep gradually goes down as you turn to the right.
The filter LDR’s can be controlled either by the envelope follower or the LFO. When in LFO mode, the envelope follower controls the LFO speed. Depending on the sweep pot position, the attack will slow down the LFO speed or speed it up. Sweep pot and Intensity of the envelope follower change the speed interactively.
Apart from the usual frequency range selection, there is one position where the caps are replaced by a resistor and the feedback resistor upped for a quite convincing tremolo effect in LFO mode. Works similar to the Tremvelope from Pigtronix. In regards to the Meatball’s State Variable Filter, I changed the first opamp to be non-inverting, so that the low pass and high pass positions are in phase with the blended signal and only the bandpass is out of phase. (instead of the opposite on the meatball)
The pedal is using buffered bypass, so that the pedal connected in the send/return path is still active when in bypass mode. I modified the blend pot to blend between send/return signal and filtered signal instead of clean/filtered. Made more sense when using distortion.
I have also included an external expression control jack connected to the outer lugs of the sweep pot. I have only tested it with a manually turned pot as I have no expression pedal available, but it should work, provided the contacts of the expression pedal are isolated from ground.