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Vintage threads from the first ten years

Re: Parametric EQ circuit

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9/8/1999 5:15 AM
Rick	Re: Parametric EQ circuit It's set up as two variable resistors, resistance goes down as the toe of the pedal goes down (filter goes up).

9/8/1999 6:08 AM

Smells like a state-variable to me. Look at the Mutron III schematic, take away all the envelope-control features, sub a dual ganged pot for the parallel resistor/photocell combo, and there's your wah. If you look at the "Super Tone Control" from either of the first 2 Anderton DIY FX books, it's the same deal.

Incidentally, many state-variables use discrete or variable resistors between op-amp stages to set the frequency. In the case of the Mutron, there is a fixed resistor paralleled with a variable one in the form of a photocell. Anderton notes that if you stick a 10K-100K pot (wired as a voltage divider on the output of the 1st and 2nd opamps) and a 1k resistor (between the wiper of the pot and the input of the 2nd and 3rd opamps), you increase the range of possible frequency sweep by about a factor of 10 or so.

Where this can come in handy is in selecting the sweet range or desired range of foot sweep. I.e. if the total range is 1000:1 (using a manually rotated pot) and the pot travel (by foot) is less than full rotation, you can use a moderate value foot-pot (e.g., dual 50k) and tack on "tuning" dual ganged pots at either end of the swept pots to set the high and low ends of the footsweep range. This will let you pick the overall band you want to sweep in and arrange for either wider or narrower sweep. If the taper of the pots is such that more responsive wahing is produced more at one end than the other end of the pots' travel, you can compensate with each of the range-setting controls.

Stated more formally, consider the voltage coming out of the pot as reflecting the ratio of Ra and Rb. In turn, Ra consists of Rxa+Rpa (where Rxa means an external resistor in series with the CW lug of the footswept pot, and Rpa means the resistance from the CW lug to the wiper), and Rb consists of Rpb (resistance from footpot wiper to CCW lug) plus Rxb (external resistor between CCW lug and ground).

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