FA-1: How much gain?

Posted: 2009/07/16 in Boss FA-1 Clone, DIY Guitar Effect Pedal Projects

A common question you might ask about a boost pedal like the FA-1 is how much gain does it provide?  Looking at the schematic, we see that the volume knob is inside the feedback loop of the first op inverting amp, IC1.

FA-1 Schematic

FA-1 Schematic

The voltage gain of an inverting op amp is given by a handy formula: Vout/Vin = -Rfeedback/Rin where in this case Rin would be R5 (15k), and Rfeedback is the 1M pot in parallel with R6 (470k).  When the volume knob is turned to the maximum, the total resistance is 320k, and 320k/15k yields a voltage amplitude gain of 21.3. To convert this into decibels, take the log and multiply by 20, in other words 20 * log(21.3) = 26.6 dB.

(These formulas I used came from Horowitz and Hill’s The Art of Electronics.)

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