Reckless Abandon

A two-channel guitar preamp pedal. Not an original circuit, but nicely voiced and versatile.

The circuit. A treble booster ahead of the gain, with a Voice switch moving its turnover between about 1.5 kHz (as in the Rat and many distortion pedals) and 500 Hz (closer to a Vox): the lower setting adds midrange before the gain, suits single coils, and changes how it feels to play; the higher one suits humbuckers.

Each channel clips through diodes in an inverting op amp's feedback loop, channel 2 with 20 dB more gain and a darker sound. After a long comparison of op amps (TL072, NE5532, LM4562, LM833) and diodes (1N4148, 1N4007, red LEDs), an LM833 with 3 mm red LEDs sounded warmest. The capacitor across the diodes matters a great deal, and so, surprisingly, does the op amp's supply decoupling: 10 µF in place of 100 nF tamed harshness and added warmth.

The EQ is a three-band passive stack with mid-shift and treble-shift switches, moving from a Marshall-like curve to a Fender-like one, followed by a James-style bass control for far more low end than a standard tone stack allows. An op amp recovers the stack's loss and buffers the output. Two true-bypass footswitches: on/off and channel. Everything mounts on a PCB sized for a 1590DD, with room to adjust the controls while playing. It's voiced as a preamp, into an effects return or a cab simulator, rather than into the front of an amp.