Runoffgroove Grace Overdrive

From ROG: A few months ago, Aron Nelson released the Smash Drive. The circuit ingeniously used an LM386 audio power amp chip to produce great distortion sounds. Aron based his work on that of Tim Escobedo. Tim's Stupidity Box was another great sounding unit and beautifully simplistic. The Smokey Amp by Bruce Zinky was another design that used a 386 for some great sounds.

Depending on your personal tastes, there may be a potential down side to the 386 designs. The input impedance is listed at 50k, according to the datasheet. This is quite low and causes some loading of the guitar signal. There is some degree of high end signal loss. This may be a favorable thing, again depending on personal preference. I chose to try a high impedance buffer before the 386 to alleviate the loading effect.

A simple Jfet buffer was chosen, due to high input Z and minimal parts count. A high value of 4M7 was chosen as the gate resistor, which helps set the input Z. The J201 fet was used, but any N Channel Jfet could be used with similar results.

The result of the input buffer is easily noticeable. There is increased sparkle in the signal. The signal hits the 386 a little harder, due to more signal reaching the amplification stages of the 386. The buffer adds no gain, but there is a perceived increase. The gain control of the circuit is something else unseen this far with the 386 designs. It is simplistic, but functional. A simple 1k pot was chosen and wired as a variable resistor between pins 1 and 8 of the 386. Looking at the datasheet for the IC, you should notice an internal 1.35k resistor. By connecting the 1k pot between pins 1 and 8, you can effectively vary the gain from near the normal condition of 20X gain to the pins 1/8 connected setting of 200X gain with the 1k pot at minimum resistance.