424 Gain Stage

The circuit uses the same UPC4570 and NJM4565 op-amps as the original recorder. Gain 1 mirrors the recorder's trim control, Gain 2 its channel fader and Volume its master fader. These three gain stages interact, so their balance sets the character more than any single knob.

Low settings give a clean tone with plenty of headroom. Pushing both gain controls moves through overdrive into broken, overloaded fuzz. Bass and Treble then shape the result.

A balanced XLR output on the side works as a DI into a mixer or audio interface. It runs alongside the standard output, so one signal can feed an amp and the other a desk. A side DIP switch lifts the ground on the XLR path if hum appears.