PEDAL BENCH
by Stompbox Labs
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Distortion1978· Pro Co Sound (USA)

Pro Co RAT

Op-amp distortion with hard clipping to ground — gritty, percussive, iconic.

⏳
Schematic pending verification

We don't ship reference schematics until they've been hand-authored against authoritative sources (manufacturer service docs, ElectroSmash analyses, GEOFEX writeups) and reviewed in the simulator. Until then, the BOM, signal chain, controls and mod notes below are accurate, and you can still add this pedal to a project — it just won't include a circuit yet.

About

A high-gain LM308 op-amp stage drives a pair of 1N914 diodes clipping to ground (hard clipping). A passive low-pass 'Filter' control rolls off treble after clipping for the RAT's signature dark fizz. Capable of overdrive to full-on distortion to fuzz-ish territory.

Signal chain preview

5 stages
Input
Input pulldown + DC blocking
LM308 inverting op-amp gain stage (gain up to ~60 dB via 100k Distortion pot)
Hard-clipping diodes to ground (1N914 anti-parallel)
Passive RC low-pass Filter control
Output level pot
Output

Quick facts

  • Hard clipping (diodes to ground) produces a more aggressive, square-wave-like distortion than the TS808.
  • Original LM308 op-amp has a low slew rate that limits fizz on transients — part of the sound.
  • The 'Filter' knob is wired backwards: clockwise = darker.
  • Used by Jeff Beck, Thurston Moore, and Kurt Cobain (notably on In Utero).

Controls

  • Distortion (100k log)
  • Filter (100k linear, reverse-wired)
  • Volume (100k log)

Mod recipes Pro

4 mods
  • Turbo RAT

    Hotter, more aggressive distortion via LED clipping.

    How: Replace D1/D2 (1N914) with 2× red LEDs back-to-back. Bigger output, more clipping headroom.

  • You Dirty RAT

    Higher gain and more compressed character.

    How: Replace D1/D2 with germanium 1N34A diodes. Lower forward voltage = earlier, softer clipping.

  • LM308 vs OP07

    Modern alternative when LM308 is unavailable.

    How: The LM308 is out of production. Substitute OP07CP — same pinout, very similar slew rate and saturation character.

  • Filter mod

    Add a 'fat' switch to flatten the high-cut tone control.

    How: Wire an SPDT switch to short out the 3.3 nF tone cap. Switch up = stock; switch down = full-range.

References

Community-sourced (ElectroSmash, ProCo schematics archive).