Electro-Harmonix has reissued their late-70s era Op-Amp Big Muff Pi that was popularized by Billy Corgan of the Smashing Pumpkins during the early 90s. Sometimes also referred to as the IC or V4 Big Muff, the original circuitry has been faithfully re-created while several practical enhancements have been added including a compact, die-cast chassis and true bypass switching.

The pedal relies on op-amps rather than transistors and three gain stages rather than four, to create its signature sound. It’s a sound that’s been described by noted Big Muff collector and historian, Kit Rae, as: "a huge, crushing Big Muff sound with more crunch...great for grungy, wall-of-sound distortion, heavy rhythm playing and heavy leads."

The Op-Amp Big Muff Pi features standard controls: Sustain, Tone and Volume, plus a Tone toggle switch for bypassing the tone circuit. It comes equipped with a 9V battery or can be powered by an optional 9-volt AC adapter.