Mike Matthews, founder of Electro Harmonix, wanted to develop a new version of the Big Muff using integrated circuit op amps in place of the four discrete transistors used in the earlier designs. He tasked Electro Harmonix engineer Howard Davis with developing the new circuit, resulting in one of the biggest changes to the Big Muff since its introduction.
Although it retained the basic Big Muff concept, the new op amp circuit had a very different character from the Triangle and Ram's Head versions that came before it. The fuzz was more aggressive, gritty, and gnarly, with a harder distortion like edge, while lead notes could still become remarkably smooth and sustaining. This combination gave the Op Amp Big Muff a sound all its own and marked the beginning of a completely different chapter in the evolution of the Big Muff.
The 1977 version featured an on and off tone switch. The final 1978 version replaced this with a tone bypass switch that completely removed the tone section from the circuit, increasing the output and adding more gain. The op amp circuit was also used in the Deluxe Big Muff.
The Op Amp Big Muff would later become famous as the primary fuzz used by The Smashing Pumpkins on Siamese Dream.
According to Billy Corgan, he first realized the potential of the Big Muff after stopping by a rehearsal of Catherine, who shared the same rehearsal space. He walked in one night and heard all three guitarists using Big Muffs, creating a massive, Sabbath like wall of fuzz. The sound made a lasting impression on him and would eventually influence the direction of Siamese Dream. You can see Billy's original 1977 Op Amp Big Muff in the photo above, still set to his Siamese Dream settings.
The specific Op Amp Big Muff Corgan used became the defining fuzz sound of the album. Nearly all of the heavy rhythm tracks and solos were recorded through that one pedal, with some songs layering as many as eight fuzz guitars. Combined with his Bat Strat, Soul head, and Mars cabinet, it created the enormous guitar sound that became synonymous with Siamese Dream.
If you listen to Catherine's album Sorry!, you can hear the sound that helped inspire what Corgan would later create on Siamese Dream.