Ever since the Op Amp Big Muff was identified as the correct version to capture the “Siamese Dream” tone, the interest in this version has been huge. It might as well be called the “Billy Corgan version”. I’ve noticed there are many differing schematics floating around the web. After having built my own clone and comparing it with the original pedal, I’ve found some errors that greatly affect the ability to capture the tone of the original Op Amp Big Muff. My current clone pedal sounds exactly like the original now. In side by side comparisons using the “Billy Corgan settings”, you can’t tell the difference. Here’s what I’ve found:

– “Any 4558 chip will do.” – False
The most over-used 4558 chip on the DIY market right now is the Texas Instruments RC4558P. This chip does not sound good in the Big Muff circuit, and a look at it’s datasheet shows why. The schematic of the RC4558P is different than the RC4558NB (original chip). The best new manufacture 4558 I’ve found for the Op Amp Big Muff circuit is the JRC4558D. It’s datasheet schematic is the same as the original RC4558NB, and it meters out at the same voltages as the original.

– Input cap – Needs to be a NOS, high voltage 150n cap. I just happened to score some of these from a local electronics shop, and the difference was very noticeable compared to the new orange drop cap I was using.

– The strange 160J “330” large ceramic disc cap in the diode clipping section. I don’t think anybody knows the exact value of this cap, most schematics I’ve seen say “150pf” or “160pf”. I originally used a new 150 pf ceramic disc cap in my clone. I later replaced it with a NOS cap that simply had the numbers “150” on it, and the difference was dramatic. Since old ceramic disc numbering schemes are inconsistent, it’s possible the value of this cap is actually only 15pf, not 150pf, which would make sense since 151 is the number used for 150pf.

-Volume pot value – This is the most inconsistently identified part. Most schematics say 50K audio taper, some as little as 10K, some 25K. First off, it’s a linear taper pot, not audio. And it’s a 100K pot, just like every Big Muff ever made. If you built a clone according to most of the schematics on the web, and it doesn’t sound right, try swapping out your volume pot for a 100K linear taper.

-741 op amp – The best new manufacture option I’ve found for this is the Texas Instruments UA741CP. Yes, other 741s do sound different in the circuit. The ST LM741 sounded dull and dark. Most builds say any LM741 will work fine, but it won’t nail the sound of the original.

The majority of other components are identified correctly on the various schematics I’ve seen, with a few resistor values differing here and there.