1977 Big Muff pi (op-amp version)

'77 Big Muff pi (op-amp)

This is one of my all-time favourite effects pedals, and I feel fortunate to have one of the originals. A late issue, mind you – a 3rd series with the rare op-amp circuit. There were only some 2000 of these circuits made; Billy Corgan has a couple, so you can hear them all over Smashing Pumpkins recordings. Somewhere between fuzz & distortion, the op-amp Muff has slightly more low end compared to the US reissues, but still has that pronounced mid-scoop inherent to the pedal.

Note the extreme settings – I’d been using it to slam a Bassballs pedal for a nasty “no-fi” sound before taking this pic. One thing of interest about this circuit is that it does away with the Tone bypass switch of earlier models; it has instead a power on/off switch. Very easy to miss when packing up after a gig, so I’ve lost a lot of batteries to it.

1977 wiring

1977 wiring

For the record, this pedal has the strangest on/off switching I’ve seen, using only half a DPDT switch (the other lugs have literally been cut off). In the off position, signal travels from the jack to the circuit input (labelled IN/BYPASS on the photo), where it shorts to the switch along the black lead – at no point is the circuit ever fully removed from the signal. The 3PDT switch in the photo is just there for comparison – don’t get confused 🙂

Once the switch started playing up, things got weird: rather than bypassing the effect, it just made it really quiet (like an angry bee in the distance). Since I’m a bass player, I ended up using a Boss LS-2 Line Selector as a blender / bypass, avoiding the issue altogether (while preserving some low end tone at the same time). Just the other day I found my 3PDT stomp switches that I’d bought about six months ago, so it was time to fix the problem.

True-bypass wiring detail

True-bypass detail

I pre-wired the stomp switch before installing it, following a diagram from the Build Your Own Clone Forums. The picture here is after installation, but you can see clearly the various elements from the diagram (and the current lack of an LED, something I may change later). I used a couple of spare legs from a broken capacitor to make the lug-to-lug connections; note the insulation over the diagonal connection to help prevent short circuits.

After that, I clipped the wires from the original switch, and removed the original (red) input wire entirely, clipping it off at the input of the circuit. This was replaced with a longer one to run directly from the input jack to the bypass switch. The black lead from the original IN/BYPASS becomes the new circuit input & all other existing wires are connected as per the diagram.

Big Muff pi after true-bypass mod

Big Muff pi true-bypass

The only time I had to touch the circuit with a soldering iron was making the connection to ground from the stomp switch. Checked all my connections with a digital multimeter once I was finished, then sealed her up and plugged in. Perfect true bypass ensued … and no effected signal at all. Hmm. Opened up the case again, checked the battery with aforementioned multi-meter: 1.3 volts. That ain’t gonna do it (and damn that switch, while I’m at it)! Popped a new battery in & delighted in the sonic destruction that resulted. All in all, a very easy modification & one I should have done long ago.