The Halo Distortion/Sustainer is a recreation of the Electro-Harmonix Big Muff Pi, first released in 1969. Originally marketed as a “harmonic distortion sustain”, it has been lumped together with fuzz effects, although it’s a very different animal than classic fuzz effects like a Fuzz Face or a Tone Bender.
While the overall circuit has barely changed in 50+ years, Electro-Harmonix was legendary for changing the part values on a whim throughout that time while never advertising the new versions as being different than previous ones. There are around 20 notable variants and many more minor variations within those, with nearly every part in the circuit being tweaked at some point or another.
The Big Muff is a pretty easy build and there aren’t a lot of things that can be messed up. There’s no biasing or gain sorting for the transistors, and no rare or specialized parts. It’s endlessly tweakable, with dozens of different variants that can be replicated by substituting a few parts. For this reason, it’s become a very popular DIY build—something of a rite of passage for those getting into the hobby. It’s also a great way to experiment and learn how simple audio circuits work, because almost any part substitution will have some sort of audible effect.
The Halo is a faithful recreation of the Big Muff, allowing any standard variant to be built. A versions spreadsheet compiles all the different variants so you can easily see which parts to substitute if you want to build a Triangle, Ram’s Head, Civil War, or more than a dozen other flavors.
Also included is a midrange switch that allows for the stock mid-scoop tone, a flat midrange response, or boosted midrange. The mid-scoop tone is 100% identical to the Big Muff circuit, but it allows two extra modes if you find that it’s getting lost in the mix.