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.
The Big Muff is something of a “rite of passage” for pedalbuilders. It’s a pretty easy build and there aren’t a lot of things that can be messed up. There’s no biasing or gain sorting, and no rare or specialized parts are needed. It’s endlessly tweakable, with dozens of different variants (some of which sound very different) that are just a matter of substituting a few parts.
The Big Muff circuit was adapted by a number of other pedal manufacturers in the 1970s. Notably, the 4th version of the Colorsound Tone Bender, called the Supa Tone Bender and released in 1973, was based on the Big Muff circuit. The next version of the Tone Bender, called the Jumbo Tone Bender and released in 1976, changed the standard Big Muff circuit by removing the 4th transistor, a make-up gain stage after the tone control. This 3-transistor variant was copied by other manufacturers as well.
The Halo PCB allows you to easily build either the 4-transistor or the 3-transistor variant.