The Protolith CMOS Distortion is based on the Mad Professor Stone Grey Distortion, a high-gain drive circuit designed by Bjorn Juhl (BJFe) and first released in 2011.
The Stone Grey Distortion was originally derived from a distortion circuit called the “CMOS Driver” from a Japanese electronics book published in the early 1980s (look for the file named “cmos-driver.gif” in this directory). It was initially called the Cliff Hanger and later the Cliff Hanger II, and sold by BJFe in extremely limited numbers beginning in 2009 before it was licensed to Mad Professor and released in 2011 as a regular production model.
Along the way, the CMOS Driver circuit was tweaked somewhat from the Japanese circuit, notably by changing the op-amp gain method, adding clipping diodes in the op-amp stage, and adding a tone control. However, the final Stone Grey circuit still strongly resembles the CMOS Driver and there is no doubt as to its origins.
Since it’s intended for high-gain operation, the Stone Grey has a significant amount of low cut. This makes it very “tight” when used with certain types of amps and playing styles, but for some it’s too thin. Mad Professor released the Custom Shop “Modernized Mod” in 2020 to help counteract this, adding 3 parallel parts on the solder side of the PCB to increase bass frequencies and gain. See build notes in the documentation for details on building the Modernized version.
The Protolith is based on the final version of the Mad Professor Stone Grey Distortion with no modifications or circuit changes.