The Solaris Germanium Fuzz is an adaptation of the classic Dallas-Arbiter Fuzz Face circuit from 1966.
The Solaris is a “deluxe” version of the Fuzz Face that adds 3 new knobs and a switch for extra flexibility. Added controls include an Input attenuation control (similar to using the volume control on a guitar to control the gain), a Body control to filter the amount of bass that is fed into the circuit, and a Contour control to tweak the midrange emphasis. (If you just want a no-frills Fuzz Face without the extra mods, the Proteus project is a stripped-down version of the Solaris.) There is also a charge pump allowing for -9V operation from a normal +9V supply.
New in the 125B version of the Solaris is a switchable pickup simulator at the input. The Fuzz Face was originally designed to connect directly to an electric guitar, and as a result it is notoriously picky about where it’s placed in the signal chain. If it’s fed a low-impedance signal (e.g. if there’s another pedal before it) then it loses much of its character.
The pickup simulator solves this problem by adding a transformer, resistor and capacitor to convert the source signal into the higher impedance that the circuit likes. It was invented by Jack Orman of AMZ and has been used in commercial versions of the Fuzz Face such as the Earthquaker Devices Erupter.
It won’t provide the same interactivity with the guitar’s tone & volume as if the pedal was connected directly to the guitar, but it will allow the circuit itself to retain the tonal character. You should only use it if you want to place the Solaris in a position other than first in your signal chain.