http://www.americaspedal.net/fx55c/
***From the manual: "The FX55C is a very high-gain preamplifier that when overdriven produces a distorting of the input signal and help sustain the notes longer. These effects simulate the characteristic distortion of a guitar amplifier turned up so high that it "clips" the signal, distorting it. The FX55 has an extra amount of gain boost in its upper range for a greater distortion effect. ... The FX55C's distortion is a symmetrical standard distortion sound with a boost of gain in its upper range."***
Factory schematic and components 2000: