The THD is an '80s metal fuzz distortion, a rework of the original Japanese HM-2. This was my first guitar pedal, which later formed its own cult following.
By using a dual-gang potentiometer, the DISTORTION knob has been redefined from the sloppy classic one. This is actually a dual-function control for two cascading gain stages: The first half of the range of the DISTORTION knob controls a full-bodied fuzz. The second half adds a further layer of compression and sustain.
The signal is processed through germanium diodes in the final compression stage. This is a crucial part of the tone, which would be omitted from commercial reissues due to RoHS.
The LO+ and HI+ are frequency boost controls. For a flat EQ response, try starting with these set to zero, rather than high noon. If you want to 'max out' all of the controls for the Swedish chainsaw tone, then you will need to reduce the VOL control to-taste, as it has an increased output level from the classic.
Japanese polyester audio capacitors are used, as well as low noise resistors, and low ESR ceramic power filter capacitors. These do not suffer the limited shelf life of electrolytic capacitors, and are distributed throughout the board in key locations, rather than one large electro filter cap serving the whole circuit. Low noise JFET input Texas Audio amplifier chips have been used.
This pedal stacks best when placed after other distortion and overdrive pedals.
N.B. this does not include the two toggle mod switches shown in demo videos of the first round build.