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. This circuit has been built in two forms. The standard four knob version is a hybrid assembly of surface-mount and through-hole parts. These parts have improved noise and longevity characteristics. A through-hole version is also available with two toggle switches: FOCUS and SI/GE. The FOCUS switch changes the bass boominess. The SI/GE switch hot-swaps the main distorting transistor for a gain-selected germanium device. This has a rounder and smoother distortion, where the silicon mode is fuller with squared clipping. Please note that some early version demos show two toggle mod switches. These are not included as standard. The four knob version uses the standard silicon mode with full ++ bass. This is built in a small-footprint Hammond 1590BS enclosure.
