The Tachyon Amp Overdrive is a recreation of the “Lead” mode of the BOSS SD-2 Dual Overdrive, originally sold from 1993 to 1998.
The SD-2 is interesting in that it’s two fully separate pedals in one box. The “Crunch” and “Lead” modes each use dual-concentric potentiometers allowing for separate settings for Gain, Tone and Volume. The mode can be changed via either a rotary switch selector, the on-board footswitch in A-B mode, or a remote switch.
Unfortunately for BOSS, the Crunch mode was pretty weak and brittle-sounding, and so the pedal was largely ignored. However, the Lead mode really shines. It’s one of BOSS’s best drive effects—while also being one of the most obscure, since it’s buried inside a pedal that was otherwise overlooked.
Despite only having three very basic controls, the SD-2 has a lot going on under the hood. It’s very linear, using five distinct op-amp stages for setting the gain, clipping the signal and shaping the EQ—a similar approach taken by Friedman for the BE-OD and Dirty Shirley pedals, or by the Amptweaker Tight Metal.