The Thulcandra Boost/Overdrive is an adaptation of the Xotic Soul Driven, a cousin of the classic AC Booster with roots in the Tube Screamer family, originally released in collaboration with Allen Hinds. This is an original trace by Aion FX.
The Soul Driven replaces the tone section of the AC/RC Booster circuit with a gyrator-based bass boost stage that emphasizes the 125Hz frequency. Using internal DIP switches, four bass boost settings are available: flat, +2.4dB, +3.6dB and +6dB.
The “Mid Boost” control is similar to the Zendrive’s Voice control or the Timmy’s Bass knob, varying the resistor in the R-C filter to ground from the op-amp clipping stage. This has the effect of raising or lowering the bass cutoff frequency as well as changing the overall gain ratio.
The Thulcandra project is faithful to the original, with the exception that the DIP switch has been replaced by a potentiometer to allow fine control over the amount of bass. (The DIP switch just selects between four pre-set resistor values, and it’s likely due to space considerations that they made it an internal control rather than an external potentiometer.)