The Aurum Amp Overdrive is based BOSS OD-2 Turbo Overdrive, notable as the first appearance of Boss’s “discrete op-amp” circuit topology which uses two JFETs and a PNP transistor to create a variable gain stage that can be controlled like an op-amp but clips gracefully when overdriven. As a bit of trivia, the OD-2 is also one of only three fully discrete circuits ever made by BOSS (no op-amps), alongside the FZ-3 Fuzz and the DS-2 Turbo Distortion.
The OD-2 was first released in 1985 as the successor to the OD-1. In 1994, it was replaced by the OD-2r. The “r” in this case stands for “remote”, since it allowed an external footswitch to change between the two channels on the fly. However, the OD-2r also corrected one one of the most persistent flaws of the original unit, which is that it could barely get to unity gain at the highest volume setting.
The Aurum is more specifically based on the “Turbo” mode of the circuit, adapting the OD-2r version of the circuit with the gain recovery stage and a couple of other minor tweaks. The non-“Turbo” mode is a completely separate drive circuit within the pedal, and generally speaking it’s unremarkable when compared to the Turbo mode.
The OD-2 was officially succeeded by the OD-3 in 1997, although the BD-2 Blues Driver can be seen as a “rebranded” in-between member of the same series since it shares much of the same circuitry as both the OD-2 and OD-3. However, all three circuits sound pretty different, and they each bring different tones to the table.
The original OD-2 uses the 2SK117-GR JFET, a low-cutoff type similar to J201, but no longer made in through-hole format. 2SK209-GR is the SMD version that is still in production and will perform exactly the same as the originals. Each of the JFETs have extra pads for soldering SMD parts, but if you don’t feel confident in your SMD skills, Aion FX sells 2SK209-GR JFETs pre-soldered to adapter boards to be used in through-hole applications.