The Heliodor Amp Overdrive is based on the BOSS® OD-3 Overdrive, released in 1997 as the successor to the OD-2r Turbo Overdrive.
The OD-3 continues in the tradition of the OD-2/r and BD-2 Blues Driver with BOSS’s signature “discrete op-amp” topology, using 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. Unlike the two earlier units, though, the OD-2 uses clipping diodes in the this stage to achieve a more traditional soft clipping.
Due to the circuit similarities with the BD-2, it’s been theorized that the OD-3 was actually designed as the successor to the Blues Driver, but that BOSS didn’t want to mess with such a commercially successful pedal and so the OD-3 was released under the Overdrive umbrella.
Regardless of what went on in the product development meetings, it does borrow ideas from both the OD-2 and BD-2 and manages to improve on both of them from a design standpoint. The Blues Driver may get more attention, but the OD-3 remains the crowning achievement of BOSS’s discrete op-amp topology.
The Heliodor is a direct adaptation of the OD-3 with no changes other than converting to true bypass.
The original OD-3 uses the 2SK184-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.