The Sapphire Amp Overdrive is based on the BOSS® BD-2 Blues Driver, one of their most popular pedals in continuous production since it was first released in 1995. It’s been used by a wide variety of artists including Mike McCready (Pearl Jam), Robert Smith (The Cure), Tom Morello, and even Prince.
The Blues Driver bears many similarities to the OD-2 Turbo Overdrive from 1988. That was the first appearance of Boss’s “discrete op-amp” circuit topology that 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. The Blues Driver starts with a circuit very similar to the OD-2, and then adds a slight bass boost to the end to give it the signature fullness, along with some clipping diodes and a fixed-position Fender tonestack for shaping the tone.
Despite its overwhelming popularity, the BD-2 is not without its flaws. On higher gain settings, the note decay can be “fizzy” or “splatty”. And the bass boost doesn’t always work well in every situation. Because of these shortcomings, it’s one of the most frequently modded pedals out there, with Keeley’s and Analogman’s mods being the most popular.
Another notable one is the Galaxie Mod from Machine Head Pedals, which is covered in the build documentation and is strongly recommended. It’s an extensive mod that replaces half of the passive components, completely transforming the pedal into something new.
The Sapphire is based on the stock BD-2 circuit converted to true bypass. It adds a bass knob to make the low boost variable, so it can be adjusted between the stock value and the Galaxie value.
The original BD-2 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 pre-soldered to adapter boards to be used in through-hole applications.