The Obsidian Metal Drive is based on the analog section of the BOSS® MZ-2 Digital Metalizer, first released in 1987. Not to be confused with the Metal Zone (MT-2), the MZ-2 was an attempt at an all-in-one metal pedal that includes onboard chorus and delay to replicate popular tones of the 1980s. However, inside this digital pedal is an all-JFET analog drive circuit that bears a strong resemblance to the BD-2 Blues Driver.
To play historian for a moment: the very first appearance of this JFET topology was in the OD-2 Overdrive, released in 1985, and the MZ-2 is only the second time it was utilized (at least tied with the DS-2 Turbo Distortion, which also appeared in 1987). The BD-2 didn’t arrive until 1995, which means the MZ-2 predated it by a full eight years.
Be aware that the analog circuit on its own is not very metal, especially compared to other BOSS circuits such as the MT-2 or HM-2. The maximum gain is not terribly saturated and the tone control is just a passive hi-cut with no ability to shape the midrange. It seems they were relying mostly on the digital effects to sell the concept. But if you remove the pretense of metal, it’s a killer drive circuit on its own and more than worth your time if you like the BD-2, OD-2 or OD-3.
We did make one circuit change in extracting the analog circuit from the MZ-2. The original unit uses an op-amp at the output to mix together the analog and digital signals, which also acts as a buffer. Since the rest of the analog circuit is all discrete, we opted to replace this op-amp stage with the transistor output buffer from the BD-2 to keep the discrete topology intact. Having studied dozens of BOSS circuits, we’re confident that this is how the engineers would have done it if there was no mixing requirement.