The OD-1 is notable for being the first overdrive effect to utilize diodes in an op-amp’s negative feedback loop for clipping purposes. Like the Distortion+, it lacks a tone control, but most who play through it say it doesn’t need one.
The OD-1 had two different versions. The original 1977 version used the Raytheon RC4303 quad op-amp (called the “quad version”). The second version from December 1980 used a single 4558 dual op-amp (the “dual version”) and changed the input & output buffers to transistor emitter followers as a cost-savings measure, as in most of their later pedals. The original quad version is said to sound better and is what the Parhelion project is based on.
While the RC3403 is long out of production, they can still be found without too much trouble. The NJM3403 is an exact substitute and still in production. They share a standard pinout with other quad op-amps such as the LM324 and LM348, so there are many other options that will perform similarly.
The Parhelion has two added toggle switches allowing for a little more flexibility. A clipping switch selects between three different sets of diodes, and an EQ switch changes the gain structure of the op-amp clipping stage for higher gain and more bass.
Note: The earlier 1590B version of the Parhelion was called the Corona. The updated version was given a new name upon release in 2021 for obvious reasons.