The Aphelion Vintage Distortion is a clone of the MXR Distortion+, one of the first op-amp-based drive pedals going all the way back to 1973.
This was also one of the first circuits to use hard diode-to-ground clipping, and while it’s a fairly simple circuit, it served as an ancestor to many other classic pedals like the Boss DS-1 and Maxon/Ibanez SD-9. It has no tone control, but instead, the gain control also changes the tone as you turn it up. Technically speaking, only the bass frequency is cut, but this has the perceived effect of increasing treble as well.
DOD released their version in a few years later in 1976, which they called the Overdrive Preamp 250 (commonly abbreviated to just the “250”). Both versions are classics in their own right, with the Distortion+ being favored by Randy Rhoads, Jerry Garcia, and Thom Yorke, while the 250 is inseparably associated with Yngwie Malmsteen.
Like the Big Muff, both the Distortion+ and 250 had a lot of circuit tweaks throughout the manufacturing run. Because of this, there’s no definitive version of either of them, although some variants were more notable than others.
The Aphelion adds two extra modifications to the classic two-knob circuit: a treble-cut switch that changes out a capacitor at the input, and a diode selector that lets you go between germanium (Distortion+), silicon (250), or LEDs.
The updated 125B version of the Aphelion has been changed slightly from the earlier version. The treble-cut toggle has been revised so that it switches out the low-pass capacitor at the input rather than the one at the output. Also, the “Compression” (input coupling capacitor) toggle has been removed since it had very little impact unless the pedal was first in the chain, and even then it wasn’t very useful.