Info about the original from Maxon:
The OD-820 was designed to provide a wide range of tube-amp overdrive tones as well as a transparent, hi-gain clean boost. As the name implies, the OD-820 was intended for use by professional guitarists with advanced playing techniques. The OD-820's expressive tone reproduces fingering and picking nuances with haunting precision, making the best use of any guitar or amplifier's character. With a full-frequency response, minimal compression and zero tonal coloration, the OD-820 won't mask a player's performance weaknesses like other overdrives can.
One of the key ingredients to the OD-820's amazing sound lies in its power section. While the OD-820 accepts 9 volts coming in, it then uses a DC-DC voltage converter (#MAX1044) to bump this up to 18 volts. This higher voltage allows for a more accurate, full-frequency reproduction of the input signal than other units can provide. This higher voltage also allows for more balanced powering of the circuit, providing stabilized positive and negative DC voltage to the overdrive, blend, and tone sections of the circuit.
Overdrive and Blend? That's right the other secret weapon of the OD-820 lies in it's blending of distorted and non-distorted signal to create its massive tone. The OD-820's Drive knob does double duty, controlling both the amount of gain and the balance between distorted and non-distorted signal. Note that we said non-distorted signal rather than dry signal this is because this signal is still processed through the tone section of the OD-820's circuit before reaching the output. So, at the lowest Drive settings only non-distorted signal is sent to the output, providing the OD-820's stunning clean boost tone.
Like the OD808, the OD-820 distorts signal in the amplifier section of the circuit rather than having a separate clipping stage, which provides a smoother, more realistic tube-like overdrive than other methods. The higher voltage supplied to the amplifier section provides slightly more clipping than the OD808 or OD-9. The OD-820 uses only JRC4558 op amps for the warmest overdrive tone possible.
And a version using a modified gain pot arrangement (described in the comments below), using two independent pots for the gain control, now labelled Gain and Clean and put together by John. Increasing the Gain pot to 500K will give you quite a bit more dirt out of this version.