The Maxon OD-820 has been touted around the boutique world for many years as a "Klon Killer" supposedly able to do that pristine clean boost with a heap of overdrive in reserve. Not many people have seen the schematic and, while it's not a Klon, it does feature many of the same ideas.
Firstly here's the description from the Maxon website:
"While many consider the OD808 to be the greatest overdrive ever created, Maxon is not a company to rest on its laurels. Instead of basking in the glowing praise, Maxon has focused on the user's feedback to come up with an all-new overdrive circuit that addresses the key shortcomings of other units - the result is the OD-820 Overdrive Pro.
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 the ProGuitarShop demo video:
And finally the schematic:
So, what have we got here. First up is a simple jfet buffer which preludes the gain control. Note that the gain control isn't actually adjusting the gain of the following opamp clipping stage it's channelling the signal between the clean blend path and the clipping opamp. What following is essentially a mixing stage for the clean signal and clipped signal. Next up is a third opamp stage - this is essentially a standard tubescreamer type tone control, nothing too special. The final opamp stage is just buffering up the volume control and keeping itself nicely out of the way ;-)
Also of note is the charge pump IC. Essentially what this is doing is providing the opamps with + and - 9VDC to ensure they run with the headroom necessary not to impart any of their own "hard rail" clipping (which usually sounds pretty bad!) on the signal.
In both electronic and aural terms the Maxon OD 820 is more transparent than the Klon Centaur - it doesn't impart the mid range texture that is a signature of the Klon. It does, however, sound just as good in it's own unique manner.
Here's a PCB layout and parts list (thanks to Dirk Hendrik and Bside2234);
|Maxon Od 820 PCB Layout|
|Maxon OD 820 Parts List|
|Maxon OD 820 PCB Transfer|
Note the parts list doesn't include part numbers for the diodes - sticking with 1N914/1N4148 would make sense as most of Maxon's overdrives go with this standard. However, other diodes are easily usable... Enjoy :-D