Maxon Pedals both the OD808 and OD-9 are based on the legendary TS808 Tubescreamer overdrive circuit but differ slightly from it and from each other. Besides different chassis (the OD808 comes in a standard Hammond-style enclosure and weighs 1 Lb. while the OD-9 comes in the custom 9-Series enclosure and weighs 2 Lbs.), the main difference between the OD808 and OD-9 lies in their switching - the OD808 uses FET (electronic) silent switching while the OD-9 uses mechanical True Bypass Switching w/a 4PDT switch.
In addition, the value and position of two output resistors in the OD808 circuit were changed from the original TS808/OD808 in order to reduce the noise created by static electricity build-up. The position of these resistors was reversed in the current production OD-9, although their values were kept the same. Sonically, the OD-9 will have a little more gain than the OD808 yet each model offers the smooth, creamy, dynamic overdrive of the original TS808 and sounds extremely similar to one another.
The OD820 features a similar overdrive circuit to the OD808 and OD-9. However, it also features a clean blend circuit that is controlled by the unit's Drive knob (at zero, the Drive knob will pass clean signal only, and the overdriven signal is added as the knob position is increased). This feature allows the OD820 to be used as a clean booster as well as an overdrive.
The OD820 runs at 18 volts which provides more headroom - the result is a fuller low-end response, less compression, and more dynamic responsiveness than either the OD808 or OD-9. In addition, the OD820 features True Bypass Switching using a DPDT switch and a larger chassis than either the OD808 or OD-9.