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Tag Archives: MOSFET

MXR Distortion III mods

DSCN0483

When the MXR Distortion III was first released I thought, great- MXR has finally decided to reissue/update the overlooked original Distortion II !  Alas the only thing close to the Distortion II was the name. The  pedal is a newer design that has more in common with other Tube Screamer type pedals that it sounds like. The design is a two stage op amp circuit with diode clipping to ground. The highs and lows are rolled off to give that signature 750Hz midrange boost found in the original TS design. It’s not a bad sounding pedal but its not distinctive. MXR later released the Classic Distortion pedal in conjunction with Guitar Center in the US. It was a Guitar Center exclusive and sold for $29.99 at their seasonal sales events. Eventually pedal enthusiasts on the internet noticed the two pedals sounded very similar. Somebody decided to investigate and discovered the circuits used are identical. The Classic Distortion has a lighter, thinner case and a different color but make no mistake these pedals use the exact same printed circuit board and component values.

Here is the schematic for the Distortion III. A popular mod to open up the pedals sound and raise the 750HZ low pass filter is to remove C4 (.022uf) from the circuit entirely or replace it with a smaller value like a .005uf capacitor. I choose a slightly different way to achieve similar results.

  • Unsolder and lift one end of C4 and connect a 2.4K resistor in series with C4 to the open solder pad where C4 had been connected. If you don’t have a 2K4 resistor try a 2.2K or a 2.7K.
  •  Solder a .0022uf film cap across R5  on the solder side of the PCB.
  • Remove the clipping diodes D1 and D2. Replace these with two 2N7000 MOSFETs with the gate tied to the drain. Place a 1N34 diode in series with one MOSFET to raise the clipping threshold on one side for a little asymmetry.
  • To be clear the MOSFETs are connected so the intrinsic body diode is clipping the signal and not the MOSFET junction itself.
  • Replace the stock LED with a nice <insert your favorite color>one.

The pedal now has a nice upper mid-range push that can be tamed with the tone control if desired. The MOSFET /germanium combination gives an open more articulated sound to chords and single notes. I think the softer clipping of the MOSFETs is complemented by the extended high end. The overall result is very good and there is an apparent increase in the overall distortion level when compared to the stock pedal.