This Week on the Breadboard: The MXR Distortion II

Chuck D. Bones

Chuck D. Bones

Circuit Wizard
A strange and interesting Dirt Box. Here's the stock schematic. The most interesting part is the parallel distortion path thru IC2.1. This is a bandpass filter centered near 55Hz with 36dB gain and a Q around 4. D3 and D4 provide limiting, but they also complicate the filtering. Their leakage can affect the filter tuning and when they start conducting, the filter freq moves around. The common lore is that MXR is replicating the resonance in a Marshall speaker cab. The effect that we get is an audible thump when RESONANCE is turned up and we pluck the bottom strings. This is all pretty cool, but what if we made the filter switchable? I replaced C6 & C8 with 6 caps and an on-off-on DPDT toggle switch. Now the filter can be tuned to 55Hz (Classic), 120Hz (Heavy) or 750Hz (Mid Boost). Classic gives us the speaker cab resonance, Heavy gives us a prominent Bass boost, and Mid Boost makes for a stuck wah midrange honk à la Michael Schenker.

These are my mods:
1. Omit the charge pump. I can run it on 18V if I like, but it sounds great running on 9V.
2. Increase R101 to 12K to make the opamp clipping in the 2nd stage more symmetric.
3. Jumper C10. No need to block DC here. The voltage is the same on both sides of C10.
4. Change C5 to 1uF film. 1uF is more than enough capacitance here.
5. Change C3 and C7 to 47pF, which are more common.
6. Change the DRIVE pot to C50K for a smoother feel.
7. Change the FILTER pot to C250K and change C13 to 1.5nF. The FILTER knob range is now more usable.
8. Change C11 to 4.7nF and R14 to 10K. This pedal was way too dark for my tastes. The FILTER knob can still make it plenty dark. If you want to go darker still, increase the FILTER pot to C500K.
9. And the Pièce de Résistance: replace C6 & C8 with a DPDT on-off-on switch and three pairs of capacitors: 2.2nF, 15nF & 33nF. I also changed the RESONANCE pot to B10K so that the filter freq changes less when the RESONANCE knob is turned.

These mods are all optional and can be used in any combination.

MXR Distortion II sch.png

I re-arranged the IC pairing on the breadboard for better flow. The first two stages are at the upper right. The clipping and filter stages are top center (the TL072 is hiding under a large 2.2nF SM cap). Last two stages are bottom center.

Knobs: VOLUME - FILTER - RESONANCE - DRIVE - Res Filter Sw.
MXR Distortion II cb mod v0.1 breadboard 02.jpg
 
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Gordo said:
Thanks for this Chuck. Did you have any issues getting above unity, especially at lower gain settings?
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No, not at all. Here's the output with DRIVE, RESONANCE & FILTER at zero, VOLUME at 10. 100mVp-p input is more than enough for some mild clipping and the output is 680mVp-p.

MXR Distortion II cb mod v0.1.png

Assuming that your Distortion II is wired properly, they only way you'd have trouble reaching unity is if you have active pickups or a booster in front of the Distortion II.
 
Chuck D. Bones said:
Me too. I ran sims before I built the breadboard and saw the 55Hz peak. So I listened to a demo on YouTube and it became clear that 55Hz is what MXR intended.
Click to expand...
I owned one of these and tried out the lore, stacked it with an op amp muff, into a cab with a G12-M Celestion speaker. It WAS pretty cool. For that one sound. I’m going to try out the circuit with your mods - looks pretty cool and way more usable than stock.
 
Gordo said:
Sorry Chuck, I’m probably looking right at it. How are you changing 3 pair of caps with an on/off/on?
Click to expand...

I knew someone was going to ask...

1791423549918.png
C6 & C8 are always connected. We either connect C6a & C8a in parallel, or C6b & C8b in parallel, or nothing in parallel. The resulting capacitance is:
35.2nF, 17.2nF or 2.2nF, in that order.
 
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Maybe This:
To fit the switch under the PedalPCB Board you will have to solder the pot legs tip flush with PCB & you may have to bend the pot legs to give you some extra height, I have done it before. Here is maximum clearance for mounting Capacitors.
Sequel II Distortion Mockup Pedal.jpg 3 Capacitors DPDT Toggle Switch.jpg
 
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Ordered the board and I've become a big fan of the output jack faceplate so tossed a mess of those in on the order. I'll redo this one per these specs.
 
You may want to try alternate values for C6 & C8, depending on where you want the midrange peak. 750Hz is a good start.

If you decide to omit the charge pump, delete IC100, C100 & C101. Jumper D101 & replace D102 with a 10Ω resistor.
 
Thinking I might incorporate switchable 18/9v and will outboard the caps for c6/c8. I was amazed at how good the original Dist+ sounded on a rig to regroup with my old 70's mates this summer and the Dist 2 always eluded me as the next generation that never was. It seemed like a good idea but all the versions I built were never loud enough (output volume). I never built the PPCB version. Lectric FX seemed like the fix but I didn't gel with it.
 
Be careful mounting C6 & C8 off-board. You gotta keep any wires that connect to IC2 pin 2 short and sweet, otherwise oscillation or noise pickup could be a problem. The wires on my breadboard are around 10cm and no problems. I personally wouldn't bother with switching the power, you'll probably like one or the other and never switch it.

I'm not familiar with the LectricFX version.
 
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