MXR Distortion + pedal has been around since 70’s and featured in many 70’s and 80’s rock and metal recordings (and many more modern ones). A true icon, used by likes of Dave Murray, Randy Rhoads, Slash, Tom Morello and many many others.
Iconic, sounding great, yet simple, great for DIY effort 😆. Let’s build one and see for ourselves.
Here I cover the basic schematic for the circuit, cover some interesting details, there’s even SPICE simulation if you want to do some analysis yourself. I’ll move on to put the effect on a breadboard and try it out.
There’s also a list of things to try-out. This is a very simple pedal, but also very easy to modify. I always do breadboarding for any effect I’m building. This way I verify the basic design – schematics are sometimes wrong. And, I also have a chance to try out different mods, see if I like something better.
I got the basic schematic from Electrosmash, here’s my version:
The schematic did not have details of the pot’s tapers. Output pot is easy – so far it has always been audio taper to control level or volume or, in this case output (whatever the label). Since I built MXR Micro Amp (or at least breadboarded it) before, I knew that Distortion pot should be reverse-log. After all, Micro Amp is based on Distortion + design, just undistorted.
After some more investigation, turns out that there’s a slight uncertainty on pot values on the original schematic. So I modified them in my version a bit, it made more sense, distortion pot value now matches gain pot value of Micro Amp. But the pedal would work either way.
For distortion pot, the only real difference is in the level of control you’d have over distortion (which you won’t be able to see from diagrams, you’d have to try it out). For output pot – the larger pot value makes the output louder (not more distorted, just the volume is higher).
For in-depth analysis go to Electrosmash, I can’t do anything better 😊. I’ll just point out some interesting things about the circuit.
There are two main building blocks – amplifier and clipping circuit. That’s it:
The amplifier block is pretty standard non-inverting op-amp amplifier. Something very similar we’ve seen for MXR Micro Amp. An extra capacitor on Micro Amp for improved stability, but other than that the topology is the same.
There are actually two main differences. One is very simple – Distortion + has maximum gain that is way greater than Micro Amp. The goal of the higher gain is to make the signal distort sooner.
The bigger difference maker for the overall sound is value of C3. Micro Amp has quite a bigger capacitor there which makes it transparent boost, it does not affect frequency response. C3 is quite smaller in comparison, and because of this, the lower frequencies are not as amplified. So, the frequency response has a distinctive hump around 1.5KHz:
The cut-off frequency is about 720Hz but the maximum gain is around 1.5KHz. Since the filter is formed by pot RV1, resistor R3 and capacitor C3, as the gain drops (RV1 resistance increases) the cut-off frequency drops as well.
Clipping circuit is very simple, and a lot of other pedals use the same topology. Despite the amplifier having a very high gain (over 200 or 46dB – compared to it Micro Amp has about 10 times smaller gain), diodes clip way sooner than this maximum gain is reached.
Diodes limit the output to their forward voltage. The original pedal used 1N34A germanium diodes which would clip sooner than silicon ones. Having said that, maximum forward voltage is 1V for both 1N4148 and 1N34A, the actual voltage depends on the current too, so the things are not that clear-cut. Anyway, germanium diodes should typically clip around 0.3V, small signal silicon diodes around 0.7V give or take.
R5 is the current limiting resistor, manages what is the maximum current going through the diodes. D1 and D2 clip the signal – for silicon diodes it is customary to use 0.7V for typical forward voltage so that’s the maximum output I’ll be getting (despite 200+ amplification).
C5 with R5 acts as a low pass filter, so it would attenuate frequencies over 16KHz (which probably does not make much of a difference to my old ears).
I thought it would be good to make a quick note about the op-amp. Warning: all calculations are very rough.
LM741 used in the effect has a relatively low slew rate of 0.5V/uS and relatively small GBW (gain-bandwidth product). This affects the frequency response. Actually, if I look at the SPICE simulation of the same circuit with LM741 and TL071/TL072 (which has 20V/uS slew rate or 40 times greater and GBW 5 times greater), the difference is apparent:
The difference comes mostly down to the GBW actually. The slew rate would limit the maximum output and would cause signal distortion, however, open-loop gain drops quickly with frequency and is lower than the maximum gain of the circuit from 4.5KHz already (as seen on the diagram). Slew rate will affect the signal from 12KHz (roughly).
TL071/TL072 is affected by C5/R5 low pass filter actually because it affects the signal from roughly 16KHz, GBW limitation would kick in after 25KHz with this circuit.
Another thing to consider is maximum output of the op-amp with 9V supply is going to be 6-7Vpp (or 3-3.5Vpeak) – both LM741 and TL072 are comparable in this regard. So clipping will most definitely happen in op-amp since 200+ gain would make signal as low as 15mV to hit the output limit.
Btw, if you wanted to use a different op-amp, you could just swap LT741 with TL071 (or TL061), they have the same pinout. With above described differences the effect will still work just fine. If you want to use TL072, the pinout is not the same since LT741 is a single op-amp chip, and TL072 is a dual op-amp chip, so some care should be taken.
Here’s a handy SPICE circuit diagram you can play with:
You can swap LM741/NS with TL072 (just right click on the op-amp and enter TL072 for Value) and see what effect this has.
Let me prepare a breadboard diagram for this. As usual, I used DIY Layout Creator. Here’s a possible diagram:
It looks relatively straight forward. The above is with DPDT wired up for bypass. As I mentioned earlier, if you don’t have LM741 you can use TL071 or TL061 instead, the circuit will work just fine since they have the same pinout.
If you wish to use TL072 (which I prefer using since I have a drawer full of them), then the pinout is slightly different:
So pins 2, 3 and 4 are the same. The only difference is – the positive supply pin (pin 7 vs pin 8) and op-amp output (pin 6 vs pin 1). Here’s breadboard diagram with TL072:
I went for minimal number of changes. Looks pretty similar, see video below for rewiring it on the fly.
Here are above diagrams in DIY format:
Things to Try Out
These simple circuits are great for trying things out. Obvious thing to try is just replacing the op-amp. Other things to try is possibly increasing C3 value – that will add more bass.
Instead of 500K reverse log pot, try linear 500K pot or even 100K pot if you won’t use the pedal on low gain as a booster.
Increasing C5 might tame some higher order harmonics if you find the distortion too harsh.
But the most obvious thing to try is different clipping options. Use germanium diodes if you have them. Or wire up diodes for asymmetric clipping by using two + one diode, or use a LED or LEDs. All of the different configurations might give you different result. This is what I mean:
Different diodes produce different clipping threshold if you will. Lower the threshold, harder the clipping. Using Schottky diode should give hardest clipping, using blue LED would give softest clipping since the forward voltage is close to the maximum we expect on the op-amp output (between 3 and 3.5V). Just keep a diary of what you’re trying out and what you like or you’ll get lost in all the different options 😁.
DOD 250 Overdrive PreAmp
I thought it would be interesting to try out DOD 250 as well. The pedals are nearly identical. Look at the schematic:
The circuits looks nearly the same. There are, however a few differences. Let me show them here:
Highlighted in red is the only significant difference. The cap at that position causes some shift in the frequency response. Components highlighted in blue should cause no audible difference. Here’s the side by side response between DOD250 and MXR Dist +:
While the frequency response appears quite different, the “hump” on DOD 250 peaks out around 800Hz (very roughly), this is still before clipping. So MXR will clip harder for frequencies over 1.3KHz (since that’s cut-off frequency – which just means audible difference I suppose), which is at the very end of 6 string standard tuning guitar fundamental harmonics range. That’s just very roundabout way of saying … there might be very little difference.
Cough … this is of course completely ignoring the fact that DOD 250 uses silicon whereas MXR Distortion + used germanium diodes … so there’d be quite a difference in output level and how hard the clipping is (at highest gain levels at least). But since I’m not going to use germanium diodes, well, there might not be as much difference after all.
Let’s do the actual breadboarding and hear what the effect sounds like.
In the end, the three versions of the pedal sound all very much alike. I’d say it would give better bang for the buck using a switch with some alternative clipping options. Different clipping options give different sounds straight away.
So for the next post – I’ll definitely go and do TL072 version with slight modifications and I’ll add an extra clipping option I think.
EDIT: 5th Jan 2021 – fixed schematics. I had diodes “pointing” the same way – that won’t work correctly, they should be “pointing in opposite direction”. Clipping schematic was fine, breadboarding diagram fine, it was just the main schematics that I messed up – fixed.