One of my favorite sites to send folks to who had questions about how pedals circuits work is a site called electrosmash. Unfortunately, their website has been gone for a bit and is always half working or has one reason or another that browser security will not allow folks to access it. Fortunately, they do have a policy that allows those with blogs to repost their content and add to it if desired, so this is a good place to do just that. With this in mind, let's discuss the MXR Distortion + (or Distortion plus) circuitry.
The M-104 MXR Distortion +, also known as the Distortion Plus or D+ is a guitar pedal that MXR designed and released between 1978 and 1979. When it first came out, the original pedal didn't have an external power jack or an indicator LED. Jim Dunlop acquired the MXR licensing rights and now make reissues of this classic MXR distortion pedal and now include both a power jack and an LED.
The MXR Distortion + uses germanium diodes, giving it a distinctive, moderately fuzzy distortion sound similar to the recordings of famous 70s rock and 80s metal bands. The design of this circuit is very reliable and was eventually used as a foundation for the M-133 MicroAmp, which is basically just a re-engineered version of the M-104 MXR Distortion+ without the clipping elements.
The MXR Distortion + uses germanium diodes, giving it a distinctive, moderately fuzzy distortion sound similar to the recordings of famous 70s rock and 80s metal bands. The design of this circuit is very reliable and was eventually used as a foundation for the M-133 MicroAmp, which is basically just a re-engineered version of the M-104 MXR Distortion+ without the clipping elements. Interestingly, the DOD OD250 is almost an exact clone of this type of circuit.
Table of Contents.
1. MXR Distortion Plus Schematic.
1.1 Power Supply Stage.
1.2 Op-Amp Amplifier Stage
1.3 Clipping Stage.
2. MXR Distortion + Frequency Response.
3. MXR Distortion + Sound Signature
4. Resources.
1. MXR Distortion + Schematic.
The MXR Distortion + circuit can be divided into three blocks: Op-Amp Stage, Clipping Stage and Power Supply:
This is a super-simple design which uses just one op-amp (operational amplifier) and a pair of germanium clipping diodes to create a distortion effect which boosts the guitar signal up to 46db, has a slight mid-hump frequency when the gain is turned up, and only has two potentiometers - Output (volume level) and distortion (gain in op-opamp). There is no tone control on it.
MXR Distortion + Circuit Layout
The original circuit is built on a single layer creme colored PCB, this PCB material was extensively used by MXR. All the components are placed on one side of the PCB that contains text which reads "hand built by guitar players". The jacks and potentiometers are hand-soldered to the board, which makes it quite labor intense during production. When Dunlop took over MXR, the PCB was redesigned using a more modern approach to the board: the power connector, LED, jacks and potentiometers are directly mounted on the PCB.
The MXR Distortion + Operational Amplifier: The 741
The 741 is a bipolar op-amp that's commonly used by many manufacturers. It was designed in 1968 by David Fullagar at Fairchild Semiconductor after Bob Widlar's LM301 integrated circuit design.
When the LM741 arrived it was quite popular, as it was the first really practical op-amp. Suddenly one was able to build a complex circuit with a good chance of it being stable, doing what it should do, and not malfunctioning over something small and insignificant. The LM741 worked reliably; the snag with using it for audio was the leisurely slew rate of 0.5 V/µs, which made full output at 20 kHz impossible.
The noise in this part is pretty high (there are no noise specifications in the datasheet), the slew-rate is slow (0.5uV/s) which cannot cope with the audio band (up to 20KHz) but this is not necessarily a bad thing. Other "mythic" op-amps like the LM308 used in the Pro-Co Rat is even slower (0.3uV/s) and seems to contribute in a positive way to the distorted sound. By comparison, other modern bipolar op-amps like the 4558 have a slew rate of 1.7uV/s (3 times faster than the 741).
In the Distortion + different flavors of the 741 IC were used during its life, the LM741CN operational amplifier that was later changed to a UA741CP and many others (JRC741, LM741, etc). Some players say they can hear or feel a difference between the old original op-amps and the new models. As usual, the vintage impossible-to-find are the best rated because of these beliefs that the vintage IC chip seem to give a thicker/smoother distortion. No double blind studies or tests exist in this area regarding guitar pedals and tone that I'm aware of.
1.1 The Power Supply Stage.
The Power Supply Stage provides bias voltage and energy to the circuit:
The curious thing in this stage is the high-value resistors used for the resistor divider. 1MΩ is rather a high value (similar pedals use 100KΩ or 47KΩ for the same operation):
- This factor will reduce the power consumption of the pedal.
- The virtual ground created (+4.5V) will have a 1M//1M = 500KΩ impedance to ground. This will help to increase the input impedance of the circuit.
- The resistors junction (+4.5V) is decoupled to ground with an electrolytic capacitor C6 (1uF) which removes all ripple from the supply voltage.
1.2 Op-Amp Amplifier Stage.
The core of the circuit is this non-inverting op-amp which provides high input impedance, voltage gain, and signal filtering:
The op-amp is configured in a classic non-inverting topology, the resistors R3, R4, and the pot RV set the voltage gain as it will be seen in the Voltage Gain Section. Several capacitors C1, C2, C3, and C4 will filter the guitar signal as it is explained in the Frequency Response Section.
- The 1nF input cap to ground (C1) avoids RF noise going into the circuit, it also helps with ESD discharges.
- The (+) input of the op-amp (pin 3) is biased to 4.5V through the R2 resistor (1MΩ), keeping the virtual ground at 4.5V and being able to amplify bipolar guitar input signals. Usually, the value of this R2 resistor is 10x bigger than the resistor that creates the virtual ground (R6 and R7). In this case, the rule is not applied but the low input current of the op-amp makes that the voltage on pin 3 is pretty close to 4.5V. When using an op-amp with higher bias current/lower input impedance, the voltage on pin 3 could be lower than expected.
MXR Distortion + Input Impedance.
The input impedance is defined by the formula:
- Zin = R1 + ( R2 // ZinOp-Amp ) + ZVirtual Ground
- Zin = 10KΩ + ( 1MΩ // 2MΩ ) + 500KΩ = 1176KΩ
11MΩ is a good input impedance, which will generally prevent loading the guitar pickups and preventing "tone sucking".
- As a rule of thumb, the input impedance of a pedal should be 1MΩ minimum.
- The Virtual Ground impedance (ZVirtual Ground) is defined by R6 and R7. ZVirtual Ground = 1MΩ//1MΩ. The fact that a very high value resistors are used to create the virtual ground (+4.5V) will help to increase the general input impedance of the pedal.
MXR Distortion + Voltage Gain.
The voltage gain is defined by the non-inverting operational amplifier and calculated as follows:
- Gv = 1 + (R4 / (RV2 + RV))
- Gv(min) = 1 + (1MΩ/ (4.7KΩ + 1MΩ)) = 1.5 (3.5dB)
- Gv(max) = 1 + (1MΩ / (4.7KΩ + 0Ω)) = 213 (46.5 dB)
46.5 dB is a big amount of gain, other distortion guitar pedals have similar numbers: Klon Centaur (40dB), Boss DS-1 (35dB) or Tube Screamer (41dB), but still the Distortion + is the higher one.
This big amount of Gain may lead us to think that the op-amp power supply rails may clip the guitar signal. But the 2 diodes (D1 and D2) explained in the next section will limit and clip the guitar waveform when it reaches 700~800mVpp and before the power supply rails get into the mix.
In the image above, the output signal voltage level is shown sweeping the volume potentiometer. The gain goes from 3.5 to 46.5dB as calculated before. The "humpy" shape of the graph is explained in the Frequency Response section.
1.3 Clipping Stage.
The last stage is the clipping stage that contains two back to back diodes (D1 and D2) connected to ground. This type of topology adds a hard clip distortion, making the audio signal to clip with a hard knee which creates a "harder" distortion.
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The R5 resistor limits the amount of current into the diodes, 10KΩ is a pretty standard value. Other pedals like the Boss DS-1 uses 2.2KΩ, the Pro-Co Rat uses 1KΩ and the Klon Centaur 1KΩ.
- The RV-Output 10K potentiometer will control the output volume using a 10K potentiometer which bleeds part of the input signal to ground. It also forms a voltage divider with R5 and therefore decreases the overall output volume as well. One could get more output volume overall simply by increasing the size of the Output potentiometer.
The MXR Distortion + Diodes: The 1N270 Germanium
The 1N270 are the germanium diodes used in the MXR Distortion + circuit originaly. Some feel that they are the most important factor for the "classic Distortion + tone". People also try other popular germanium and silicon diodes. They all sound subtly different mostly due to the threshold at which they clip at. Also, changing diodes here will change the overall output volume.
Mods addition
If you're DIY'ing this circuit there are several changes you can make that are quite simple, and of course a person can complicate the circuit as much as they'd like! Some simple things to do:
Remove D1 and D2 hard clipping diodes, and instead add anti-parallel diodes in parallel with R4. This turns it into a bit more of an overdrive.
Another easy modification is to simply change the size of C6. The graph below shows a frequency curve of different size capacitors in this circuit, at the C6 location.
Another easy thing to do is increase bass or decrease bass with the Distortion control all the way "up". Simply increase C4 to add more bass, or decrease C4 to take away some bottom end.
And, for the more advanced DIY'ers, you can always add all kinds of different circuits. This could include active EQ or passive EQ with additional gain (or volume) boosting stages after to even more clipping stages.
Comment below what your favorite type of MXR Distortion Plus mods are, or your favorite "clone" pedals based on this type of circuit!