Analog D15T0RT10N+ With Digital Setting

As other thousands inventions (like Penicillin, Viagra and Dynamite!) the now popular Distortion for electric guitar can be considered a consequence of misuse of pre-amp, op-amp and clipping circuit: all of these were designed to have clean and safe signal but Guitarists, you know, are the most insubordinate people so they abuse it and invented ROCK!

There is a pletora of project and commercial devices (Maestro, Fuzz Face, DOD, MXR Distortion+, Electro-Harmonix Big Muff Pi, BOSS DS-1, etc.) available, as well as a lot of good internet info on how to build your own distortion: this project is aiming to solve one of my biggest problem while I was experimenting, that is to swap component and make A-B comparison on the actual sound of my guitar.

The ugly resulting pedal (see the picture to believe me!) has an all-Analog circuit for the distortion itself, and a digital part to insert and memorise 4 circuit variations.

Part Value

R1 10kΩ

R2 1MΩ

R3 1KΩ

R4 470KΩ

R5 10kΩ

R6 1MΩ

R7 1MΩ

R8 220Ω

R9 1kΩ

R10 220Ω

R11 470KΩ

R12 100KΩ POTENTIOMETER

R13 100KΩ POTENTIOMETER

R14 220Ω

R15 2.2KΩ

C1 2.2nF

C2 2.2nF

C3 47nF

C4 1µF

C5 2.2nF

C6 1µF

C7 220µF polarized

C8 47µF polarized

C9 100nF

C10 33nF+33nF

D1 1N4148

D2 1N4148

D3 1N4148

D4 LED WHITE

D5 1N4148

D6 LED YELLOW Distortion

D8 LED GREEN Distortion

D19 1N4148

T6 BC547 NPN TRANSISTOR

IC1 µA741 OP AMP

DIS1 TUXGR 20X2 LCD display 20x2 characters

ARDUINO NANO

REED D10_MEGA REED

REED D6_ASYMMETRIC REED

REED D7_COMPRESSION REED

REED D9_BASS_BOOST REED

RELAY K6 TQ2-5V RELAY

SWITCH_SET PUSH-BUTTON

SWITCH1_ON/OFF_DIST.2 FOOT-SWITCH

+9V Barrel POWER_JACK

X1 AMPLI (OUT) Jack connector

X2 GUITAR (IN) Jack connector

The schematic is a combination of 2 parts:

1 - ANALOG Distortion Circuit (bottom)

this is a potpourri of circuits from DOD, MXR, DIY, Encyclopaedia Britannica etc. I have played with, around the uA741 op-amp.

There are very informative articles on internet that you can easily find on the subject.

The most important (and to me, original) part is the fact that some components (Capacitors, Resistors and Diodes) can be inserted/excluded to change the configuration of the circuit and, as consequence, the kind of distortion generated.

2 - DIGITAL Control and Display Circuit (top)

the heart of the digital part is the ubiquitous ARDUINO nano that oversee the following tasks:

1. TRUE bypass RELAY to guarantee that the circuit is completely isolated when not in use. This is particularly important to avoid unwanted digital noise and/or signal contamination when no-distortion is selected
2. Switches Input and LEDs Output
3. LCD Display
4. REED command to insert the mods (I have tried with CD4016 or similar but the noise introduced is unacceptable).

For this code you have to install the Libraries:

Arduino.h

avdweb_Switch.h -> to take care of Single, Double and Long press of switches

LiquidCrystal.h -> to manage all the Messages on the LCD display

EEPROM.h -> to memorise the setting

The basic idea is to use REED relays to insert/bypass part of the circuit.

These are the mods:

1. COMPRESSION adding C9 in parallel to C2
2. Hi-Gain adding a 470K resistor in series with potentiometer R13 to have a MEGA-Distortion through a higher gain of the uA741
3. BASS adding C10 in parallel to C3
4. Add a LED in series with one clipping diode to have ASYMMETRIC distortion

and they are activated by toggling the foot switch stomp button:

1. Press ONCE to activate Distortion set 1
2. Press LONG to activate Distortion set 2

In each set use switch SET to change parameter that will be automatically saved in its set

1. Press ONCE at the time to select which Mod activate
2. Press LONG to toggle it ON OFF and memorize it

I have used the D13 (build-in LED) as memory check - useful during troubleshooting to avoid writing too many time the EEPROM (if you run a wrong program the EEPROM could be fast written thousand time and loose the capability to work properly!)

I put as many comments as possible to have the program clear and, as you can see, the most tricky part is the management of the display: most of the troubleshooting time was spent to overwrite characters to have readable clean lines!

If you want to use the project for some other mods, please consider that COMP and BASS work in Normally Open mode (if active the mod is ON) while MEGA and ASYM work in Normally Close mode (if disable the mod in ON).

Arduino nano is a wonderful tool but, as I learned by experience, it's a very noisy digital piece of equipment: better to keep away from high gain op-amp!

So, I build the D15TORTION in the longest metal box I had available (a broken ethernet hub) keeping all the digital stuff (nano and LCD display) on the left and the analog circuit on the right, achieving a very reasonable result in terms of interference and a good compromise as ergonomic while using the foot switch and the pots.

One negative aspect of this solution is the footprint on the pedalboard, even if the horizontal development could help the placement.

I have utilised as much as possible components that I have spared in years, so the quasi-vintage LCD display is one of the stuff left in a drawer: probably an OLED display will cost less and look more modern, if you like it...

In the attached pictures you'll find:

1. draft of the printed circuit board (I have used a prototype PCB) - please note that the relay is doubled but you need just one as per schematic. All the connections to the digital parts are missed but, I believe, easy to follow in the schematic.
2. the overall view that shows the separation digital (left) - analog (right)
3. the analog circuit with all the components named (see, only one relay!)
4. the digital section, including the power supply connector

I have connected the Arduino nano to the 4 REEDs with a piece of flat-cable to facilitate the assembly.

It's a relative simple circuit that just requires some patience to be built. Alternatively you can use a larger box to facilitate the assembly.

Ps.: probably you have noticed a funny piece of steel under the foot switch and yes, you're right, it is a former Credit Card from a famous American brand. I've just retired and I don't have anymore the Company Credit Card so I've been asked to damaged it to disable its possible misuse. I found this application better than any other kind of disposal!

Tested with wave generator and oscilloscope, you can see the "usual" clipping of the sinusoidal input: introducing Bass, Compression and MEGA the signal will be further deformed and, with ASYMMETRIC... just asymmetric!

Anyway the best assessment can only be made by earing some sound...

I have played a Telecaster in the pedal to a small Pignose (no other effect, to have the driest sound possible): you can see in the audio.mov file the difference when BASS and MEGA are active.

Overall the pedal produces a decent Distortion/Fuzz output, very usable for rock music.

Reducing the level of distortion it sounds like a moderate crunchy valve clipping but... just vaguely!

The final scope of this project was to have a practical tool to try different circuit solutions around the MXR Distortion+/DOD classic op-amp pedals, and I'm very satisfy from this point of view. So I recommend you try your own alternatives using this digital add-on as enabler to your imagination.

Enjoy!