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Building MXR Distortion Plus

After trying it out on a breadboard, it is time to finalize my version of MXR Distortion Plus. (I dubbed it MyXR Distortion ++ … lame, I know 😂).

I’ll do the final schematic and bill of material, then do the soldering layout for a protoboard. I’ll plan out the enclosure, and finalize the enclosure. I’ll finally assemble it and do test and demo.

Final Schematic

I tried out several things when I did the effect on a breadboard. I finally settled for a TL072 version (I have quite a few of these op-amps lying around) and I decided to add a toggle switch to toggle between two clipping options: “standard” silicon diode pair and a red LED pair. See the breadboarding post for all options I tried out.

Here’s the full schematic:

MXR Distortion Plus schematic modified to use TL072 op-amp and with extra clipping option
MyXR Distortion ++ circuit schematic (click for full image)

The difference from the one used in breadboarding is U1 – I used TL072. In the breadboarding video I tried it out along with LM741, sounded pretty similar. Since I changed the op-amp I added C7 (47p) to make the output response near identical. I tried out several values and decided to go with that value.

I also added R8 which is often added to reduce thump when switching the pedal on and off. I used 2.2 Meg resistor and I increased resistance of R2 to 2.2 Meg to keep the input impedance roughly the same. Neither should affect sound at all.

Toggle Switch

The biggest difference is addition of a toggle switch to support 2 different clipping modes – one with “standard” silicon diodes and one with red LEDs:

Schematic showing clipping options used
Clipping option toggle switch

In one position, silicon diodes are connected to GND and LEDs are disconnected and essentially taken out of the circuit. In the other position LEDs are connected to GND and silicon diodes are out of the circuit.

The Rest

Other than that, there are 2 more parts to complete the schematic fully.

Remaining TL072 wiring up

That part above is for the unused op-amp from TL072. As I mentioned I have a drawer full of them, so TL072 it is for most builds 😁. I could’ve used it for something more useful, and I might in future for a different build. But for now – wiring it up to minimize impact of it. Also, I added C9 – decoupling capacitor which is customary used for decoupling power supply for integrated circuits.

The last remaining part is wiring up stomp switch and power:

Schematic showing how to wire-up stomp switch and power
Wiring up 3PDT and power

The above is just a standard way of wiring up 3PDT switch for the pedal I use. There are other options, but this one is simplest and as effective.

One thing to note, I’ve put 1K* for R10, that depends on the LED used and how bright I want it to be. In the final build I used 4.7K, I tried out couple of options and decided that’s the right level for me. The purpose of it is to limit current, but brightness depends on the current, so it also controls brightness. Higher the value, less bright the LED is. I’m just saying – if you’re using high brightness LED, 1K might be too low value and you might not be even able to look at the LED straight since it will be too bright.

Bill of Material

Here’s the final list of components I used:

DesignatorsComponentNotes
27969PSLA
(1590A Hammond)
Enclosure
Protoboard SBB170 solder-in breadboard by Proto Advantage (fits into the enclosure … nearly)
Jumpers and wires24 AWG solid core wire and 28 AWG stranded wire (24 AWG might be slightly easier to use)
Anything thicker might be hard to stick into the protoboard.
J3Lumberg 1614 09DC Power Socket. I usually use Cliff FC681473, but could not fit it into the small box
J1Switchcraft 112BX
(Alternative: Switchcraft 12B, 11, 111X)
In – mono Jack (I don’t use battery so mono. But I used stereo, it’s fine, I just ran out of mono jacks)
J2Switchcraft 112BXOut mono Jack (I used stereo, it’s fine, I just ran out of mono jacks)
RV1TT Electronics
P160KNP-0EC15C500K
Distortion pot: 500K anti-log pot (solder lugs, metal shaft and bushing)
Alternative pot: Bourns PTV111-1415A-C3103 for example (but I’m not sure it would fit into the small enclosure)
RV2TT Electronics
P160KNP-0QC20A100K
Volume pot: 100K log pot (solder lugs, metal shaft and bushing)
Alternative pot: Alpha RV16AF-10-15K-A100K-3LA for example
KnobsThis depends on what pots you use. I used 6mm straight shaft knobs with screw. If using Alphas you might need knobs that fit 6.35mm straight shaft for example, or if using split shaft – you’d need push on split shaft knobs.
S1SF17020F-0302-21R-LTaiwan Alpha 3PDT latching foot switch
SW2Mini Toggle Switch On-OnJust a random miniature toggle switch – on-on SPST I had around
C1, C51nF (0.001uF) Metal film PET cap
C210nF (0.01uF) Metal film PET cap
C347nF (0.047uF) Metal film PET cap
C4, C61uF Electrolytic cap 25V
C747pF Ceramic C0G (NP0)
C8100uF Electrolytic cap 25V
C9100n (0.1uF)Ceramic X7R
D1, D21N4148Small signal diode
D3, D4LED5mm Red Diode for clipping (try out what works best for you)
D5LEDLow current red LED
R1, R510K 1% 250mW metal film
R2, R82.2M
R34.7K
R4, R6, R7, R91M
R104.7K LED current limiting (and brightness determining) resistor
U1TL072TL072IP
DIL Socket8 pin DIL socket so I don’t solder IC directly to the board
List of components for MyXR Dist ++

There couple of things to note here. I used a two pin power socket, which is fine since I didn’t use battery and I don’t need switched pin. I had to use a socket that is smaller than my usual Cliff socket. This also means, the socket had to be installed prior to soldering because it fits differently from the Cliff one.

I used 16mm pots for this, they are barely fitting the enclosure, hence some decisions had to be made during the assembly to be able to fit everything in. So far I’ve done single pot pedals in this small enclosure, two pots are a stretch, and I should’ve gone for a bigger enclosure (1590B for example). In the end, I managed it … but it was not a pleasant experience to be frank.

Board Layout Plan

As usual, I prepared one possible layout with DIY Layout Creator:

Protoboard diagram for MXR Distortion Plus pedal
Protoboard diagram for MyXR Dist ++ (click for full size)

This was a struggle to fit in. If you look at the bill of material, there is a lot of components, by far the most complex pedal I covered so far. (And by all means, it’s not a very complex pedal as they go).

I had to solder indicator LED and accompanying resistor directly to the foot switch, I had no room on the protoboard (actually, I could’ve fit it to the board, I had trouble finding where to fit it inside the enclosure, so it was easier to solder it directly to the switch).

Enclosure Planning

I did usual enclosure planning in Inkscape:

Enclosure planning diagram
Enclosure planning

Very tight, barely fitting, my toggle switch is slightly slimmer so I managed to fit it all in the end. But it was very tight. Likewise it required me being a bit more careful than usual since there was no much room for error.

Here’s the original SVG I used for this:

Preparing Enclosure

I did standard stuff with drilling the enclosure first using the above template. Before doing anything else I made sure all components fit, would’ve been pointless if I could not fit them all.

If you look at the first photo from the gallery, 1590B is way bigger (relatively speaking) compared to the two 1590A enclosures. I regretted not going for a bigger enclosure by the end of the build.

For this build I decided to go with using universal primer in spray can, then I used some acrylic paint as top coat, then I used some acrylic paint markers for the design and finished it off with several coats of varnish in spray can. Here’s the process and the final result:

Aaahh, Picasso would be jealous.

I’ll prepare a very crude video as an assembly log with all the steps I took at some stage (I hope).

Final Assembly

The final assembly was a bit tricky due to space constraints. Nothing that I haven’t really covered before but just with an added complexity of that toggle switch. That may have been a touch too much. But somehow I managed:

I hate all the wires. They are making everything take a lot of time assembling. Also, very hard to fit everything in.

I had to trim some of the board with a Dremel tool to be able to fit everything in. I’ll invest some time to maybe come up with a PCB I could use for small builds.

Test Ride

Here’s how the finished pedal looks like:

Finally, here’s how it sounds:

Pedal in action

I like the pedal. Nice sound, not a huge level of control, but it is simple and works just fine. Some noise can be heard, and funnily enough, when I turned off the lights – noise was almost completely gone!!!! (check it out around 6:30 mark – just so you know – switching off lights does not help in all situations! 😎)

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