TUTORIAL Distortion 250 - MXR Dist + - The Modder's Dream!

[BuddytheReow]

Damn, I miss breadboarding. The ability to make a circuit on a whim without sacrificing solder or anything else except your time. My wife is out for most of the day hanging out with her family for a girls' day, so I thought I would lock myself in the basement and get things done around here, haha.

Anyways, here is the PPCB Distortion 250, which is the MXR Distortion+. Released in the 1970's and was pretty much unprecedented at the time, this used the first opamp in a guitar pedal. It is relatively transparent and has stood the test of time. Just take a look at sweetwater, guitar center, sam ash, or your local music shop to see what I mean. They're still being sold today, new and used, and for good reason: they get the job done. The schematic is incredibly simple and is screaming for some mods if you inner mad scientist wants to try this out.

Here is my finished layout, the schematic, and "official" notes on the store website. I tried to stay true to the original, but I swapped the pot values/tapers a bit and i'll explain. It will still work just fine if you use the stock values. Ready to get started?

 

[BuddytheReow]

First things first with any of my layouts, I start with the main IC and the power section. That's this section here on the schematic if you're relatively new to breadboarding/schematic reading.

First, make sure you've got power and ground going to the main rails. Install the IC (I used the stock 741 chip) at a reasonable location about a third of the way to the left of the breadboard. A 47uf cap goes in the rails themselves for DC filtering. Power (I've got 9v approximately) goes to pin 7 of the chip and pin 4 to ground. Simple, right?

 

[BuddytheReow]

Now let's work our way to the VREF section. R6 and R7 (both 1M) act as a voltage divider to split the power in half. Additional power filtering is done with C6. It looks a little cramped in the picture, which it is. You can get all this in using 1 column of your breadboard. It helps save space for other tinkering. One end of one resistor goes to power and the other resistor's lead goes to ground. Mind the polarity of C6 (the shorter end goes to ground).


We've got both our power sections ready. Let's connect VREF to the IC using R5 and goes to pin 3 of the chip. Notice I've made a gap in column 31 of my breadboard. I'm going to throw a jumper wire through there later. You'll see .

 

[BuddytheReow]

Now let's work on the input signal portion from the schematic here. I've already mentioned R5 coming from VREF. It's purpose is to bias the chip and "tricks" it to turn on and pass AC signal, which comes from your instrument.


It's a little cramped again, but the input wire (orange) has C1 (1n) going to ground and C2 (10n) acting as a coupling capacitor and cuts off a bit of low end in your input signal. From C2, R1 is inserted to pin 3 of the 741 chip. We are making a non-inverting opamp configuration and this is only a piece of it. A separate write up will be done eventually to talk about this type of layout.

 

[BuddytheReow]

Keeping things simple here, lets work on the feedback loop of the opamp. I'll only mention R2 here and the layout is very important in order to get this to work correctly.


Essentially, we are connecting the output pin of the opamp to the inverting input. That's pin 6 to pin 2 (notice the - sign on the opamp). I used a few small jumpers here to make things very clear when you're doing this on your own. We'll finish off the feedback loop in a moment.

 

[BuddytheReow]

OK. Now let's finish up the feedback loop of the non-inverting opamp configuration. Why do we need this? It's a separate writeup, but essentially we need to "select" a corner frequency to tell the chip what to amplify. In this case we have C3, R3, and the GAIN control.

I won't use any math here, although I should, the gain here is defined as the relationship between R2 and the combination of R3 and the GAIN resistance. The smaller the total resistance of R3+GAIN (anywhere between 4.7k and 1.0047M), the more gain the chip will produce. Connect these 3 things right where R2 meets the jumpers to go to pin 2 of the chip. See the pic to know what I mean. My white cable goes to pin 2 of the GAIN pot and pin 3 (brown wire) goes to ground. I decided to use a C1M pot instead of a B1M pot to have a bit more control over the gain, but either one will work here. Taper's really don't matter in terms of if the circuit will work or not. Tapers only change how the circuit reacts as you turn the know one way or the other.

 

[BuddytheReow]

Let's throw C4 from the output pin (pin 6). This is a great time to test out your circuit to see if it works. I did it on mine to make sure I'm not giving you guys a crappy layout. Attach your output pin right after the cap here:


If your signal isn't louder when the gain knob is maxed out, check your layouts to make sure there are no gaps and all your components are fully mounted in the breadboard. Also check to make sure your pot is tight on the protoboard (I love this thing). If you've got issues then ask them here, message me, or start a troubleshooting thread. 90% of the circuit is done here.

 

[BuddytheReow]

Alright the last section. We've got C4, R4, 2 diodes acting as hard clippers, a low pass filter using C5, and the volume pot. This section is where the main "distortion" sound happens. What we've done previously is simply gave ourselves the ability to amplify an input signal.

Remember in post #3 where I said I make a gap to throw a jumper wire? Well, here it is. Right after C4 to start the final section. R4 (10k) connects the jumper wire to the diodes. For the diodes, I went with the stock value of 1n270 that I had lying around. Then, a small jumper in order to attach C5 to ground. Also at this point you should connect your volume pot (that's column 43 in my picture) with pin 3. Pin 1 goes to ground and pin 2 is your output. Based on the "official" notes for the PCB, I changed the stock value of A10k to A100k.

 

[BuddytheReow]

And, viola! She's done. It should be mentioned that this isn't a very loud pedal, but it is a good one and a classic for a reason.

 

[BuddytheReow]

MODS

Considering how bare bones this schematic is, nearly every single component (except the power section) can be swapped out for a different value and will affect the overall tone. Here are some of my takes on this circuit.



-Diodes can be substitutes for different values. You can try 4148s, LEDs, MOSFETs, or anything else your heart desires. You can tack on 1 additional diode to make asymmetrical clipping.

-C3. Try increasing or decreasing its value to alter the corner frequency. Increasing it's value will allow more bass to come through and give it some beefy low end.

-C2. Increase or decrease to allow or remove more bass from the input signal.

-C1 and/or C5. Again, try increasing or decreasing the values. Or better yet, pull em out and see what happens! That's the beauty of breadboarding!

-On the breadboard, this circuit is dying for a tone control of some kind. Insert your favorite tone stack (Rat, BMP, TMB, James/Baxandall) right after the diodes. If you want an active tone stack you'll need to install a chip or transistor to work off of.

Of course, I have to mention Brian Wampler's take on this classic circuit and his many mods. You can read about it over here. See what you like and let's see your tasty creations!

BuddytheReow

 

[szukalski]

I love it. This is invaluable! Thanks a lot.

I've been getting into vero a lot lately, I love to solder, but the breadboard should be that first step..

 

[BuddytheReow]

I love it. This is invaluable! Thanks a lot.

I've been getting into vero a lot lately, I love to solder, but the breadboard should be that first step..

I was like you and went to vero/stripboard first. I then wanted to understand how circuits worked from a practical standpoint rather than just reading textbooks, online articles, forums, etc. Breadboarding was the next step to see, "what happens when I pull this resistor/cap out?" "What happens when I increase the resistance here?" "Can I tweak the tone stack to my liking?" It's a lot of playing around to see what works and what doesn't work.

 

[Fingolfen]

I just finished up my first breadboard project - taking a Wampler Mod Distortion+ and messing around with it until I was happy with the tone control (which meant I dumped his concept and added my BMP tone control)... your board is a hell of a lot neater than mine... I've got a fair amount of spaghetti... Steggi helped...

 

[BuddytheReow]

I just finished up my first breadboard project - taking a Wampler Mod Distortion+ and messing around with it until I was happy with the tone control (which meant I dumped his concept and added my BMP tone control)... your board is a hell of a lot neater than mine... I've got a fair amount of spaghetti... Steggi helped...

View attachment 36396

Does adding the BMP tone control make the pedal too quiet for your taste? Do you have a recovery stage at the end? A booster, perhaps?

 

[KR Sound]

This is great. Thanks for documenting the process!

 

[Fingolfen]

Does adding the BMP tone control make the pedal too quiet for your taste? Do you have a recovery stage at the end? A booster, perhaps?

Yeah, there's a gain recovery stage. If you look at the etched PCB the muff tone is really just the first two resistors and capacitors with the pot behind them - the other four resistors, cap, and transistor are the gain recovery stage.

 

[Feral Feline]

Great to see you back at your Breadboard, Buddy.


To all the breadboard-brethren and sistren, this circuit can be modded to the Blue Clipper and...well, dozens of variants.

One that I saw and liked is to make the IC a dual op-amp IC (TL072 or 4558 etc) and use the IC's second op-amp as the gain-recovery stage after whatever tone-stack is employed.
Of course, once you add a dual op-amp you can't really use a Dist+/250 board to transfer the breadboarded circuit to it, you'll have to transfer to Vero or Perf or...
Transfer to the PedalPCB Permanent Breadboard.

Oh, and if you really like the ol' school sound of the LM741 chip, but want that tone-stack and recovery,
the LM1450 or MC1450 is basically two 741s jammed together into a single dual op-amp IC.


[EDIT:] AHH CRAPOLA! I musta been posting half asleep again; Even though I do have 1450 in some OLD notes about the op-amp Muff, along with the 1458, my old notes might've been a typo — what I meant was what Chuck has in his post below about the dual op-amp IC — good luck finding my mentioned LM1450 (MC1450), but you can easily find Chuck's:
LM1458
SORRY FOLKS! Chuck for the win. [/EDIT]

 

[Chuck D. Bones]

1458 is also a dual 741.

You have to make C2 <1nF to choke off any bass. C3 is much more effective for tuning the bottom end.

 

[Fizz]

Great job Buddy! This is the way to do a tutorial

 

[equinox]

I like these MODs to keep it vintage, but give it a little more gain +46dB => +60dB