MXR Distortion Plus

[adding_to_the_noise]

I just finished building my very first diy project, an MXR Distortion Plus. Everything works fine except that I cannot seem to get enough distortion/gain. Even at high volumes it doens't help much. Any suggestions on what my problem might be, or how i can get more gain?

[Transmogrifox]

You may have something like a bad solder joint, or you forgot something somewhere.  Maybe your gain pot is the wrong value.

It's hard to know if your lack of distortion is due to an error you made, or if it's just due to the fact that the mxr dist+ doesn't offer the amount of gain you're looking for.

To add more gain, you can make the gain pot  a larger value, or make one of the resistors to ground from the - op amp terminal smaller, or both.

Another way to get more gain is to build another MXR dist+ and put the 2 in series.  Then you'll have the gain squared.

If that's too much gain, make the AMZ mini booster and stick that on the input.  That will make it crank a little more.

[Fret Wire]

I second the advice to double check all wiring and soldering. On that circuit, zero resistance on the gain pot is maximum distortion, so the gain pot value is not your problem. Make sure you have the diodes going in opposite directions. What did you use for diodes?

Better yet, read this sticky, and post the results.
http://diystompboxes.com/sboxforum/viewtopic.php?t=29816

[Mark Hammer]

1. Did you build it from JUST a schematic or did you use a PCB layout like the one at Tonepad?

2. I hope to goodness you aren't using a 4558 plugged into a 741 layout (or vice versa).  The original uses a 741 but because the 4558 is more or less the same price and makes layout a little easier (all the pins for the gain section are on the same side of the 4558), you can often see layouts using a 4558.  Don't confuse the two.  Different pinouts.

3. The original uses a 10k output pot, but you will get much more output level if you use a 50k or 100k pot instead.

[Fret Wire]

The original uses a 10k output pot, but you will get much more output level if you use a 50k or 100k pot instead.

Mark, I believe 50k is the original value, and 10k was a short lived, mid-period substitute value. I've seen scripts with 50k, but I've never ran into one with a 10k. My early block logo (non-LED) is also 50k.

Tonepad has two layouts for the D+: a straight 741 version, and the dual D+/Microamp version that uses a dual op-amp.

[Mark Hammer]

Mark, I believe 50k is the original value, and 10k was a short lived, mid-period substitute value. I've seen scripts with 50k, but I've never ran into one with a 10k. My early block logo (non-LED) is also 50k.

Tonepad has two layouts for the D+: a straight 741 version, and the dual D+/Microamp version that uses a dual op-amp.

My confidence in the reasoning of MXR is restored.  The one I traced in the late 70's had a 10k pot.  Glad they smartened up.

[Fret Wire]

I think they did some of the "buy what's on sale cheap" like E-H did, though not to the great extent E-H did.  

[adding_to_the_noise]

http://www.diystompboxes.com/pedals/MXRDIST2.GIF
This is the schematic I used, and i used the 1N34 diodes. I didn't use the tonepad layout, just a perfboard from radioshack.

[petemoore]

That's the DIST+ Schematic allright.
 The circuit should sound at least pretty good, hard to tell for sure what your'e hearing, but the tone should at least be satisfyingly distorted with some increase in volume over unity, even using Ge diodes for clipping the output.
 IIRC when I tried a 10k output pot and 1n34 diodes, the sound was still just a little above unity.

[Transmogrifox]

...zero resistance on the gain pot is maximum distortion, so the gain pot value is not your problem. Make sure you have the diodes going in opposite directions.

Oops.  My bad.  Yes, I didn't even look at the schematic and dumbly remembered it being a TS type gain control pot.  You really can't get much more gain out of this circuit, since the 741 bandwidth is pretty bad compared to modern standards, and certainly not good enough to really get much more gain without getting muddy.  It's already suffering from slew rate limiting in it's current max gain setting.

Ok, now looking at the schematic, this is a hard one to mess up and still have sound come through, so here's the way to see if it's at least doing what it should do:

Turn the gain all the way down, then play.  If it's a little distorted, then that's good.  It should be possible to make it sound somewhat clean.

Now turn it up just a little bit (maybe to 2 O'clock).  If it sounds completely clean, you don't have the diodes connected at all.  

Now as you turn it all the way up, you should be hearing increasing distortion.  If it sounds a little blatty and with some futzy octave crunch and seemingly bad sustain, you likely have the diodes wired in the same polarity and it's behaving as a 1/2 wave rectifier.  

If it sounds like a distortion pedal, then you're ok.

To get more gain, add a booster on the input whether a MOSFET booster or a mini-boost or something--or just make another one and chain them together.

[petemoore]

There's a certain value to taking and analyzing Voltage measurements and audio probe info.
 I take them 'anyway' just so I know my confidence that 'x' circuit sounds good is supported by the #'s...I just like to see what numbers 'this' one comes up with.

[adding_to_the_noise]

Thanks for all your help, I checked it this morning and dicovered a bad solder joint on the diodes. I re-soldered them and it works great. I'm thinking about replacing the diodes with LED's though, which kind works best, small or large?

[Steben]

only the colours affect the tone, not the size. 3mm's are good for clipping, since 5mm are bigger to install and maybe somewhat more expensive.

[MartyMart]

I left mine with the pair of 1N34A's for diode's and just used a 100k vol
pot.
That seems fine, I have enough gain/volume from it and changing the
diodes to LED's didn't seem "right" for  DIST + circuit !
It was "louder" but a bit "hard" sounding .....

Marty.

[petemoore]

The farthest I got toward 'softening the clipping' on the DIST+ Was an Si-->l and two Si's l<--l<--...opening up the opamps path to the output...
 I agree with Marty, just doesn't seem "DIST+ Like'.
 Now...boosting the Input...that's another matter altogether...

[Steben]

2x 1N914 sounds good, better than 1N4148.
Or double the germaniums.

[dv8]

There are no rules as far as clipping diodes go, feel free to arrange them to taste.

After several builds of this circuit, clipping diodes are YOUR taste.  Phillips' Liquid Drive uses 3 Ge diodes, YJM overdrive uses 2 Si diodes, etc..

It depends alot on your amp and the way you're gonna use the pedal (as an overdrive or as distortion), and finally what sounds good to you.

You can use any combo of LEDs, 1N914, 1n34a, 1n4148, or even 1n4001s in asymetrical or symetrical fashion.  If you need more volume, try LEDs.  In my setup, LEDs are much different than 1N914.  1N34a to me, are more for overdriven sounds.  Mix and match is always different.  try one 1N34a and a LED.

[Mark Hammer]

Whatever you use, remember that part of your tone comes from pushing the front end of your amp, and part comes from the pedal.  If you use diodes that provide more/easier clipping in the pedal, that will also result in less output signal from the pedal because of the ceiling the diodes set.  It may well be that using diodes that provide modest clipping in the pedal, but also permit a HUGE output level to punish your amplifier's preamp, may be what you really want.  LEDs will not yield serious fuzz on their own, but they *will* permit output levels that are some 5 times greater, on average, than what Ge diodes will permit.  

Think about the synergy between pedal, amp and guitar.  It's not ALL in the pedal.

[petemoore]

Gain Hog always goes for seeing what the booster on the input does...seems like it always increases Distortion Intensity [on any 'boost/clip type distorter] and Volume...can make SC's sound like HB's [not really, but for discussion] or Si diodes sound more like Ge's...
 I just added another bypass switch for the M.B. on my /MiniBooster/>/Liquid Drive/ box. Nice Box...LED's, Reverse Polarity protection, input PS Switching...