MXR Distortion+

[littlegreiger]

Hey,  I've found three different schematics for the MXR Distortion+.  I found them on Tonepad, General Guitar Gadgets, and another site.  Here are the sites:
Tonepad
http://users.chariot.net.au/~gmarts/ampovdrv.htm
General Guitar Gadgets

Can someone please tell me which one is correct?

[cd]

They're all essentially the same.  Any differences between them are trivial, and don't affect the sound (i.e. pulldown resistor, cap to prevent oscillation, etc.)

[Fret Wire]

One thing they share in common, is that they list MXR's least used, and least effective gain (1M) and volume pots (10k).

Correct is 500k rev. log for gain, and 50k log for volume. If you build your own, use 100k for volume. It'll help insure you hit unity gain, especially if you use ge diodes.

The original also used a 741 IC and 1N270 diodes.

[bobbletrox]

Is the main difference between the vintage and modern ones just the germanium diodes?  I used to have a vintage Distortion+,  so it'd be nice to be able to get the same tone if I bought a new one and switched the diodes.

[Fret Wire]

Yup. Just use the 741 and 1N270's and you have the original script/early block logo sound. They also used ceramic caps, tantalums for the 1uf's, and carbon comp/carbon film resistors. The important thing is the 741 (it'll clip without any diodes) and 1N270's.

The later models with Si diodes just need the diode swap. If there is a pf smoothing cap across the diodes, that needs to be removed.

[Steben]

One thing they share in common, is that they list MXR's least used, and least effective gain (1M) and volume pots (10k).

Correct is 500k rev. log for gain, and 50k log for volume. If you build your own, use 100k for volume. It'll help insure you hit unity gain, especially if you use ge diodes.

The original also used a 741 IC and 1N270 diodes.

If you have got a lot of stompboxes, off course it's better to place another diode in serie with each existing one, and use as small a volume pot as possible (and stick to the 50k one). Large values will add up in each box more and more noise.

[Fret Wire]

True to a point, but the question was about the original script sound. And with the two Ge 1N270's, you need at least the original 50k volume.... 100k's better to insure unity gain, plus some boost. The 100k doesn't add any noise that I've heard, nor do I think that one specific pot change is going to degrade the rest of an effect chain. The more effects a chain has, the more noise, that's for sure.

[thomas2]

how can i build a bass overdrive out of dist+ (or dod overdrive preamp 250 in this case).. would changing the input and output caps to bigger value do it?  i was also going to add high and low bypass switches.. can this be done by adding switchable caps between the input and the output of the circuit? also a mod to get more distortion from the circuit would be cool.. :lol:

[bobbletrox]

there are some mods for the dist+ on this page thomas:
http://www.dominocs.com/ToneWorks/shop.html

[Mark Hammer]

how can i build a bass overdrive out of dist+ (or dod overdrive preamp 250 in this case).. would changing the input and output caps to bigger value do it?  i was also going to add high and low bypass switches.. can this be done by adding switchable caps between the input and the output of the circuit? also a mod to get more distortion from the circuit would be cool.. :lol:

There is a lengthy post of mine somewhere in the archives here about bass response in this circuit and the role that the distortion/drive pot plays.  I can't find it but the short version is this.

When an op-amp is configured the way the Dist+ is, the gain can be determined by the resistance to ground from the inverting input pin.  In this case, the Dist+ sticks a pot there which serves as a variable resistor to adjust gain.  As the resistance gets smaller, gain increases (one of the reasons why you probably don't need a 1m pot there and can probably easily live with anything between 100k-500k without altering the "original" sound of the pedal).

The problem with this is that the bass response/rolloff is determined by the combination of that resistance and the cap that ties it to ground.  As the resistance is decreased (and gain increased), the bass is also rolled off until you reach a point where at maximum gain (resistance equal to JUST the 4.7k series resistor) the rolloff is down by 3db at 720hz.  I gather the design intent is that you get a two-for-the-price-of-one effect by going from cleaner full bandwidth to buzzier thinner distortion with a single control.

The fix to this is simple, increase the cap.  If you change it to .47uf (nonpolarized will be easier to avoid mishaps with but polarized will likely be smaller), the low end will be nice and full throughout much of the drive pot's range, with bass only being down 3db at 72hz at maximum gain (even less rolloff at less than full gain).  That ought to work nice for your bass.

Certainly changing the .01uf input and 1uf output caps CAN have an impact on the bass, but whatever they provide the potential for is negated by the ground cap in the op-amp....unless you change that cap.

The use of a 50k-100k output volume pot is highly recommended if one wishes to be able to get any sort of volume boost at less than maximum drive/dist.

The simplest way to get LOTS more distortion is to cascade two copies of the circuit.  There are a few ways you could do it but probably the easiest is to feed the volume pot output of the first copy of the circuit to the 10k input resistor.  The additional .001uf cap to ground on the 2nd circuit's input is not necessarily needed.  For simplicity's sake, you can probably omit the gain pot on the 2nd stage and simply replace it with a 47k resistor.  That will give you a gain of 22 in the 2nd stage which will be more than adequate to reclip the signal when you pass it by the 2nd set of diodes.  All advice about changing the .047uf cap to ground applies here too.  Using a .1uf or larger cap will be fine (.1uf will result in a 2nd stage rolloff around 33hz).

Finally, stick a pair of Si diodes in the second stage instead of the Ge ones.  That will give more output and yield more tonal variety.

Here is the important part: stick a cap in the feedback loop of the 2nd stage.  There are few things that will result in harsh, fizzy, noisy fuzz than reclipping higher order harmonics.  Feh!! as my grandmother would say.  Since your intent is to use it with bass, my recommendation would be to stick a 100pf cap in there, which will produce a rolloff of high end starting around 1.6khz.  That will smooth it out nicely and turn it from a raspy shriek to a nice snarl or growl.  I suppose you could go as low as 33pf if you wanted something that would work for guitar and bass, but I wouldn't go any lower than that.

What you'll end up with is something sort of like the new E-H Double Muff, where the quality of distortion from the 2nd stage will depend on what you do with the first one.  You could, say, max the output of the 1st stage but keep the drive down low.  This will feed the 2nd stage with a moderately colored signal that will be reclipped by the Si diodes.  Since the 2nd stage adds gain, how hard the 2nd stage clips will depend on where you set output level 1.  Drive stage 1 hard and max the output 1 control and you'll get ridiculously fierce distortion from stage 2.  Although the feedback cap in stage 2 rolls off highs, it is receiving a stage 1 signal that can vary from normal tone to sizzling, so you can shift tonal emphasis as well.

All in all, I think you'll find a great deal of flexibility in just 3 simple controls.  Make sure both output level pots are 100k.

[Fret Wire]

Nice idea Mark. Tonepad has the Stereo MicroAmp layout that can be adapted to cascade two Dist.+, or booster/Dist. +.
http://www.tonepad.com/getFileInfo.asp?id=51

Bubbletrox: I've seen that page before. I think it's more helpful for existing units. I've never seen the need to increase gain, though. Clipping diodes and bass response are the two most important mods to that pedal. Lowering the output resistor helps the reissues, volume wise, if your going back to Ge diodes. Fortunately, the reissues have a 47k log vol., so they can handle the switch back to Ge diodes.  If you're building your own, or can replace the pot, 100k on the volume pot is the way to go.

I guess we got off the original question again.

[thomas2]

HUGE THANKS MARK! so.. how about those switches?  :wink:

[thomas2]

thanks to you too fret wire.. we posted at the same time  :lol:

[Mark Hammer]

HUGE THANKS MARK! so.. how about those switches?  :wink:

De nada.

My advice is to see what you get from the circuit as described, and worry about the switches at a later point.  With different diodes in each clipping stage and the capacity to vary how hard each stage is driven, you'll be able to reap the qualities of each diode type easily without any need for switches.

I suppose if you bought a pile of toggles and need to justify all the money you spent, you could consider switching the .047 cap to ground from suggested larger value to stock.  That would further add to the tonal flexibility.

[thomas2]

actually i prefer small slide switches.. i'm gonna use the switch to change the cap, that's for sure. i was also wondering how i could let some of the lower freqs trough with a switch so there would be an option to get the distortion only in mid and upper freqs. :twisted: i'm gonna build a monster.. the enclosure is 14cm x 14cm x 4 cm, so there's plenty room to add switches and stuff.

[littlegreiger]

I'm building mine from the Tonepad schematic using 1N4148's for diodes.  What pots should I use?

The schematic from General Guitar Gadgets says that I should use a 1M Reverse Log instead of the 1M linear.

I found that the schematic from the other site is different from the Tonepad and General Guitar Gadgets ones.  Did anyone else notice this?

[Fret Wire]

Use a 500k rev. log for gain, and a 100k log for the volume. Other tapers will work, but this will adjust the most "normal" to your ears for lack of a better word.
As Mark explained, maximum distortion is when the pot is at zero resistance. Whether you use 100k, 500k, or 1Meg, you get the same dist. It's the other end that changes. A 1M pot will go from relativly clean to full distortion. But the adjustment is less fine. A 100k pot will cover less of a range, but be more precise. I'd just stick with the 500k rev. log. You'll get a good range of adjustment that will increase gradually.

The 100k volume, with the Si diodes will insure you'll have no trouble hitting unity gain-plus with most rigs.

[littlegreiger]

Would 1N34a diodes work in this pedal?

[Fret Wire]

Yup...... 1N34a, 1n270, 1N4148/914, LED's, 1N4001, they'll all work. The Ge 1N34a's will give close to an original "script" type sound like the Ge 1N270's. That's one of the reasons I recommend the 100k vol pot. If you go Ge on the diodes, you'll still have enough volume.

[petemoore]

I think I might like the increase distortion mod...thanks for posting that link
 the 4k7 to 2k2 and .047 to .1uf...
 I wonder though...
 if it could be implemented with a switch like this.
 string a switch, a 4k7, and a .047uf ...parallel to the existing 4k7 and .047uf...
 so basically you'd have paralleled two sets of R/C there, but each set would be separate...not connected between the resistors and capacitors.
    4k7  >   .047uf
                     
    4k7  >  .047uf
 where the '>''s  would not be connected.
 To the left of the two paralleled R/C's would be the gain pot, to the right would be the opamp pin.