Interesting Distortion + idea...

[jedhiggs]

Hey guys, just opened up my mxr distortion + and was examining the volume and distortion pots.  The volume is 47K while the distortion is a 470K pot.  Since everyone knows the output on these are hurendous, could you just switch the pots?  Ideally, giving the volume more range while lowering the distortion range.  Any one think this would work, or should I just be messing with the 2 diodes?

[delbowski]

hi... welcome to the forum 

i think you are thinking about this the wrong way.  if your volume pot is 47k, that means that you have a resistance of 47k when the pot is turned all the way counter-clockwise (volume all the way down), not clockwise.  using a larger value will not help you get more range out of the pot, in fact it will give you less range.  if you used the 470k pot, the volume would still be turned down when fully turned counter-clockwise, but as you turn the knob clockwise you will not hear any increase until you reach the 47k mark on the pot's travel.  you would get the same amount of volume, but it would be bunched up at one end of the pot's rotation.

the same goes for the distortion pot.  you have a resistance of 470k when turned all the way counter-clockwise, not clockwise.  you wouldn't be lowering the maximum amount of distortion you could get, but rather limiting the minimum amount. 

anyone care to verify what i'm saying, or if i'm totally off base, set me straight too?  hope this is making sense. 

del

[jedhiggs]

Ok, well that makes sense to me.  But ultimately, I'm really just trying to increase my output volume.  A few guitar player friends of mine would also like this done, so i've volunteered ha.  thanks for your reply.

[LyleCaldwell]

Remember the distortion pot is reverse audio taper, so you don't want to just swap parts.

I don't know if the circuit of yours is similar to http://generalguitargadgets.com/diagrams/dist_plus_sc.gif, but changing the volume pot won't do much (see delbowski's post).  I have a similar circuit with mostly the same values (500K for gain instead of 1M, and 50K for volume) and it has a lot of volume available but less distortion on tap.  You could try going to a 500K reverse audio taper pot for distortion, seeing if you like it.

[dave_neal_59]

You are limited by the clipping diodes because they are after the gain stage.    You can either add another gain stage (major surgery) or change the diodes (minor surgery).   You can either go to LEDs or diode pairs (two diodes in series) to get more volume out of the pedal, however, it will change the distortion sound.  The best thing for more volume and similar sound is to go with two pairs of the same diodes that are in the pedal.  I don't own an original so I am not sure what is in the stock pedal.  Near the bottom of the DIY FAQ is information on putting parts in series.

[petemoore]

  What type of diodes are in it?
  If they're GE...diddling with adding Ge to them or substituting Si or two will increase output, that's where I'd start.
  Larger value volume pot I'd think would increase output a little, since at max volume there'd be less shunted to ground through the potwafer.
  Making the gain knob smaller value would still allow Max Gain [because that's when the pot is set to 0 ohms on the DIST+], and disallow lower distortion settings, but then again that's a reverse taper pot...unless you don't mind turning the pot CCW to increase gain...so if you got a smaller value with a taper you like....well since I always have a DIST+ set sorta near max gain, that'd give a 'finer tune' control that'd include max gain and near max, but not lower gain settings [I never use]...I put a 100k there, that goes to plenty low gain for all my purposes.

[LyleCaldwell]

I used 1N4148s and had a lot of volume available.

[jedhiggs]

Very interesting ideas all around BUT dig this.  So my unit has germanium 1N270 "glass package" diodes.  In an attempt to mess with them, I was trying to remove them delicately and I ended up breaking them.  They were soldered on both sides and found it very difficult to remove them unharmed.  So i left the legs intact and connected to their original points and re-assembled the unit.  When tested, the unit was doing very interesting things.  I'll let you know for sure tomorrow what the exact results were when I can crank up my Silvertone amp.  (SOME people are trying to sleep right now, haha)  SO I guess what I'm asking all of you is what effect should removing the 2 diodes entirely have??  Thanks for your posts

-jedhiggs

[anti-idiot]

Ok, well that makes sense to me.  But ultimately, I'm really just trying to increase my output volume.  A few guitar player friends of mine would also like this done, so i've volunteered ha.  thanks for your reply.

another thing u can do is, d'ya see the resistor and cap before the gain pot? i think they are 4.7 and 0.047 rspc.. put another resistor and cap (same values) in parallel and u'll increase ur output.

[squidsquad]

Removing diodes oughta give you more volume & less distortion...and that may be what you wanted.
But it won't distorte as much as it used to.  Maybe check out the articles by Jack Orman showing how to use a pot to dial-in diodes:  http://www.muzique.com/lab/sat.htm

[Transmogrifox]

The only hope to increase output on the MXR Dist+ is to mess with the diodes or remove them entirely like you have done.  The Ge diodes explain the low output.  Replace them with Silicon diodes, and that will bring the volume up a good bit.  Then if that's not enough, put 2 Silicon diodes in series for each polarity (like previously mentioned).  That would make a total of 4 diodes...or for the asymetrical sound, do two in series going one way and just one going the other way...or you can build a booster pedal and chain the MXR dist + into its bypass loop so the booster only boosts the MXR output. 

Removing the diodes alltogether will give you a really hot output that will drive the hell out of your amp--and if that's why you have a distortion pedal (to drive the hell out of your amp), then this would be just what you want.

[Jay Doyle]

i think you are thinking about this the wrong way.  if your volume pot is 47k, that means that you have a resistance of 47k when the pot is turned all the way counter-clockwise (volume all the way down), not clockwise.  using a larger value will not help you get more range out of the pot, in fact it will give you less range.  if you used the 470k pot, the volume would still be turned down when fully turned counter-clockwise, but as you turn the knob clockwise you will not hear any increase until you reach the 47k mark on the pot's travel.  you would get the same amount of volume, but it would be bunched up at one end of the pot's rotation.

Well, I hate to disagree but this is wrong. Of course I am thinking of the old R.G. drawn schematic that I saw on AMZ (http://www.muzique.com/schem/mxrdist2.gif) that has a 10k resistor feeding a 10k level pot but the idea is the same.

A volume or level pot set up in this way is nothing but a voltage divider. If you take out the 10k resistor feeding the level pot, the only difference you are going to get between a 10k and a 470k level pot is the amount that the volume pot loads down the output of the opamp. If it was a transistor circuit, this would most likely make a noticeable difference, with the 10k loading down the transisor's output, but with an opamp, the output impedence is so low that a 10k or 470k would not make a difference, at least not audibly.

BUT...

In the Dist+ you have that 10k resistor before the 10k level pot. With a 10k level pot this essentially means that the maximum voltage out is 1/2 the available voltage because this 10k resistor forms a voltage divider with the 10k output pot and the junction of the two splits the voltage out to 1/2 the original amount and then you can only get that amount if the level pot is cranked. If you were to take out or jumper that 10k resistor before the pot you would effectively DOUBLE the amount of voltage available to the output. Replacing the 10k level pot with a 470k would make the 10k resistor have less of an affect in terms of voltage division. I don't want to do the math but it would be significant. Though it would also be noisier and probably brighter due to parasitic capacitance.

But switching the pots would mean less gain available from the opamp.

If you want more out of the circuit, jumper that 10k resistor before the level pot.

BTW, there is a similar situation in the Ross Comp and the Dynacomp, take out that 10k resistor before the 50k level pot and you will have 15% to 20% more voltage available at the output.

Hope this helped.

Jay Doyle

[Mark Hammer]

Actually, I think everyone (including Jay and Jed) is a little bit right and a little bit wrong here.

1) As Jay correctly notes, the 10k+47k pot acts like a voltage divider, which is why a 47k output pot is "louder" than a 10k pot in this case, and why a 100k pot is only a little bit louder than a 47k.  If you had good measurement devices, 500k output pots would provide a small increment to output level, but not enough to notice, really, and probably offset by the loading effect of that high an impedance.

2) As several correctly noted, there is an issue of pot taper for the gain/distortion pot that makes it inappropriate for use as a volume pot.  But is it inappropriate to sub a 47k pot for it?  IMHO no.  Why?  It made perfectly good sense to use a 741 op-amp-based pedal as both a "clean" booster and a distortion pedal.....in 1977!!  As such, a 500k reverse-log taper pot was helpful in dialing in low gains that did not produce clipping.  It does not make sense to use this pedal a your "clean booster" in 2006.  The first 420k of that pot are for attaining low gain settings that do not produce any distortion.  It's really only the last 47k or so that make anyn useful difference in distortion amount so why not just use any old 47k pot without having to assure some fancy-pants taper that assures how that 47k wilkl be distributed across the pot's rotation?  If you want "clean", make yourself a booster, or at least use a better op-amp and install a diode lift switch to reduce all clipping.  If you want the Dist+ for a distortion, then set it up for distortion the way it should be, and forget all that vintage hogwash.

3) Missing one of the clipping diodes, the Dist+ will still "work" and will still clip.  It will only clip one half-cycle of the signal, though.  That will yield an audible harmonic boost, but will also deliver a louder signal than stock because there is no ceiling on signal level imposed by the diode for half the signal.  That will also allow for more dynamics, though at the cost of audible fuzz.  Feel free to sub a silicon diode (1N914/1N4148) for the 1N270 for the time being if you want distortion.  You may even grow to like the one-of-each combination.

So, bottom line: leave the output pot where it is, but feel free to replace the 500k with a 47k/50k.

[Jay Doyle]

Mark,

You are right in that I was a little bit wrong. I forgot to take into account the taper of the 470k pot. My apologies.

BUT, I do believe that jumpering that 10k resistor, even with a 47k pot, will give you more output, about 20% more if my math is right. Though due to the way the ear works, that will probably not be all that much.

Jay