MXR Dist+ Mods

[phillip]

I know...it's probably been done to death  BUT...has anyone tried using a 4558 in a Dist+ instead of the LM741?  I noticed that the Ross Distortion, which is another version of the Dist+, uses the 4558.  Does the 4558 sound any better than the 741?

Also, will a 1M linear taper pot work in the place of the 1M reverse log distortion pot without much difference?

Any other tips or tricks for this circuit?

Thanks!
Phillip

http://fuzzcentral.tripod.com/

[petemoore]

The one I'm using now has a 741 in it. Socket the diodes on this one and you'll be able to use the ckt to taste/test them.
 GEO sectret life of pots article is a good read using a 1meg there [audio or linear] will get the same range of resistances however the right one will have a more even attenuation, the other one works, but some bunching of it's effect at one end of the travel and little or no effect at the other end as you say turn it down...no biggie...that's how mine works, and since I set and forget it's no big deal.
  The more you socket the more you can 'semi breadboard' it, I like my input caps socketted...then after thorouugh testing [with other boxes too] sometimes switched incap value..
 You could look at the New Clipper Mod that has a 100k bias pot...Runoff Groove,
  I use my Dist + as like a 'dirty clean' booster.
 Another similar build that gets meaner [at least In My Exp] is the Ibanez Sonic Distortion.
  Isn't the Dist+ basically a Micro Amp with some diodes clipping?

[phillip]

How's the Volume control on the Dist+?  I read somewhere that even when it's wide open, it's only at unity.  Is that true?

Phillip

[RGW]

How's the Volume control on the Dist+?  I read somewhere that even when it's wide open, it's only at unity.  Is that true?

Phillip

I built a DOD OD250 which is very similiar, and immediatley noticed the limitation of the Volume control. It really depends on the type of clipping diodes you use. If you use Ge they are going to clip the signal alot sooner. The smaller amplitude of the output signal results in low volume output. I tried red LED's (which clipped at around 1.4V) and the output level increased dramatically, to the point of providing unity gain when the Volume control was at 12 o'clock. But you also get a different sound. More of a crunchy overdrive than the smooth fuzzy distortion you get from Ge diodes. I also tried 1N914's and they were stronger than the Ge's, but still had far less output than the LED's. Maybe closer to the Ge's in the "saturated distortion" type tone, too.

Another thing is the fact that some schematics show a 10K log output pot versus 100K log on others. I used a 100K log on mine, which should give a stronger output than the 10K log.

take care,
Robbie

[phillip]

If I use a 1M linear taper pot for the Distortion control, will RG's tapering resistor trick work on the Dist+ to get the reverse log taper that's needed?  

I'm not too clear on the math formula that's used to figure out the degree of tapering...could someone help me out?  Math was always my worst subject  

TIA!
Phillip

[phillip]

Also, when RG says Clockwise lug and Counter-Clockwise lug, which lug is which?  Assuming that we're looking at the back of the pot from inside the enclosure, and the lug farthest to the left is 3, the wiper lug is 2, and the lug farthest to the right is 1.

Thanks again!
Phillip

[brett]

Hi.  First, I believe RG is working from the front of the pot.  Concerning the maths - His article works on using ratios of resistor to pot values, so for a 'b' value of 3, use a resistor that is about 1/3 of the value of the pot.  (b=3 is about right for a "pseudo" log taper).  So for a 100k pot, use a 33k resistor.

Also, concerning the original question of a 4558 chip versus a 741, I believe that the 4558 is virtually a twin 741.  The improvements, if any, were not great.  For a significantly different (better?) sound from a D+, try a TL071.  It's quieter and cleaner and subs straight in there.

Cheers.

[brett]

Almost forgot, instead of 2 x 1N914 (=1N4148) diodes, try an extra germanium in one direction (ie a 1N4148 antiparallel with a 1N4148+Ge).  Lifts the output a little and gives a nice, slightly asymetric feel.  Also, some people prefer 1N4001s (or similar rectifier diodes).  Personally, I like the smoother sound of 1N4148s.  Cheers.

[phillip]

So, given the formula  

Rt = 1,000,000/5

Rt = 200,000

and a guesstimation of how the resistor should be hooked up with the Dist+ Distortion pot, would this be correct?:



THe problem is that the signal from the 5K6 resistor is going into lug 1 in the Dist+ (CW lug if viewed from the front of the pot), while the wiper lug and lug 3 (CCW if viewed from the front) are connected together.  This is opposite of how RG's is drawn, with the signal going into the CCW, and the wiper and CW tied together.

Phillip

[Chris Goodson]

Hey Phillip, did you figure out how to get the 1M linear taper pot to work right?  I put one of those in mine and would love to be able to make work the way it should without having to order another pot.  I read the Geo article but I'm still not sure what to do. :oops:

[brett]

Philip, thanks for the diagram.  To me, the diagram looks good.  So it's just a matter of what lugs where.

I'm a bit fuzzy-headed today, but shouldn't lug 1 should go to ground?  Not to the 5k6 resistor.  Then, maximum resistance (about 180k) will be achieved when the wiper is turned fully counterclockwise (contacting lug 1).  And minimum resistance (and max gain) achieved when the wiper contacts lug 3 (at earth).  I might be wrong (lack of coffee), but try it this way.  If I'm wrong, it should be easy enough to unsolder and swap the wires to the two lugs.

cheers

[J. Luja]

the problem with that setup is you've only got 180k max of resistance now, so your minimum gain will be much higher than if you had just used a 1M pot. you could however adjust the value of the cap and feedback resistor to achive the same frequency response and gain of the original.

the real problem is that what you want is a variable resistance that decreases quickly and then more gradually as you turn the pot clockwise. using a tapering resistor only works if you need a resistance to increase quickly and and then more gradually with a clockwise turn

so ignore the first paragraph, because unless i'm mistaken, it's just not gonna work

[RDV]

To make a fake reverse 100K for the EA Tremelo speed control I used a 10Meg resistor and still only had a resistance of about 98K or so(though this worked fine).

Regards

RDV

[Mark Hammer]

In the grand scheme of things, the pot taper does NOT matter.  It will NOT change the sound or working of the pedal.  What it MAY change is your ability to accurately dial in the sound you want, but even that depends on what sorts of sounds you like it best for.  The illustrated mod for adjusting taper may help some folks stretch out the zone of favourite tones to a larger proportion of the pots rotation (as opposed to having only 10 degrees of rotation be useful and the rest moot).

The gain of a non-inverting amp is given by the formula

(feedback resistance + resistance to ground)/(resistance to ground)

You can see from that that unless the feedback resistance is zero, the gain will always be greater than 1.  Indeed, the classic non-inverting unity-gain buffer has the input and output shunted (zero ohms) and *nothing* between the inverting input and ground [zero+infinity divided by infinity = 1].

Anyone who has ever foolishly tried to turn a Dist+ into a "clean booster" by lifting the grounds comes away sorely disappointed, because the Dist+ can never BE "clean" due to the 741.  In this respect, it is an implicit double clipper, in the same league as the Big Muff, since the signal leaving the 741 on the way to the clipping diode pair is *already* clipped.  If you switch to a "higher quality" op-amp, what you end up with is a tone that is more purely derived from the clipping diodes.  That is neither better nor worse,merely a different tone.  What you gain is potentially the ability to have a clean booster by lifting the diodes.