MXR Distortion + problems

[brianisgood]

I built this:
http://www.aronnelson.com/gallery/main.php/v/DRAGONFLY-LAYOUTS_0/album17/album06/mxr_dist_plus_001.jpg.html

After a long day of debugging (ending up in checking against the circuit diagrams), I think that it's wrong and there should be a break in the track just below pin 4 of the IC (on the 2nd strip up from the bottom).  Anyway, making a cut this fixed mine and it's great now.  I'm convinced that this is an error in the image but can it really be??? -  this image has been in the gallery a long time and I've not found any complaints.

My next problem is the lack of a reverse log 1M pot for the drive. I have the same problem reported many times with only the last 10th of a turn doing anything. OK - so I've read 'Secret life of pots' and anything else I can find on how to make a 1M linear act like a reverse-log BUT I just can't get it working!  This pot has the middle AND end grounded.   I have been using a 220k resistor and tried all combinations of wiring on the pot and although I did get one strange setup where the graph would have been U-shaped (sound at both ends but not in the middle) i can't make it work at all. 

Please don't redirect me to more 'secret life of pots'-type pages - I'm sure I've read them all.  Please just let me know how I can wire this 1M linear pot and make it more like a reverse log (if it is at all possible).  Thanks 

[Mark Hammer]

Use a log pot and wire it backwards, so the resistance gets large as you rotate clockwise.  That will give less gain going clockwise, but with the same degree of change per degree rotation as a reverse log pot.

Alternatively, do you really want to use the circuit for clean-ish boost?  The requirement for reverse log is because you don't start getting any diode-clipping until you get the pot resistance down to around 50k or so.  Reverse-log simply gets you through that first 950k quickly, leaving the remaining 50k distributed over a good chunk of the remaining pot-rotation.

If you have no need for anything clean from this pedal.  Just stick a linear 100k or 50k pot in there and be done with it.  That will work like a 1M reverse log that has already been turned halfway up.

[brianisgood]

Thanks Mark,

Does this mean that it's not possible to simulate a log pot when two of the pot connections go to ground?

I think I'll try a smaller pot.

With 1N270 diodes, I get a great sound but it does reduce the boost/output a lot - if I reduce the 10K resistor at the output (near the Vol pot on the image) will this increase the output?

thanks!

[Fender3D]

With 1N270 diodes, I get a great sound but it does reduce the boost/output a lot - if I reduce the 10K resistor at the output (near the Vol pot on the image) will this increase the output?

thanks!

With Ge diodes you get ~0.3 Vpp, the 10k resistor+ the 10k volume pot will half the signal when at max volume...
you may increase the volume pot value to 100k and get more output signal (always within those ~0.3Vpp range)

Again the original D+ had a 500K distortion pot, as Mark said you can decrease its value at will...

[brianisgood]

Many thanks - I'll try this later today!

[brianisgood]

Sorry if this appears stupid but I can't see how increasing the value of a pot will let more signal though.  My simplistic view is that when a pot is at zero resistance, all the signal gets though - is this wrong?  It would be great if you could explain this in simple terms for me.

[served]

More resistance is between the Ground and Signal source. If the value is too low, more signal goes to ground. If the value is very high, no signal goes to ground and everything goes to output.

[Fender3D]

Murphy's law is widely better known than Ohm's law indeed...

In this case your signal passes through 10K + 10K (resistor and pot) and ground;
Picking it at junction, you have half signal Vout=Vin*10K/(10k+10K)=0.5Vin
With my greater pot value you have Vout=Vin*100K/(10k+100K)=0.91Vin

[brianisgood]

I'm a bit more confused now.

Is it essentially because the second part of the pot (R2) is larger so less leaks to ground?



e.g. in a 10k pot with R1=0, R2 = 10k, R2 lets more through to ground than in a 100k pot R1=0, R2=100k, R2 being greater prevents as much leakage?

[GibsonGM]

Yup. A resistor will always allow SOME signal thru, but a large one will let much less get by...

[brianisgood]

Excellent - I understand now.  Thanks!

Where should I post about the MXR Dist + vero being incorrect? (I'm sure it's missing a track cut)

[Mark Hammer]

The "Gain" pot on the Dist+ provides more gain  the smaller its resistance is.  The resistance in question is that between the wiper and one OR the other outside lug.

Why does the schematic show 3 lugs being used?  Simple.  It is traditionally considered a good practice to insure against the potential for noise produced via intermittent contact by placing a wire between the wiper and the other outside lug.  Obviously, the wire connection between wiper and outside lug contributes no resistance.  However, should the wiper momentarily go out of contact with the resistive strip inside the pot, the maximum resistance will never be more than the value of the pot.

This practice is not absolutely mandatory.  If you see a PCB layout with three holes for a pot, and two of them are in contact with each other, you have the choice of running three wires to the pot...OR two.  Three is safer, but two is easier.

[brianisgood]

Thanks Mark - that's great to know. That's the kind of info. I've searched for but have been unable to find.

I just tried lots of pots on the Dist + and settled on a 470k Linear.  The log pots got a smooth transition when used backwards but I would never be happy with it working that way.  I also tried setting up a linear pot as a reverse log as in 'secret life' but it just isn't the same.  I'm not sure it works at all but it coulkd be the 47k interfering with the equations. I'll try and get a reverse log (I know it will annoy me until I do...).

I also tried bypassing the 47k resistor and got some nice screaming at full gain - presumably the amp is being pushed too much.  This explains why I get a whistling from my LM308 Rat at full gain (funny, it doesn't do it with a TL071 or 741!). 

Thaks again for you help!

Andy

[zombiwoof]

I don't know what kind of pots you are using, but Small Bear has 16mm solder lug reverse audio in 500k and 1M, here:

http://www.smallbearelec.com/Detail.bok?no=116

The 500k would be close enough if you like the 470k linear you are using.

Al

[brianisgood]

I've searched and can't find any here in the UK - I'll look at the shipping charges from Smallbear now...

thanks!

[Mark Hammer]

Thanks Mark - that's great to know. That's the kind of info. I've searched for but have been unable to find.

I just tried lots of pots on the Dist + and settled on a 470k Linear.  The log pots got a smooth transition when used backwards but I would never be happy with it working that way.  I also tried setting up a linear pot as a reverse log as in 'secret life' but it just isn't the same.  I'm not sure it works at all but it coulkd be the 47k interfering with the equations. I'll try and get a reverse log (I know it will annoy me until I do...).

I also tried bypassing the 47k resistor and got some nice screaming at full gain - presumably the amp is being pushed too much.  This explains why I get a whistling from my LM308 Rat at full gain (funny, it doesn't do it with a TL071 or 741!). 

Thaks again for you help!

Andy

The whistling is a characteristic of the 308 apparently (assuming it is what I am thinking of).  There is a nice article in Art Thompson's "Stompbox" book where the designer of the Rat explains the origins of the Rat, and the peculiar characteristics of it.