MXR distorsion plus gain pot wiring

[stomp bomb]

hi, I'm trying to rebuild the Mxr distorsion+ but I have some problems with the 1M rvlog pot, I don't exactly know how to wire it.



I mean I wired the signal coming from pin 2 of the IC to the first pot pin (with the pot pins pointing down), and the second and third pin to ground. is it right?

thanks

[Mark Hammer]

How does a potentiometer work?  The shaft of the pot moves the wiper (labelled '2' in the picture) around.  As it goes clockwise, the resistance between 1 and 2 is increased, and the resistance between 2 and 3 is decreased by that action.  How MUCH those resistances are changed by X degrees of shaft rotation is what is meant by pot "taper".

In the case of the Dist+, we're only concerned about using one of those resistances, but which one.  We want the gain to increase as we turn clockwise.  The design of the Dist+ uses the so-called "ground leg" of a non-inverting op-amp to alter gain.  As that resistance goes lower, gain is increased.  That means we want to use the resistance between lugs 2 and 3, since clockwise movement reduces that resistance.

Were you building a Tube Screamer, that fixes the ground-leg resistance of a non-inverting op-amp, and elects to use the feedback resistance instead to vary gain.  In that instance, increasing that resistance increases gain.  So in that case, one would use the resistance between lugs 1 and 2 to vary gain in a manner whereby gain would increase as you turn the knob clockwise.

The punchline is that it is always good practice to imagine the pot and whether rotating in a given direction produces the desired change in resistance.

[stomp bomb]

so I have to connect log 2 of the pot to the signal coming from the IC and connect log 1 and 3 of the pot to ground?

[Mark Hammer]

Ignore lug #1****.  It makes no difference to the circuit whether lug 2 goes here or 3 goes there.  What you're making use of is the diminishing resistance between them.  Remember, if you were to replace the pot with a fixed resistor, it wouldn't make any difference which way the stripes went on that resistor, would it?  And in this application, lugs 2 and 3 are being used exactly like that.  If it was being used as a voltage divider (like a volume pot), THEN we'll talk about which lug goes to where.  But for this Gain control, it doesn't matter.


****It is considered a more "professional" practice to wire up lug #1 directly to lug#2 as a sort of backup, and you will often see this in circuit diagrams.  But in this context, it's completely safe to just ignore it.  The circuit is only interested in the resistance between 2 and 3.  If you were building the pedal for a big ticket client who was taking it on a world tour, I suppose you'd aim for the more professional practice.  But then, you wouldn't be asking the questions you're asking, would you? 

[stomp bomb]

got it.
thanks man

[Rob Strand]

Click to expand image:

[Mark Hammer]

There you go: "As seen on TV!".