MXR Phase 90 speed pot backwards

[tom42107]

Hello, I just finished the General Guitar Gadgets MXR Phase 90 Script Logo pedal and it worked right off the bat. I etched my own pcb and added the required jumpers. The problem is the speed knob is backwards. Faster speed counter-clockwise and slower speed clockwise. I know this is an easy fix, or is it. I question it because I followed the wiring diagram to the letter and it worked right away, other than the speed knob. I did use the 500k reverse log pot. The diagram shows the the back of the pot labeled 3, 2, 1, left to right, which I assume is correct. Do I change lug 1 to lug 3 or do I swap lugs 1 and 2? Just not sure what happened. Thanks.

[Mark Hammer]

Sometimes, that is actually better.

The relationship between the resistance setting of the Speed pot, and the actual speed of the LFO is not a linear one.  That is, the difference in speed between a pot setting of 400k vs 500k is not the same as the difference between the pot set to 101k vs 1k.

This is complicated by human perception.

Hearing a relatively smooth change in speed as you turn the knob, requires a pot that changes resistance in a logarithmic fashion.  If you want the pot to increase speed as you turn it to the right, then the pot should have an anti-log or C taper, so that it decreases resistance from max fairly quickly, and once you've gone past halfway, smaller and smaller changes in resistance are produced.

If you use a log, or A taper pot, you get the reverse; resistance changes slowly, initially, and small degrees of rotation produce larger and larger changes in resistance as you go clockwise.  This is how one would wire up a speed control to make the LFO slower as you go clockwise.

Neither of these is right or wrong, just different, and either counter or congruent to the user's expectations.  Since most folks expect somethng labelled "Speed" to go faster as you turn it clockwise, a C taper is preferred.

Assuming you are staring the shaft in the face, and the lugs are pointed downwards, with a C/anti-log taper, the lugs of principal interest would be the one in the middle, and the one on yur right, such that, as you rotate clockwise, the resistance is getting smaller (and the speed faster).  If you are using a log/A=taper pot, the lugs of principal interest are the middle, and the one on your left, such that resistance gets bigger as you go clockwise.

[tom42107]

Thanks Mark for the quick reply. Based off of what you said, and the fact that I have a C log pot, I need to move lug 1 wire to lug 3.

[Mark Hammer]

Yup.

Note that, realistically, only twolugs are really required.  However, over time, it has become considered a good practice to wire up the remaining lug to the wiper, such that the "unused" lug can serve as a dependable source of continuity.



The wiper is a moveable contact whose electrical/physical contact with the resistive strip is subject to many sources of physical obstruction and discontinuity.  Should one or moroe of those sources cause the wiper to momentarily lift off the resistive strip, breaking contact and then restoring it, you would likely hear that as crackling or some other sort of noise.

When the unused lug is connected to the wiper, you now have two points of contact with the pot on that "side".  The connecting of lugs 1 and 2 makes them electronically identical when everything works fine, and renders lug 1 effectively redundant.  Of course, should some dirt or whatever get inside the pot and the wiper momentarily loses contact with the resistive strip, lug 1 will still have your back and maintain contact, suich that you go from whatever the initial resistance was to 500k and not from that first resistance to open circuit.

This is not a required practice, just a good backup plan.

[tubegeek]

When I wired a "speed" pot backwards for anti-log behavior, I labeled it "sloth."