Phase Switching
This mod will take any two pickups that normally sound full and rich and turn it to a thin and shrill sound.  Why would you want that?  It's great for reggae or funk where you need a thin sound.  Also, this sound will cut through a lot of effects or distortion that would otherwise make your tone too muddy.

Incidentally, Brian May from Queen has a phase switch for each pickup on his signature guitar (although this is a bit excessive, to be honest, as one is adequate).



So how does it work?
This is roughly what the signal from a pickup looks like.  The x-axis is the time and the y-axis is the amplitude (volume) of the signal.
Now, when two coils are activated together, the signals you normally want to hear (those produced by the guitar strings) are added together, but if you wire the pickups out of phase with one another, they cancel out all of the sound produced by each except for the differences between them (the difference in the sound between the two pickups).

This subtraction/cancellation is why the volume drops when you have phase cancellation.


Phase switching with a 3-pickup Strat-style guitar
You would perform this modification to affect the middle pickup of a guitar with 3 single coils (e.g., a Fender Strat).

The illustration shows how to put a phase reversing switch on a push-pull pot (I don't have the pot wired in this picture; it can serve any function).  You don't have to use a push-pull pot as any DPDT (double pole-double throw) switch will work.

The bottom configuration (the way it would be if you had the push-pull pushed) is the normal phase (what I call "in phase").  The top (pulled) configuration produces a phase cancellation between coils.

The (-) wire is the black one.  The (+) is usually red.  The wiring running to the 5-way switch goes to the same place on the switch as the middle pickup was originally connected (I didn't bother drawing this as different guitars have different types of switches).



Phase switching with a guitar with humbuckers
Just follow the same scheme as above with the two leads from your pickup.

Phase switching within a humbucking pickup:
This modification can be performed on one or several humbucking pickups within the same guitar.  The theory is the same, it is just that the phase switching is performed on what would be considered half of one pickup.  The sections below describe the wiring within individual humbuckers.

Series, out of phase
This scheme throws the two coils out of phase with one another.  There is less output produced than standard wiring because the pickups are effectively canceling one another out except for the differences in their respective sounds (which isn't much if the two are close to one another).
 
Side by side comparison

At left is the standard in-phase wiring.  At right, the bottom coil has been electrically reversed to be out of phase, but the connections are the same.

Parallel, out of phase
This is much the same as the above wiring scheme.  Once again the coils almost completely cancel one another out.  This is useful for more exotic applications, but nothing you would use all the time.

 
Side by side comparison

At left is parallel in-phase wiring.  At right, the bottom coil has been electrically reversed to be out of phase, but the connections are the same.


So how many phase switches do you need?
You really only need one less than the total number of pickups on a guitar.  For example, if you have a standard Les Paul, you only need one phase switch for the two pickups (unless you're going to throw in coil taps... but that's a whole other story).  Similarly, if you have a Strat, then two phase switches would be adequate for achieve every possible phase combination.  Remember, if you reverse the phase of two pickups, then they're right back in phase with one another.

Now, as far as I'm concerned, if you have a Strat, then you would be perfectly fine to only install one phase switch for the middle pickup.  That's the pickup that is used in combination with the other two on a stock Stratocaster.  If you do a pickup add-in or a parallel blender, then, yeah, you will get the neck and bridge pickups, but do you really want to go overboard?  Okay, I do a lot on this site, and even I haven't installed more than one phase switch on a guitar.  Save your time, energy, parts, labor, and control cavity space for more useful mods.  I think there are a few on this site that might work for you.



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