A common issue when mixing pickups from different manufacturers is when they don't play nicely together - the volume drops and the tone becomes thin and nasally. So here's a brief overview of the reasons for that and what can be done about it.
What Is Phase?
In very simplified terms, in regards to the signal generated by a pickup, phase can be used to describe the direction of the waveform of that signal when the string is moving in a particular direction.
When two pickups are in phase, the signals combine to reinforce each other. When they are out of phase, they cancel each other out.
The reason we can still hear something from two out of phase pickups is that they generally won't be the same specification, and as they are in different positions and therefore generating slighty different signals, only parts of the two waveforms will cancel each other out.
What Affects Phase In A Pickup?
There are two factors that influence the phase of a pickup:
The four combinations of these two factors give us the following phase polarities:
How To Test The Phase Of A Pickup
Any digital multimeter with a voltage setting can be used to test the phase of a pickup.
If the pickup isn't installed in a guitar, attach the black connector to the ground wire, and the red connector to the hot wire.
If it is installed in a guitar you can still test it: Either plug in a guitar cable and connect the red connector to the tip of the jack plug, and the black connector to the sleeve. Or, simply cut the end off a guitar cable and peel back the insulating material to access the two conductor wires. The black connector attaches to the outer shielding, the red connector to the inner hot wire. Plug the cable into the guitar and make sure the pickup you want to test is selected, and the volume control is fully open.
Set the multimeter to the smallest voltage range (usually 200mV), and place short length of some ferro-magnetic material across the poles of the pickups - a steel shafted screwdriver is perfect for this.
With one eye on the multimeter, pull the screwdriver up sharply. The multimeter should indicate either a positive or negative number - this indicates the phase polarity of the signal being generated by the pickup when a string is moving away from the pickup.
How To Change The Phase Of A Pickup
To swap the phase of a single coil, we generally change the direction the current is flowing through the coil by swapping the hot and ground wires over where they attach to the pots or switch.
Be aware that single coil pickups with covers or baseplates will need those covers or baseplates to remain connected to ground. If your hook up lead has three conductor wires, the yellow cloth push-back or bare wire will generally be the ground wire for the cover or baseplate and should remain attached to where it was, only the black and white wires need to be swapped. If it only has two conductors, some modification of the pickup will be required.
To swap the phase of a humbucker, we can change the magnetic polarity. Remove the cover if it has one, loosen the baseplate screws, slide out the magnet, rotate it 180 degrees and slide it back in place. Re-tighten the baseplate screws, replace the cover, and you should be good to go.