Guitar Wiring Explored - Humbucker Internals

In the previous article, we took a first look at adding switches to a guitar. The mod we made was a simple “add neck pickup” switch for a Strat. But one of the main reasons players add switches to their guitars is to explore the tonal versatility offered by four-conductor humbuckers. Before we can start looking at those mods, however, we need to learn a little about what makes humbuckers tick.

First let’s take a quick look at a single-coil pickup:

There are some magnets (the pole pieces) and a very long coil of wire wrapped around them. The string disturbs the magnetic field generated by the magnets and this induces a voltage change into the wire. By then attaching each end of that coil of wire into our guitar circuit (through the white and black wires pictured), we get that sound into our amp. Usually, single coil pickups have the South pole of the magnet pointing at the strings. The exception is of course RWRP pickups, which are of course North polarity.

Now let’s look at a humbucker.

 

It’s two single coil pickups stuck together right? Well, very nearly, yes. We have two coils, and each coil has a wire attached to each end of it. On most traditional humbuckers, one coil has “slug” pole pieces, and the other has “screw” pole pieces. The magnetic field, however, is North polarity on the slug coil, and South polarity on the screw coil.

What four-conductor wiring gives us is a wire for the start and end of each coil in the humbucker. The colors used for this in a Seymour Duncan pickup are as follows:

 HumbuckerColorCodes.gif

If you’d like to see what colours other pickup manufacturers use, there’s a handy diagram here.

In normal humbucking mode, both these coils are active. However, they are not connected in parallel, as we have been doing with our individual pickups up until now. They are connected in series. So our path from ground to hot starts at ground, goes around one coil, then around the other coil, and then out to hot. So to get this with our four-conductor humbucker wire we just connect the end of one coil straight to the beginning of the next one, right?

Well, nearly. In order to achieve hum-cancellation we would expect one of the coils to be reverse-wound. But actually both coils are wound the same way. It’s an easy fix though – we just reverse the “start” and “finish” wires, which pretty much amounts to the same thing. We go around the North coil the other way. Because the magnetic polarity on the coils is opposite to each other, reversing the wiring direction through one of the coil has the effect of reversing the voltage from that coil twice in relation to the other coil – so the string signal is in phase across both coils.

Of course, the fact that the wiring direction is reversed is also what gives us the hum cancellation – the hum is induced in the coil by external fields, which aren’t affected by the pickup’s magnetic polarity. This means that the hum is reversed only once in one of the coils, and when the two signals are connected together, it means that the hum cancels itself out.

The one other thing we need to be aware of is that there’s always a bare wire in there, in addition to our four coil wires. That’s for shielding and grounding purposes, and always goes to ground.

Knowing all this, we can now see what we need to do to turn our “two pickups stuck together” into a regular humbucker. We use green (and bare) for ground, then connect red and white to each other, and then black is our hot output. If we do that we can use the humbucker just like any pickup with only two wires. If that’s all we’re planning then we would also tape off the red and white wires so they didn’t short out against anything else in the circuit. If we ever get a “single-conductor” humbucker, all these connections are made internally, such that the hot output is the wire in the middle and the ground connection is the outer braided metal.

However, it also leads us to the point where we can easily work out how to “split” the humbucker – that is, to make it so that only one coil is active. This is a good way of achieving single-coil tone and output level and is the most popular wiring modification for humbuckers. Of course, with a pickup like the P-Rails, it’s actively designed to be split as part of normal use.

To start with, we’ll look at how to leave just the slug coil active. For this change we don’t need to disconnect the red and white wires from each other. All we need to do is have our switch wired so that, when we activate it, it grounds both of those wires. This cancels out any voltage difference between the green and red wires (the start and end of the screw coil), meaning that the screw coil is now having no effect on the sound.

We’d wire it like this – a DPDT switch either in the form of a mini toggle or a push/pull pot.

 201_Diag_2.png

The red and white wires from the humbucker are connected to the switch to allow the coil split. As usual, the green and bare wires are grounded, and the black wire is used as the hot output from the pickup.

Note the use of the grounding symbol in this diagram – that simply means you would connect the wire to any grounded point – usually the back of a pot.

I hope this article has taken the nystery out of those four wires coming out of your humbucker cable – and inspired you to think of your humbucker as more than just a single pickup.

 

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