How do humbuckers work?

Humbuckers are great for their warm, powerful tone, and best of all they "buck" the hum that single coils are usually are prone to.  60-cycle hum comes from electronics that surround us every day like florescent lights and power transformers, and it sure doesn't sound very good when it comes through an electric guitar amp.  That's why humbuckers were such a valuable invention when music was getting louder in the 1950's, but how do they work?

Humbuckers are basically two single coils arranged side-by-side (fig. 1), each with six magnetic poles or slugs (one for each string).  They are wired in a way that cancels the unwanted frequency and keeps as many of the good frequencies as possible.

fig. 1

Most modern Stratocaster guitars (fig. 2) have a humbucking option built in.  The middle pickup is reverse-wound/reverse-polarity to the neck and bridge, so that when they are used in combination they will cancel any hum.  This only works because the two coils are so far apart and the harmonic frequencies are so different, the only quality that is the same about them is the hum, so when the two coils are opposite, they cancel the hum. 

fig. 2 the middle and bridge or middle and neck pickups are hum-canceling together

 Humbuckers function a little differently than this. Since the coils are so close together (as close as they can possibly be without being on top of each other), they are sensing very similar sounds and they need to be out of phase to cancel the hum.  Humbuckers consist of two coils (both wound counter-clockwise) with opposite magnetic poles facing up, usually wired in series.  

Some people don't like how humbuckers aren't as clear or bright as single coils, but what a lot of people don't realize is that there are many different ways of building humbuckers to change the tone.  The original Seth Lover PAF humbucker (which is the baseline against which all other humbuckers are judged) consists of 5000 uniform winds per coil (not scatterwound), bar magnets mounted underneath, adjustable poles on one side, slugs on the other, topped off with a nickel-silver cover.  All of this makes for a pretty dark sounding pickup.  Which might sound great in certain guitars, but some guitars beg for bright, twangy tones.  You could use scatterwound coils (for a brighter, clearer sound) and solid Alnico polepieces much like a Strat pickup.  This is the formula I used for my Apex Humbuckers.

fig. 3 my Apex Humbucker

You could also have an underwound humbucker (say with 4000 winds per coil) for a brighter tone, and you could wire these coils in parallel (instead of in series) for pickup with a clear tone and absolutely no hum.  

So there are lots of different options when it comes to getting different tones out of humbuckers, they don't always have to be dark sounding.  Most modern humbuckers come with 4 conductor leads so you can split the coils, and you can look up a wiring diagram here to experiment a bit.  Knowing your color codes of your pickup wires will be very helpful too, here is a link to color codes of a number of popular humbucker manufacturers.

Eliminating electrical noise in your guitar

For many of us who love the sound of single coil pickups (eg. classic Strat or Tele) we are familiar with 60 Hz hum, radio stations, and cell phone signals coming through our amp.  This is electrical interference that our single coils are especially susceptible to, and yet we refuse to give up single coils, we just love the tone too much!  Humbuckers are a great way to fight this 60 Hz hum, if you have ever played a guitar with a humbucker and a true single coil you can easily hear the difference, but a lot of players complain that too much tone is lost when the hum is "bucked."  The truth is, you can stick with your old single coils as long as you follow a few DIY procedures that are guaranteed to reduce the noise to an absolute minimum.  Even if you have humbucking pickups these steps will help reduce the overall noise of your rig.

Step 1.  Shielding Paint

With the pickguard and electronics removed from the guitar, you can paint shielding paint from Stew-Mac on the inside of the control cavity and the pickup routs.  Two or three coats (allowing it to dry overnight between coats) should provide enough coverage.  All grounding material must come in contact somehow with the back of the potentiometer (ground).  Connect a ground wire from the paint to the back of a potentiometer or paint up to a screw hole so the paint comes in contact with the copper tape on the back of the pickguard (see step 2).

Step 2.  Copper Tape

This tape (also from Stew-Mac) is great for sticking to the back of pickguards and totally blocking out any interference.  The tape is grounded by coming in contact with the switch and potentiometer casings.

Step. 3  Leads

Keeping all electrical connections to an absolute minimum length will reduce the chances of interference.

Also, twisting the positive and negative leads from your pickups (as seen on this Jazz Bass pickup) will help cancel interference.  It is debatable whether this makes a noticeable difference in passive equipment, but it won't hurt, some like it for aesthetics and wire manageability.

Step 4.  Final details

Having a good quality, shielded instrument cable free of cracks and keeping it as short as possible will help tremendously in avoiding interference.

further reading:

Eliminating Troublesome Hum & Buzz Created By Electric Guitars by Bruce Bartlett