A: Generally speaking, there is nothing in a guitar that actually generates hum, generally you can assume that the hum is being induced into the guitar, or it is being added to the signal of the guitar.
Experimentation is your friend. Here are some possibilities â€“
RECEIVING BROADCAST HUM
There is something in the environment which is â€œbroadcastingâ€ RFI in the room where your equipment is located. Common sources of Radio Frequency Interference include motors (vacuum cleaners, refrigerators, and automobile distributors), and transformers (high-intensity lamps, fluorescent lights), and from big magnets like speaker coils or television sets. The sound from autos are likely to vary in frequency. The sound from lamps and lights and refrigerators are likely to be be consistent, and at 60 cycles per second, which is what we normally call â€˜hum.â€™
The RFI can be picked up either by strings (antennas) or by pickups (coils) as the signal is induced into the circuit created by the guitar and its parts, or into the cord (usually not possible if cord is properly shielded on both the guitar and the amp end), or into the amp.
The RFI is then *not* filtered by the humbucking pickups. Normal hum *is* filtered by humbucking pickups. In our shop, when we build the Mobius Megatar Tapping Basses, we do our lab work two feet under a fluorescent light, just to â€˜hearâ€™ if there is a problem, and this hum is normally filtered out.
So the best way I know to get an idea that strong RFI in the environment is some part of the culprit is to get the hum going, and then change the orientation of the instrument. If you hear hum while the instrument is flat on the table, but not when itâ€™s upright, or if you hear hum while the instrument is upright facing east but not when itâ€™s facing north, then probably there is a strong RFI source in your environment. Remember that it may be behind a wall or a ceiling or floor. Wood and sheetrock is no barrier to Radio Waves.
If the instrument seems the same in all orientations, then consider the cord and the amp. Try moving them to a different room or part of the room.
Sometimes you will hear hum, and then you touch the strings with your hand and the hum vanishes. Or you touch the (metal) tone/volume knobs, and the hum goes away. Iâ€™ve been told this has something to do with the â€˜capacitanceâ€™ of the human body, and I think that means that the body soaks up alternating signals. But thatâ€™s too vague. It doesnâ€™t get us anywhere. I suspect this is a meaningful clue; I just donâ€™t know what it means.
A GROUND LOOP
Hum can be caused by something called a â€˜ground loop.â€™
We can think of an electrical power plug as have a left and a right prong, or we can think of a signal having a signal and a ground wire. If youâ€™re not using two amps then the question of their having the same side (left or right) as the â€˜groundâ€™ should not be an issue, though it is *possible* that an input jack on your mixer *might* be reversed from the others ones *if* repair work was ever done on the mixer.
Likewise, if some of the inputs on the mixer are for â€˜line levelâ€™ like synths, and others are specifically built for magnetic inputs like guitar, or others are specifically for mikes (and either with or without phantom power) then it is possible that all inputs are not equal. Sometimes there are switches associated with the inputs.
HUM SPECIFIC TO TAPPING GUITARS AND BASSES
On specialty tapping instruments, like the Chapman Stick, Warr Guitar, and Mobius Megatar, usually two outputs are provided. One for the bass pickup(s) and one for the melody pickup(s).
In Mobius Megatar and other instruments that Iâ€™ve seen, the bass and the melody have the same ground.
Normally, the ground is common throughout the guitar â€” The pickups cavity is grounded all around, the pots and pickups and outjack are all grounded to the same place. Even the strings are grounded to the same place. Therefore, the stereo cord goes into two mono cords, and the two mono plugs â€” one for bass, and the other for melody strings â€” have the same ground.
On Chapman Stick or a Mobius Megatar tapping bass, these output mono jacks, have signal on the metal tip, and ground on the metal sleeve of the jack, and itâ€™s the same ground all the way back to the pickups and guitar components and the shielded cavity.
Therefore, unless one uses a mutant and miswired stereo cable, the sleeves of the two mono jacks should both be ground. A voltmeter connecting the two sleeves should show zero resistance.
HUM COMING FROM MIXER OR AMP WITH TWO MONO CABLES
But if a hum is coming from any mixer or amp when two common-ground mono cables are plugged into two inputs, it might be suggested that there is either (a) a 60-cycle induced hum that has been induced across the two signal tips; or (b) that the signal/ground is reversed on one of the mixer/amp signal paths relative to each other.
This article is not capable of analyzing or diagnosing the device you are using, but a fast way to find out if thatâ€™s the source of trouble is â€” carry your Megatar or Chapman Stick to Guitar Center. plug it into a bass amp, and plug it into a Guitar amp, and play it. No hum? Then there is nothing wrong with the guitar or the cable.
Get them to stop the kid playing Stairway to Heaven with his amp set to eleven while you do this test.
ELECTRICAL OUTLETS AT YOUR HOUSE OR ON THE GIG
The electrical outlets in the wall are â€™spozed to have proper grounding. However â€¦ maybe they donâ€™t. Even if they have three prongs, maybe somebody just stuck those part on the wall and maybe that third, grounding wire isnâ€™t connected up to an actual ground.
If you have more than one amp or effects, best to plug them all into the same outlet. Or carry your own multi-outlet strip.
An easy way to create a ground loop in the power is to have two devices that have two prong plugs. Plug one in rightways and the other one reversed. Just about nearly always youâ€™ll hear hum, and with the right equipment you can shock yourself seriously. Not even a joke.
Plugs these days are â€™spozed to have one fat prong and one thin one, so they cannot be reversed, but it doesnâ€™t always work. And remember â€¦ some human may have wired up that plug. Oops.
SINGLE COIL PICKUPS
Single-coil pickups are part of a radio-receiver circuit. Remember the coil of wire you made when you built a crystal radio in cub scouts? Signals can be induced into coils rather easily. They are natural â€˜receivers.â€™ Any source of RFI is likely to sound through single-coil pickups. Darn.
Specialty touchstyle basses like the Chapman Stick, Warr Guitar, and Mobius Megatar, designed for two handed tapping, will usually have the instrumentâ€™s gain turned higher than a standard guitar, because weâ€™re just tapping gently on the string instead of strumming like all get out.
Turning the gain up increases the signal, but also boosts the background noise, including hum, as well, so dealing with hum may require a bit more attention, if you want a quiet sound.
SORRY INSTRUMENT DESIGN
Cheap instruments, in some cases imported strat knockoffs, sometimes do things so sloppily that they do not have any shielding around the electronics. The cavity around the electronics and pickups should have a solid conductive material all around these components. Itâ€™s called a â€˜Faraday Cageâ€™ after Michael Faraday I suppose, and it keeps those nasty vibes from annoying the components.
I have heard, but donâ€™t know whether it is true, that the basic Fender Stratocaster design has ground-loop errors in the basic design. I am dubious. Not Leo! But for certain, a guitar maker could make a blunder, I suppose. If itâ€™s on one guitar but not another, take it to a whiz guy.
The teeny signal is running on a wire across (to the signal) a vast distance to get to the amp. Lots of signals in the air could disturb it, but it is â€™spozed to have a big fat woven wire all around it â€” again a Faraday Cage â€” protecting it from bad vibes. And this big fat woven wire would optimally be connected to ground on the tapping guitar, and to ground at the amp.
But is it?
A fancy name and expensive price tag may not be the best cable. However, again, a trip to Guitar Center with the noisy guitar and cable, and if you try some of their cables and the noise goes away â€¦ itâ€™s a definite clue.
[This post adapted with permission from the Reducing Hum article at the Two-Handed Tapping website.]