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Note: This article applies to 110 / 120v electrical sources common to the United States. Higher voltage sources in other countries would need to refer to electronics specialists in their area.
There it is again... that annoying hum from your guitar amplifier. What is causing that? It seems to come and go. Sometimes it's louder, sometimes softer. Is it the guitar, the amp or something difficult to identify?
CAUSES OF HUM
There can be many reasons for hum or noise in a piece of equipment (listed here in order of "easiest to identify and correct"):
* Jack-related ground loop issues
* Electric line ground loop issues
* RFI / EMI (radio frequency or electromagnetic interference)
* Bad line power ("noisy" electricity)
* Bad instrument or amplifier
... and stuff that really is difficult to identify. Fortunately the most common causes of amplifier noise are easy to identify and correct-- although it may cost you a bit to fix it.
JACK-RELATED GROUND LOOP ISSUE
This is very easy to test. Plug a regular guitar cord into your amp (but not into your instrument). If you hear hum / noise, unplug the cord. If the noise vanishes, you have a jack-related ground loop issue.
Fixing this problem is reasonably low-cost (about $20 or so). You will need to purchase an instrument/microphone ground loop eliminator. Models are made by Pyle, Behringer and other music companies. You plug your instrument cord into this device and then run another cord to your amplifier. If the hum is instrument or cord related, this will very likely correct the majority of the problem.
Note that this may not fix issues in which there is something wrong with the instrument (an internal short or grounding issue). For that you'll need to have the instrument inspected and repaired. But if you tested with a raw guitar cord and the problem disappears when the cord is unplugged from the amp, a jack ground loop eliminator will very likely be the solution.
ELECTRIC LINE GROUND LOOP ISSUES
Testing for this issue is easy if your amplifier has a 3-prong electric cord. (If your amp has a 2-prong cord, you can skip this section.) Sometimes interference from "ground loop" causes noise problems (other items on the line).
IMPORTANT! FOLLOW THESE INSTRUCTIONS FULLY! To test for this use a common 3-prong to 2-prong adapter (a little plug you can find in almost any store). If you plug the amp in using this adapter and the noise vanishes, you have an electric line ground loop issue. Whether the hum vanishes or remains, once the test is performed (2 seconds or so), immediately turn off the amp and remove the test plug. A 3-prong amp should never be operated with a 2-prong adapter for any length of time. This is for testing purposes only.
Resist the urge to keep using this easy and inexpensive device to fix the issue. There is a reason your amp has 3 prongs; using a 2 prong adapter for a length of time can cause grounding issues and result in blowing out your amp or (worst scenario, however rare) starting a fire or severely shock the player. Just don't do it.
What you will need in this case is a power ground loop filter, which can take care of such problems. It's not an inexpensive fix ($60 and up depending on your source) but it will most likely fix this electrical problem.
RFI/EMI ISSUES
Anyone who has ever lived near a radio or TV station or near a power generator will likely have experienced this kind of problem. This can also happen in areas with florescent lights, heavy machinery, microwave ovens, computer equipment, large video monitors, or other sundry electrical equipment that can produce radio frequency or electro-magnetic interference.
There are many solutions for such, depending on the source, direction and seriousness of the interference. Shielded cables in your instrument and all cords (including the power cord) may be required. Many guitars are already shielded from RFI/EMI, but many aren't. You can shield them by opening the electronics area and lining it with aluminum or copper tape (available in electronics stores or online).
If you are working with an acoustic or other non-metal bridge guitar, chances are your strings aren't grounded. (Sometimes the bridge/strings aren't grounded in standard electric guitars, if it was built incorrectly or a wire came loose. Always check for string grounding.) In an acoustic, remove the strings and saddle, drill a small hole in the saddle slot and run a grounding wire underneath the saddle (between the saddle and bridge) to your guitar jack. Then replace the plastic or bone saddle with a metal one (brass or aluminim). Metal provides an excellent saddle and will ground every string. Metal saddles to match your guitar are avaialble from most online guitar pro shops.
In the case of an amplifier an inelegant solution is to cover it with aluminum foil. A slightly more elegant solution is to build a "Faraday Cage"-- which is basically a mesh-wire box to surround most of your amplifier. Either method may block interference and allow your amp to work noise-free. You will still need a good-quality shielded guitar cable and power cable.
Note that these are measures for "extreme" situations of electrical interference. Less-extreme is to try to locate the source of interference and increase the distance between that source and your music equipment.
NOISY ELECTRICITY
Sometimes noise is caused by a bad electric line. Basically the quality of electricity coming from your socket is unstable, causing erratic behavior. In such instance the best option is to have an electrician come out and check your house, because such problem can shorten the life of all of your electric appliances.
But for your instruments and amps there are several solutions. The least expensive are voltage regulators or a standard UPS (uninterruptable power supply)-- basically a computer battery backup. A voltage regulator tries to clean up your energy line and can smooth out your current. A UPS puts a battery between your electricity and amplifier, which also acts as a voltage regulator. A UPS is also the very best way to prevent electrical surges and even direct lightning strikes. (There are recorded cases where the UPS was disabled or even melted down, preventing damage to other equipment.) I personally have UPS devices on all of my sensitive electronic equipment.
However, either of these devices may be insufficient for really bad line noise. For such noise you may need a professional musician's line noise filter, which can run a hefty $200 or more. There are many options in this area, so shop around and see what works in your case. Starting with the inexpensive and work up if necessary. Purchasing from a retailer that allows returns in case of failure might be a wise and low-cost solution. If you can get by with a voltage regulator or UPS, that will be your least-expensive answer to this problem.
INSTRUMENT / AMPLIFIER ISSUES
Both instruments and amplifiers degrade with age and need repaired or replaced. Some items are simply built poorly or have a problem develop during the manufacturing process (a weak solder point, bad part, etc). If you perform the basic tests listed above and the noise still persists, you may need to have your equipment tested for noise issues.
This is most easily done through the process of elimination: keep replacing equipment (cords first, instruments, amplifiers) until you find the piece of faulty equipment. Always test cords first; it's amazing how often a problem is simply a matter of a bad connector cord.
An electric guitar can be tested by plugging a different electric guitar into the same amplifier, using the same cord, and seeing if the noise vanishes. If it does, your guitar is the issue.
An amplifier can be tested in the same manner. Have a friend bring over his/her amplifier and test it in the same environment, same electric outlet, same guitar cord and instrument. If the noise persists it's not likely to be the amplifier. If it vanishes you'll have a good hint your amplifier needs repaired or replaced.
SUMMARY
Most causes of hum fall into four areas: jack-related ground loop, electric source ground loop, RFI/EMI problems, equipment issues. These can be tested and fixed at various degrees of expense. Start by performing the easiest tests to isolate the source of the hum / noise and correct that source once it is found.
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