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Hum and buzz when plugging in an amp. Annoying and frustrating. Where is it coming from? Is it the git, the amp or something else?
Many articles have been written on this. Doesn't hurt to re-hash once in a while. This is a step-by-step process for locating and eliminating hum, starting with easiest-to-check.
TEST FOR GROUNDING ISSUES
Touch the metal jacket on your guitar cord when it's plugged into the amp. If the hum vanishes you likely have a grounding problem somewhere.
TEST THE CORD
Poor guitar cords are notorious for creating hum. Try a different cord. If hum still persists, you may want to invest in a quality, shielded cord. They cost a bit but there's a reason for that: they help eliminate external sources of hum. If the hum disappears it's money well-spent. If not, it's still money well-spent.
TEST THE AMP
Try a different amp if you have one and see if the hum disappears. If hum vanishes, you just located the source without much trouble. In one instance I thought the hum was my guitar; it turned out instead to be the wall wart (AC Converter) that fed my Roland MicroCube. How did I find out? I unplugged it and ran the cube on batteries and the hum vanished. Another way would be to try other wall warts and see if they do better (once I went through six wall warts until I found one that worked properly. They are notoriously poor in manufacture). If your amp doesn't use a wall wart it may be something in the internal wiring. That's a bit trickier and out of my ability to trace (without risk of painful or fatal electrical incident). Usually in such case I just buy a new amp. Unless it's an expensive amp it'll cost as much to have the amp repaired.
TEST THE INSTRUMENT
Test two or three gits if you have them and see if the hum remains, vanishes, or changes. If the hum remains across all gits, then that almost insures the problem is either with the amp or the environment (unless of course, you have 2 or 3 problem gits-- not likely). If it's the instrument, see below on how to correct that problem.
TEST LOCATION / ENVIRONMENT
If hum persists, eliminate the environment as a possibility. Hum could be caused by something in your home or workshop. I've heard of entire houses that had hum in the wiring itself... unstable electric flow.
First use an outlet ground tester to make sure the outlet is grounded. If all shows green you're good to go. If it shows ungrounded, you'll need to ground your outlet-- or switch to another outlet that is grounded.
If your outlet is grounded, take the git and amp to another location and see if you still have the hum. First move to other rooms and test the system. If the hum persists move to another place entirely... a local park with electric outlet or someplace totally away from your home area where you can test your guitar and amp. If the problem is in your home-- your entire block or area of the city up to the central transformer could potentially be affected. Eliminate the environment as a cause. If you go elsewhere and the hum disappears, you'll either need to just get used to the hum or invest in a hum eliminator device (plugs between your amp and wall socket to stabilize voltage and eliminate hum).
Do you have fluorescent lights? Those little twisty bulbs? Computer equipment nearby? Heavy electronics nearby (microwave, television, etc). Those are known sources of RFI (radio frequency interference). You can try isolating your music setup from proximity to those,
HOW TO FIX A WONKY GIT
If the hum problem turns out to be the CBG... something isn't connected properly somewhere. You'll want to make sure items that need to be grounded are connected to the negative (center) pole of your guitar jack-- either directly or through a central grounding wire.
* Pickups. "Humbuckers" are so named because the double-coils naturally inhibit hum. If you're using a single-coil pickup (common with CBGs) the pickup itself could be the source of hum. Google "shield coil pickup" for lots of information on eliminating pickup hum.
* Pickup cover. Some pickup covers are metal and have to be grounded. If they're not they can cause significant hum. Solution: ground to the pickup ground wire.
* Sound wiring. Sometimes the instrument wiring is simply hooked up wrong. Double and triple-check the wiring layout and see if all the wires are where they're supposed to be.
* Strings. If you have a metal bridge or the strings run across metal somewhere (such as the tailpiece), that's easy to fix. Just run a ground wire straight from the jack to the metal. If the strings don't touch metal you may need to add a metal strip to the tail or bridge and ground to the jack in order to ground the strings. They're steel / nickel and are basically horizontal antennas... very sensitive to interference. The tuners are also metal; you could probably use your git to pick up television signals in a pinch. So ground the strings.
* Fancy do-dad. If you have a license plate or other metal decoration on your git that's any larger than a bottle cap, it could be it is picking up interference. If there are metal decorations (especially LPGs), ground them.
* Faulty pot grounding. Sometimes a ground solder on a pot may look good but actually not be grounding correctly. In other cases the pot itself is burned out and not grounding correctly. In some cases the pot was faulty right from the factory (I've had that happen twice). If you've tried everything else and it still hums, I recommend:
a) Unsolder and re-solder all ground connections on the pot (being careful to not get the pot too hot).
b) If it still hums after that, replace the pots one by one and see if one of them was faulty.
THE FINAL, LAST-DITCH OPTION
If you've done all of the above and hum still persists, yet it goes away when you touch the guitar cord jacket... procure for yourself a "grounding wrist band". You can buy these online or at computer stores in the form of "anti-static wrist bands" or make one yourself out of a loop of flexible wire or mesh and a gator clip. When playing, wrap it around your wrist or run under your shirt, then clip it to grounds somewhere on the guitar. That should solve the hum problem no matter where it's coming from. (Note: I am not responsible for sudden unforeseeable amp electrical kickback that results in loss of future generations.)
That's pretty much the extent of my knowledge in this area at this time. If anyone has anything else to add, please do. In my experience if you do all the above, chances are the hum is going to vanish at one point or another and your problem will be solved.
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