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When I plug my combo practice amp into a normal wall outlet I pick up a local public radio station, just loud enough to be annoying. The only way I've found to reduce it is to plug it into a UPS or to wrap the input cable around the amp. Anyone have a technical explanation of why this happening and/or any solutions?

Edit: Crate combo amp (pretty cheap), and Epiphone Les Paul with Humbuckers. Tried testing again and plugging into UPS doesn't complete eliminate radio, but does diminish it. Radio sounds still happen when guitar and cable are NOT plugged in. Implying the problem must be in the amp itself and not necessarily from the cable/pickups acting as an antenna - although I can't imagine that helps.

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Does it happen when the guitar isn't plugged into the amplifier? –  Anonymous Jan 14 '11 at 2:45
Guitar cord could be acting as an antenna and picking up signals at a frequency that resonates with the amp speaker...when you wrap the cable around the amp, the signal constructively interferes and you hear nothing. the UPS most likely gets rid of extraneous signals? not sure...was a physics major (shame)! –  Jimi Oke Jan 14 '11 at 2:51
"it's the power cord picking up the signal!" That's not very likely. The powercord is carrying 115 VAC which would overwhelm any radio signal received. The guitar cord, which carries the signal from the guitar to the amp is a more likely culprit as a poorly shielded cable would act as an antenna, with the radio signal being amplified by the pre-amp. Replacing that cord would (dis)prove that theory. If the sound occurs without the cable plugged in I lean toward it being the amp itself, or its grounding. The UPS helping could be because of grounding issues too. –  Anonymous Jan 14 '11 at 9:35
Incidentally, this exact thing can be heard at the end of RATM's Sleep Now in the Fire — see en.wikipedia.org/wiki/Sleep_Now_in_the_Fire –  Anonymous Jan 15 '11 at 21:39
The difference might be in the A/C wiring of the apartment. OLD buildings often didn't have grounding, or, if it was added as a retrofit, was done wrong. Though the outlet has a ground and the amp does too, if that ground is not connected or was wired wrong, you can get noise. You can also get electrocuted too, which is why it's always good to carry an A/C line tester, and check the outlet before you plug in. Getting shocked hurts, and can kill. –  Anonymous Jan 16 '11 at 20:02

3 Answers 3

Sounds like your amp's shielding isn't good, as radio interference shouldn't happen like that.

You don't say what brand it is, but you really shouldn't hear interference/noise/radio from the amp, except maybe for hiss if your gain is turned up.

A good amp technician can check all the solder joints and make sure the grounding is good. They can also add additional shielding to help cut out radio waves.

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Shielding is likely the issue. Radio interference occurs due to lack of shielding or poor grounding of anything from the guitar to the amp, to even pedals. I remember when I built a custom fuzz pedal and screwed up the ground wiring to the case of the pedal. When I plugged it into my chain I was suddenly listening to latin Polka. When I removed it the radio interference was gone. –  Jduv Jan 16 '11 at 17:21
"...I was suddenly listening to latin Polka." Nothing like the blistering sounds of shredding concertina set to 11. Maybe some of the components were "Hecho en Mexico"? :-) –  Anonymous Jan 16 '11 at 19:57

Does this happen when your guitar is plugged in? If so, is your guitar use single coil pickups?

If both of the above are true, then you are witnessing one of the common fallbacks of a single coil pickup, and one of the main driving factors for the creation of the humbucker dual coil pickup.

A pickup is just a magnet surrounded by coils of wire, where when a string is plucked in a pickup's presence, an alternating current is generated, essentially producing a audio signal. Consequently, due to the coils of wire, the pickup also acts as an antenna, making it liable to picking up stray radio frequencies (usually producing a hum). To rectify this issue, the dual-coil humbucker was introduced, where two magnets are placed in series with reversed polarities. Since the coils are connected in series and out of phase, noise and interference are significantly reduced via common-mode rejection, reducing the amount of hum you hear.

Not sure if I'm on the right track here, especially since plugging the amp into a UPS helps fix the issue...

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Not true at all. My amp picks up stations regardless of if I use my Les Paul or my Telecaster. It has nothing to do with pickup type. –  Jduv Jan 16 '11 at 17:19

Usually when Ive picked up background interference it was due to poorly wound coils in my guitar pickups. If u talk near the pickups an you can hear your voice through the amp that's a good sign that it is the case.

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Why would plugging in to a UPS reduce the problem if it was poorly wound pickups? –  Anonymous Jan 14 '11 at 9:29
Also, I suppose that hearing your voice through the amp when you talk close to the guitar has to do with the (mechanical) wave generated by your vocal cords just slightly vibrating the strings, since your voice alone can't do anything with the magnetic field generated by the pickup. Radio waves are eletromagnetical waves, a whole different realm just to start with. –  Rafael Almeida Jan 17 '11 at 20:45

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