Instrument Repair

Why would a guitar amplifier emit a loud humming sound whenever you touch your thumb to the tip of the phone plug?



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This is normal on guitar amplifiers because of the very small signal from the guitar pickup one will need a high gain pre amplifier to boost the signal to drive te main amplifier, if you touch the tip of the phone plug you are actually feeding a 50Hz or 60Hz signal that is picket up by your body that act as an antenne for the magnetic fields from the 50 or 60Hz mains, that is then amplified by the preamp to a large enough signal to drive the main amp. Also: In terms of electromagnetic fields, the human body is oddly noisy. The hum you get from most single coils is actually from the player, this is why touching the strings (they are metal and connected to the bridge piece which is grounded) can make the hum go away. This is part of it.