You discovered for YOURSELF one of the OLDEST sore spots for the Phonograph, the Cart and it's associated wiring are the WORST for picking up spurious signals. they are the BEST antennas for radio frequency signals ever invented. Though you might find some success looking at the things that are mentioned by the other people here, and they are not wrong in looking for the source of the radio signal, your real problem is the Cart. while these other people are spending all of their time looking for the source of the signal, yes, they are looking for sources of radio freq signals BUT, without you touching the lamp, thus creating an antenna, you wouldn't have the problem. Those offending source signals are whiteout harm or offense without YOU as the transmitting antenna. They can not propagate a signal W/O an antenna.
So I was changing a lightbulb last night…
I had just finished listening to a record and decided to change a lightbulb that had gone out directly above my turntable. When I started unscrewing the bulb I noticed a faint buzz coming from my speaker. I then turned the volume way up on my amp and tried again. Turns out the buzzing was happening when my hand touched the metal light fixture, not the lightbulb.
At first I thought the tubes in my phono stage were picking up an EM field from the light fixture and out of curiosity I grabbed a piece of foil and covered the phono stage and then tried tapping the fixture again, same buzzing. Then I switched the input on my amp to my DAC and tried the tapping, no buzzing. Switched the input back to the phono stage and tapped the fixture, buzzing continued. Then I covered the tonearm with the foil and the buzzing went away almost completely.
So it appears my tonearm is picking up some sort of signal from the light fixture but only when I touch the fixture. If I turn the lights off there is no buzzing when I tap the fixture. The setup is in the basement and I use LED lights that are recessed in the ceiling and wired to an “LED” dimmer switch. The audio gear is on a dedicated circuit.
Any of the knowledgeable folk on here have an explanation for what’s going on? Doesn’t effect SQ AFAIK since the buzzing only occurs when I touch the metal fixture. Seems odd but I thought it was interesting and maybe a chance for me to learn something from the members.
I uploaded a video of this happening to Imgur that I’ll try pasting here:
Cheers