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I just bought an acoustic/electric mandolin (Ibanez M511SE-DVS) that I decided to test out with the only guitar amp that I have (a Baja BG-10). It works fine, but I noticed that when I have my finger on the volume knob, I receive (what is most likely) an FM signal to a local radio station.

Given that I don't have a background in electrical engineering, in layman's terms what are the reasons that a guitar amplifier would pick up a radio signal and feed it back through the output (speaker)? This behavior only happens when I touch the volume button.

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  • \$\begingroup\$ Your body acts as an aerial. Replace the volume knob with a plastic one, and repeat the experiment :D \$\endgroup\$
    – Laszlo Valko
    Commented Oct 11, 2014 at 20:30
  • \$\begingroup\$ See electronics.stackexchange.com/questions/71727/… \$\endgroup\$
    – pjc50
    Commented Oct 11, 2014 at 20:54
  • \$\begingroup\$ @pjc50 I've already read that question, but there is a primary difference between mine and theirs - I only pick up radio signals when I touch the device, specifically the volume knob. I know from that question that it's receiving radio waves arbitrarily, but why they feed back when I touch a specific knob is what I'm interested in. I edited the title and question body to make this more clear. \$\endgroup\$
    – Chris Cirefice
    Commented Oct 11, 2014 at 21:01
  • \$\begingroup\$ When you touch it, the antenna becomes large enough to receive the signal clearly enough for it to be discernible/audible. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Oct 11, 2014 at 21:04
  • \$\begingroup\$ @IgnacioVazquez-Abrams I figured that out, but why my body acts as an extention is what I'm questioning :) \$\endgroup\$
    – Chris Cirefice
    Commented Oct 11, 2014 at 21:05

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The capacitance from the volume control knob through the pot is large enough to allow RF to couple through - from your body, when you touch the knob - to the volume control circuitry, where it gets rectified/detected (turned into an audio signal) and sent through the rest of the amp to the loudspeaker.

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