I installed a Banana Pro (like Raspberry Pi), plugged an usb audio stick in, and a microphone in it. I run this thing with a properly dimensioned usb power supply (tried different ones). This way, there is a constant strong humming noise in the audio signal from the microphone. This happens with two similar of those installations.

When I connect the outer part of the usb receptacle with ground, that noise disappears completely. Other parts may work as well, but usb is easily reachable.

For me this raises these questions:

  • Is this a typical phenomenon? What's causing it? Can I practically do something in order to avoid the cause?
  • If not, is it a valid workaround to actually connect it to ground? Or can that be a dangerous thing?

Added details:

The stuff is assembled this way:


It doesn't contain more (though BananaPro has some more ports). 230VAC comes from the wall socket. As PSU I tried two with 1.5A, one with 2A, both with a usb socket output. They are all 'ungrounded'. One was bundled with mobile phone, I don't know details about them, and also not about the usb audio stick. The microphone is then plugged in via typical 3.5mm TS jack.

The (*) is the place I connect to ground of the wall socket in order to make the hum disappear. To be exact (but I'm unable to paint it that detailed): The outer metal hull of the plug/socket.

  • \$\begingroup\$ First thing would be to try a different power supply. \$\endgroup\$ – Chris Stratton Jul 29 '18 at 16:20
  • \$\begingroup\$ ... what I did. \$\endgroup\$ – ginger Jul 29 '18 at 16:21
  • \$\begingroup\$ But what else is connected? Presumably you have an amp and speakers. Do these have a mains PSU? Is it ungrounded too? \$\endgroup\$ – Transistor Jul 29 '18 at 17:02
  • 1
    \$\begingroup\$ Add a diagram of your setup and be clear about where you put the wire to stop the noise/hum. \$\endgroup\$ – Andy aka Jul 29 '18 at 17:30
  • \$\begingroup\$ yes, it's common, and yes, a common ground will fix it. that's why old audio equipment (esp phono pre-amps) had ground screws on it. if you remove the galvinic connection between the devices, like running the laptop on a battery, the noise will go also away. The common ground you found is a perfect way of dealing with poor wiring's effect on audio. \$\endgroup\$ – dandavis Jul 30 '18 at 17:40

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