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I am experiencing a hum in my audio line from my computer. I suspect it's from a ground loop. My monitor (which house the speakers) is a 2 prong plug is plugged into a separate outlet from the power strip of which the PC is plugged into. I thought this prevents a ground loop but I would still like to test.

I just got a multimeter, and I have no idea how to use it. All the examples I see on ground loop testing are not for PC/Monitor jacks.

Can anyone give me a step by step. I have this multimeter: Amazon Link

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  • \$\begingroup\$ Nothing to do with electronic design. Question should be closed. \$\endgroup\$ – Leon Heller Jan 23 '12 at 18:04
  • \$\begingroup\$ @LeonHeller I was border line on this question. I decided the answers would be equally the same if the question were to read "I designed XYZ and I think it has a ground loop, how can I find if the noise I am getting is from a ground loop?" \$\endgroup\$ – Kellenjb Jan 23 '12 at 18:46
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It's not a ground loop. It's something else. A loose connection, a broken connector or cable, or just crappy equipment. Also, plug everything into the SAME power strip. If that doesn't cure the problem then it absolutely isn't a ground loop.

"Ground Loops" almost never happen in the real world, and they absolutely don't happen with normal equipment plugged into the same wall outlet (via a power strip).

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  • \$\begingroup\$ Why do you think so? I've been told that ground loop are a serious problem in measurement system, at least to take into account. \$\endgroup\$ – clabacchio Jan 23 '12 at 20:32
  • \$\begingroup\$ @clabacchio Ground loops form when the GND connection on one piece of gear is at a different voltage potential than another piece of gear and that gear is connected via a communications cable that is not isolated. This would cause significant current to flow over the GND wire inside the cable. If the gear is connected into the same outlet, and the gear is not putting lots of current on the GND connection of the power cord, then the voltage potential of the GND on the gear will be at almost identical levels and very little current will flow on the GND wire inside the cable. \$\endgroup\$ – user3624 Jan 23 '12 at 21:30
  • \$\begingroup\$ Maybe we are talking of 2 different things. What I know is that a ground loop is when you have a ground path or plane that forms a closed loop that acts like an antenna, taking noise from outside. That's why grounding shielded cable has to be done accurately, and ground paths in PCBs should be separated as far as possible. \$\endgroup\$ – clabacchio Jan 23 '12 at 21:41
  • \$\begingroup\$ I checked wiki and it says your same thing...so what I know must have a different name, but it's been presented to us as ground looping... \$\endgroup\$ – clabacchio Jan 23 '12 at 21:45
  • \$\begingroup\$ David, I think (but its not really clear) that OP said the equipment with the speakers "is plugged into a separate outlet from the power strip of which the PC is plugged into". If the two outlets are not connected to the same home run back to his breaker box, a ground loop like you described does seem possible. \$\endgroup\$ – The Photon Jan 24 '12 at 2:18
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3.5mm audio jacks include 1 or 2 signal lines and a ground. Hopefully there isn't any looping happening on the sound card or amplifier and speaker assembly.

I think you're hearing the magnetostriction effects from either the 120 Hz rectification or the ~kHz switching, from one or all of the PC power supply, the monitor power supply, or by way of induction from the environment, meaning 12VAC cords, nearby equipment or wiring in the walls, to the preamp section (sound card, cord, etc.). This can be verified (or contested!) by measuring the frequency of the hum with the meter's frequency counter feature.

Try the monitor with another computer; try it with an outlet on another circuit (ie: supplied through a different breaker) -- probably not the same wall. Check and play with the power supply connections (12V, 5V) to your motherboard, and, if they're separate cards, the sound and video card supply connections. Wiggle the PCI[X, e] connection, too, if the sound or video card are separate. This is just looking for poor power contacts. Minimize the distance the video line (PC to monitor cord) lies parallel to nearby power cords. Use a digital video interface standard like DVI (in digital mode, since it can carry analog VGA, too), DisplayPort or HDMI instead of VGA (analog) for error protection and correction.

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