[skypilotpete]

I have my computer speakers some distance from my computer, so I have to use an extension cable. I use a standard 3.5mm audio extension cable. However, I am being driven nuts by intermittent interference in the form of an electronic hum through the speakers. Sometimes it doesn't happen for ages, then it just suddenly starts. If I move the various cables behind the computer around, I eventually get it to stop, but I can't work out what is interacting with what to cause the interference. It has just occurred to me that perhaps the extension cable should be specially shielded in some way as it is surrounded by a whole lot of other electrical cables.

Can anyone advise me on this?

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I am afraid you are a victim of "eddy currents" - assuming you do not have a faulty sound card. These are magnetic fields generated when electricity flows through a conductor and they certainly do cause interference in adjacent conductors and circuits. You are absolutely correct that shielded cables is a solution, but there may be other solutions you can try, and should check first.

• Ensure all wall power outlets are properly wired and grounded - use an AC Outlet/Ground Fault Indicator Tester.
• Make sure all connections, including the sound card, if not on-board, is securely fastened to ensure maximum conductivity.
• Replace any broken or worn connectors and frayed wiring.
• Ensure all connections are clean of dust, dirt, and skin oils.
• Separate the routing of power and "signal" (audio and video) cables as much as possible - especially the audio cable from the computer to the speaker's internal amplifier input as that audio signal is small, yet to be amplified, and susceptible to interference. Ensure the audio wires do not run near power transformers, monitors, fluorescent lights, or other interference generating devices. Wires feeding the actual speakers should not be an issue as that signal is amplified and much less susceptible to interference.
• Where separation is not possible, cross the wires at sharp angles (near 90°) - this cancels the effect, and is the theory behind unshielded twisted pair (UTP).
• Replace the entire cable length with a single cable of the length needed - no extension cables. These extension cables are generally of "low-fi" quality, the two extra connections means added resistance and opportunities for interference, and just the extra cable length degrades the signal.
• Remove the extension cord and move the speakers closer - you may need to do this just for testing purposes to see if problem exists without the added length.
• Use shielded audio cables with quality connections and construction.

[Digerati]

I am afraid you are a victim of "eddy currents" - assuming you do not have a faulty sound card. These are magnetic fields generated when electricity flows through a conductor and they certainly do cause interference in adjacent conductors and circuits. You are absolutely correct that shielded cables is a solution, but there may be other solutions you can try, and should check first.

• Ensure all wall power outlets are properly wired and grounded - use an AC Outlet/Ground Fault Indicator Tester.
• Make sure all connections, including the sound card, if not on-board, is securely fastened to ensure maximum conductivity.
• Replace any broken or worn connectors and frayed wiring.
• Ensure all connections are clean of dust, dirt, and skin oils.
• Separate the routing of power and "signal" (audio and video) cables as much as possible - especially the audio cable from the computer to the speaker's internal amplifier input as that audio signal is small, yet to be amplified, and susceptible to interference. Ensure the audio wires do not run near power transformers, monitors, fluorescent lights, or other interference generating devices. Wires feeding the actual speakers should not be an issue as that signal is amplified and much less susceptible to interference.
• Where separation is not possible, cross the wires at sharp angles (near 90°) - this cancels the effect, and is the theory behind unshielded twisted pair (UTP).
• Replace the entire cable length with a single cable of the length needed - no extension cables. These extension cables are generally of "low-fi" quality, the two extra connections means added resistance and opportunities for interference, and just the extra cable length degrades the signal.
• Remove the extension cord and move the speakers closer - you may need to do this just for testing purposes to see if problem exists without the added length.
• Use shielded audio cables with quality connections and construction.

[skypilotpete]

Thanks for a fantastically detailed response!

[Digerati]

Let us know how it turns out.