Not sure when the hum started, but I know that it wasn't always there. The speakers are Harmon/Kardon desktops and there is an accompanying subW. The hum is not responsive to volume regulation, nor is it responsive to shutting down the computer and the monitor. The only thing that stops the low hum is to turn the speaker off or to unplug it. The speaker's plug is plugged into the same power source as the computer. Moving the speaker's plug to different positions on the power bar, makes no difference. Hum still there. All cable/wire hookups are in good shape.
Does the system use a conventional (analog) plug from the computer audio connector to the speaker system? (I.e., not USB or optical-digital)? If so, it may have come loose or developed a bad connection at one end.
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What they said. Also could be the power supply for the speakers is failing, and letting too much ripple from the AC mains into the analog amp circuitry. Do these speakers have a separate power brick, or is the power supply inside the sub?
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I have mine running into a splitter. I one pair that is 2.0, then another pair that are newer that is 2.1. The older pair is over a decade old. The power is in the speaker on the older pair. I bought the newer pair because I thought the older pair was going out, but then they turned back on eventually.
Another thing to look for is analog signal cords running parallel to power cords for any decent length. It's fairly easy to induct a 60Hz hum into a speaker if it's simple bell wire up against a power cord.
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On my home theatre setup, the subwoofer automatically turns itself on when it gets a signal from the receiver and turns itself off after not getting any signal for a little while. This means that after turning the receiver off the subwoofer is still on for a minute or so with the lead to it acting as an antenna so it hums. When the receiver is on I have no hum as it is actually holding the output to the subwoofer at 0V.
I wonder if the same thing may be happening with your setup?