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why do my speakers hum on my computer

Speaker hum is usually due to (1) RF interference along the cable, or (2) something called ground loop. (1) can be addressed by using a shielded cable, RF chokes (the magnets you see at the ends of some cables), or using unshielded twisted pair instead of RCA (a proper adapter will break the audio signal into two duplicates, invert one, and send both over the UTP cable; any interference will cancel out when one signal is inverted and the two are added together again). (2) is tricker. It's caused by the electrical ground on one piece of equipment being at a slightly different voltage than at other equipment.

This causes something like a DC current between your equipment, which your speakers interpret as a hum. The quick and dirty fix is to use a long extension cord (with a 3-prong plug for ground) so your speaker system and computer are plugged into the same wall outlet. The fact that you're not getting the hum with your phone (it is electrically isolated from your home) suggests that (2) is the culprit. As a quick and dirty test, haul the computer over next to your amplifier and speaker system. Plug it into the same outlet (use a power strip if you have to - all that matters is that the equipment shares the same ground).

Use the same cables and see if it takes care of the hum. If it does, you can just buy a 20 ft extension cord to go with your 20 ft RCA cable. Though that may necessitate you switch to a shielded RCA cable (the unshielded cable is basically a 20 ft antenna which will pick up the 60 Hz AC traveling over your new 20 ft extension cord).
My guess - you have amplified speakers, which are plugged into the wall to power the amplifier, but input cables are not yet plugged into the MOBO's audio ports.

Several possibilities here, all predicated on the fact that when you say "the speakers work on other computers", they are being tested in another room/building, on 'another computer'. If the wall plug is in the same room, in the same building, I'm with zipzoom in suspecting the bong! 1. You have a crappy/nonexistent ground in the wall plug you're using; four dollar outlet tester available at any 'big box' hardware store will check this. 2. There is something else on the circuit (and it doesn't have to be in the room - many buildings are wired with circuits extending to two/several rooms) that is electrically 'noisy'; suspects are (test by unplugging/unscrewing/removing): anything with a motor - fan, space heater, etc. ; a bad or just 'peculiar' CFL ( ight) - some of the 'cheapies' made in China, Malaysia, et al are incredibly crude; a defective or cheap wall transformer for some random POC that, like every other damned thing these days, was designed too poorly to have an internal power supply; a light dimmer/ceiling fan speed controllerwhose rectifier filtering is 'leaky'.

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