This 20ft Premium RCA Audio Cable offers high-quality construction to ensure crystal-clear audio for your home theater system or audio applications. The RCA Audio Cable features gold-plated connectors, a durable yet flexible PVC Jacket for easy installation and excellent strain protection. Backed by Lifetime Warranty, our Premium 20ft RCA Audio cable is guaranteed to provide long-lasting performance.
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).