One good source for understanding hum problems is the rec.audio.pro FAQ. (link?)
The section on audio interconnections
explains ground loops and various strategies for fixing them.
Here are some strategies that can help:
- Use only balanced (3-wire) connections, with all shield wires disconnected at one end to avoid spurious signals travelling through the shield. This will usually give better results but it's expensive and sooner or later you need to connect to some unbalanced
device, like your computer soundcard.
- Since you will probably end up using some unbalanced (2-wire) connections, try connecting the chassis (metal case) of ALL your equipment together with some big copper braid or other very-low-resistance wire. The theory here is that this provides a path of least resistance for spurious signals travelling through ground wires, so they stay out of the audio signal cables. It may also help, once you've done this, to disconnect each audio cable's shield at one end, since that ground path is now redundant and can only make things worse.
Here are some more reader tips on ground (earth) loops: