After seeing the quoted passage below I dont think that the long leads are the cause of the issue.
Originally Posted by yiancar
i get noise / hum on the left channel ( balanced amp btw) which is not affected by changing the volume. when i move the amp case off the psu case the noise dissapears . i use r core transformers and the transformers are directly below one of the two amp bored of each channel
no its not shielded. i cannot understand how noise is picked up only from the right channel, i mean both the psu and amp are symmetrical.
So when you move the amp away from a device that one could safely assume radiates EMF the noise goes down.
Not much question what is wrong.
Build a long umbilical, like you should have as the only real reason to build an external PSU, and put the PSU 6ft from anything you care about.
Alternately you could try lining the lid of the PSU with mu-metal foil, but for the cost I would not bother.
Why one channel and not the other: one of your transformers leaks more flux than the other.