Diy helph

Hi,
Ive just put together an amp / preamp combo. Every thing sounds great but it has an audible hum when the amps input cables are connected to the preamps outs, the hum is not effected with the volume, its a set volume.
The weird thing is if i connect either a left or right channel only via the cable theres no hum?
Its only when both channels are connected.
Anyone got any ideas?
Regards
Mike