You can cure this with a ground wire from an output jack to the console, or by a jumper at the patchbay from a shield of the device to a shield from the console. When All Else fails, Isolate There are some connections that will hum, no matter what. Many devices are simply not up to the specifications of professional audio, but they must be used just the same. Guitar amps and computer sound cards are common offenders this way. The worst part is, sometimes patching one of these bad boys into the system can make noise appear everywhere.
Isolation transformers are the only solution.