So, I've rescued my old Pioneer deck from the loft. It used to get its power from a Pioneer amp that was part of a stacking system but that's long gone. I've bought a 12 volt dc power supply and configured the power connector to be centre negative which is correct for this deck. I've also bought a pre amp from eBay to provide the correct phono power levels. Hooked it all up and connected it to my sound bar and there's an annoying 'hum'. There's no ground pin on either the deck or the pre amp but I've found a ground point inside the deck and wired that to the case of the pre amp - no difference. I've even tried adding an external earth connection to this lead and if anything, it's worse. The case of the pre amp doesn't appear to be connected to the circuit board anywhere so I suppose connecting ground to the case is a bit pointless. Any ideas ?