Originally Posted by NutFlush
1 When you connected just the Oppo to the amp were there any other cables connected to either the amp or the Oppo? It is vital to disconnect all cables from the two components. If you leave any connection say from the amp to your set top box you may have a ground loop going through the set top box (in through the cable from the amp and out into the antenna or through the power plug even when switched off, because the ground is NOT cut off even then).
2 Also make sure the subwoofer is disconnected (not just turned off).
3 This should leave you with: 2 power cables, 1 audio cable from Oppo to amp, several speaker cables. Nothing else!
4 Are you using 2 power cables that both have a ground? Does the amp have a ground receptable for the power cord?
5 Does your amp allow a "ground lift" (physical switch or maybe in the setup)? What model is it anyway?
6 Try placing the OPPO as far away from the amp as possible. Maybe the amp induces a current in the OPPO.
Originally Posted by markg
1. By "amp" presuming you mean headphone amp as the Oppo would never go to the power amp directly. With only the 1 pair of analogue RCAs from Oppo to headphone amp, its all fine (when the power amp is on too). The STB issue was what seemed to occur in scenario 2. I might try disconnecting STB and antennae entirely though just to eliminate that possibility.
2. Yes, was disconnected
3. Are you suggesting trying with Oppo and power amp on, cable to headphone amp (but headphone amp turned off(, speaker cables and that's it? That's what I had last night + inputs from the Meridian to the power amp, which I had not disconnected. But it was this scenario that worked ok.

I thought you use a home theater amplifier = "amp". Are you using the headphone amp as a pre-amplifier that bundles the signals from CD, STB, LP-player, ...? If so then disconnect anything that is attached to the headphone amp except the Oppo and the power amp. Put all 3 devices into the same power board and turn them on. Is there any humming? If it's gone try adding the other components. Start with the STB. I bet that as soon as you connect it to the antenna you'll hear the hum again. If this is the case you need a ground filter for your antenna cable. You can get this for little money in any electronics shop (maybe 20-50$).
Originally Posted by markg
4. Yes as far as I know (3 pins, Aust plugs); 3 pins on the IEC end too. Ground receptable?

I meant: does the socket on the amp where you plug in the power cord have 3 pins also? If so then the chassis is grounded by the 3rd cable inside the power cord.

For now I wouldn't use a ground lift (it's not a real disconnection but rather a resistor & capacitor combo; more info here: http://www.epanorama.net/documents/groundloop/groundlift.html ). Try the antenna solution first even if it means spending some money, because that would actually remedy the problem!

Enjoy! smile