Hum with VTL's

I have a Wadia 7 & 9 Transport/DAC/Preamp and a pair of VTL 450 monoblocks all fed by a PS Audio P600.Using the balanced connections there is a very loud hum coming through each channel (does not vary with volume). It does not accur using the RCA connections and also stops when using the balanced connections (toggle switch on bal) but
the RCA cables also plugged in. I read somewhere about checking the continuity from the earth pin to the phono ocket body and both my amps have continuity. I would be grateful for any advice on where to go next with this. Having both the balanced and RCA cables plugged on stops it but I'm not sure if I'm curing the problem or the symptom.
Using the RCA cables grounds one chassis to the other. Using the balanced evidently does not. Either the balanced sockets are floating on one or both ends or your balanced cables don't have a connection from end to the other for the body of the shells which would ground them. If using the RCA cables grounds everything and that solves the hum then I don't see why you can't do that. If all the equipment has three prong AC plugs then that would also ground them together.

BTW, your equipment draws more current than the P600 is designed to provide.
You have a ground loop. Try to connect everything to a outlet strip then use another outlet once. this fixed my prob.
Herman Thanks for advise.
I've tried different cables and they are the same so it's not them. Does this mean the balanced sockets should all have a grounding to the chassis of each commponent and I have one or more that does not have this connection ? Could this be a design issue or would it be a defect ?
Similarly as all my components have 3 pin mains have I lost earth continuity to mains on a component. If I have a design conflict between components which my bridging with he RCA solves then I can live with that I just don't like the idea of having something that is not right or has a defect !!
On the P600 it normaly shows an output reading of 450-500 Watts as it's supossed to be a 600 watt unit I thought it should be OK ? Mind you it does get a bit warm !
Many Thanks Ade
Evidently there is a ground path missing somewhere without the RCA cables. There is no standard for XLR connectors or balanced cables. Some have a line connected to chassis ground and some do not. Evidently somewhere in your system it does not since using the ground from the RCA connectors gets rid of the hum. I wouldn't call that a ground loop since making the connection quiets the hum.

I don't see all of this as a problem, but if you are curious and since you have an ohm meter, with the P600 unplugged from the wall, check continuity from the Wadia chassis to the VTL chassis in each configuration;

1. with the RCA
2. without the RCA

Check from the Wadia and VTL chassis' to the ground pins on their power cords, The cords, from one ground jack on a P600 outlet to the others. By the proces of elimination you should figure where the ground is missing. However, I don't know how your equipment uses the ground pins on the power jack or XLR connectors, so if the piece is isolated it may not use it. Everything may be normal and your particular setup just needs that chassis to chassis ground that is missing. To save money, use a cheap set of RCA's or go to Radio Shack and get some cheap RCA plugs and some thin interconnect wire. Take 2 plugs and solder a wire from the ground lug only of one RCA to the other and use that as your ground path.

Thanks again, I've done some more checking and I'm getting more confused ! I've found that the wadia 7 has no chassis continuity to earth, while this can't be right I'm not sure how it would effect things as the link from the wadia 7 to 9 is optical. Also one of the amps has a poor earth connection as you have to wiggle the socket to get the continity ! again I'm not sure this is relevent as I still get the hum with this earth path there though it does reduce it. There is no continuity between the wadia and amp chassis with either the XLR or RCA cables .
I guess I should sort out the faulty earths I've found and check again Regards Adecook
It's not suprising that the 7 chassis is not grounded. If everything inside is isolated from the case and it uses an optical output there is really no need to ground it.

It is a bit unusual to have the RCA shells not connected to the chassis but who knows? After you fix the bad socket try touching a wire between the preamp chassis and the amp chassis and see if that gets rid of the hum.

Hum like this can drive you nuts and sometimes the solution defies reason. Years ago I had a Phase Linear preamp, a Yamaha power amp, and various sources and had a low level hum. I was using a clip lead to try different schemes, connecting different ground points from one chassis to another. It would not go away. I just clipped the lead to a screw on the preamp chassis while I thought about it and left the other end hanging in the air touching nothing. The hum dissapeared. I know that sounds impossible but it was easily repeatable and I had friends verify it just to make sure I wasn't insane.

Good luck.