Signal and safety grounding - frame to signal resistor?

[ToolGuyFred]

Here's a question for people with a similar system set-up to me. I have my Foreplay preamp in the corner of the room with most of my audio sources and ParaSEX power monoblocks next to each speaker. While this means only a metre or so of speaker cable (more inside the cabinet following the horn than out of it) I have 10 metre pre to power amp cables. Unfortunately, the power amps are on a different mains outlet to everything else, giving the perfect ingredients for a HUM LOOP.

The solution I am using at present means the safety (mains power) ground at each power amp is disconnected. (I use a switch inside the amp in the mains ground line.) While this gives a fault current path back to the ground bus of the preamp which will be sufficient to trip my earth leakage circuit breaker, I am concerned the (relatively) high impedance of the signal cable will result in a lower fault current with the possibility of significant reduction in the trip speed of any mains safety devices.

For my next power amp project, I am considering reinstating the safety ground at the power amps for the chassis and separating off the signal ground for the audio, linking with a resistor. This should allow the audio to be referenced to ground but give the lowest audio ground impedance through the audio interconnect rather than the house wiring. I have seen a number of circuits for various purposes using this method but I have seen resistors between 330 and 10K ohms. Is there anyone out there using this method with Bottlehead gear and what value of resistor works for you?

 

[Paul Birkeland]

Since you have several outlets in one room that aren't on the same breaker, I would recommend running a pair of long power cords from the outlet that powers your preamp.

Another option is to use 1:1 input transformers on your power amps, which will break the ground loop that you had with everything connected safely.

I've seen a lot of different components connected between signal and safety ground on various schematics, but I always leave them hard connected.

 

[ToolGuyFred]

Just a small note in case you aren't familiar with UK domestic electrics. My wiring is fairly standard:
Individual breakers for high current items; shower, oven etc.
Breaker for all 1st floor lighting
Breaker for ground floor lighting
Breaker for socket outlets - yes, just one 30A breaker for all mains outlets.

In many parts of the world you will find a breaker for each outlet but in the UK we normally have one or two "ring mains" with a single breaker. The "ring" element means higher current handling as there are two routes the current can take, essentially in parallel. The power cord for each  device is protected as every UK plug is fitted with a fuse.

So, I have most of my HiFi in one corner of the room on one outlet and the power amps powered from an outlet in the opposite corner of the room (on the same breaker but the two possible ground paths gives me loads of hum).

Perhaps a transformer volume control with separate primary and secondary windings?

 

[Paul Birkeland]

Yeah, that is a rough condition for what you're trying to do.  A pair of input transformers for your power amps should solve problem quite easily. 
-PB