On Thu, 30 Nov 2017 12:00:07 +0000, Dave Plowman (News) wrote:

In article ,
Johnny B Good wrote:
I have a dedicated AV radial circuit here. With the earth for that
going light back to the house earth. Not sure that bit makes any
difference, though, but no big deal when installing it.

It shouldn't. Assuming you only have a single safety earth connection
to
the common signal ground return, the whole thing could be left to float
at whatever induced mains voltage happens to occur (including half
mains from filters or even a full contact to the live). It won't make
any difference although I'd be rather leery of actually touching any of
the kit if bringing your hand to within close proximity generates even
the slightest hint of mains hum.

Can you run that by me again? You can obviously get a potential
difference between two earths when any current flows. Like say between
the earth to a socket and the one to a light switch in the same room.

Provided all the Hi-Fi kit is bonded together via the signal earths and
only one point of connection to the signal earths is used to connect to
the ring main earth, it won't matter how much voltage is induced into the
mains earth. Indeed the whole lot can be left floating without detriment
to the audio signals passing between the separate components, even to the
point of having a connection (intentional or otherwise) to the mains live
where the only detriment would then be to the health of anyone grounding
such a high voltage to a real earth via their body.

Some of the internal signal wiring might be less than completely
screened from external electric fields which normally wouldn't be an
issue until you introduce a source of high voltage gradient (240v 50Hz ac
between you, at earth potential and the amplifier or TT wiring at 240v)
hence my semi facetious remark about being leery of inducing a faint hum
whenever you place a hand close to a less than completely screened piece
of Hi-Fi kit.

For line level signals, whatever minuscule currents might be induced
into the local safety earths of tape decks, tuners and other line out
sources will be way down in the noise. In any case, a lot of such kit is
usually double insulated, allowing the screen connections (ground
returns) to remain decoupled from all but the one and only connection
point to the safety earth, typically the main amplifier.

The one and only item of Hi-Fi kit that remains vulnerable to such hum
induction is the old fashioned TT cartridge wired directly into a remote
RIAA equalisation pre-amp located a metre or two away in the main
amplifier. The proper way to deal with this problem is simply to install
the RIAA pre-amp within the TT itself so that when the equalised signal
from the TT has to negotiate the link to the main amplifier, it can do so
at line level and appear as just another aux input signal labelled TT or
'Phono'.

--
Johnny B Good