In article , Dave Plowman (News)
wrote:
In article , Serge Auckland
wrote:
A better way - if possible - is to lift the grounds from the mains
connections of all units *except one*. And then make sure all
screens are connected at both ends and used for ground path return.
A practice I find dangerous, in that if the audio connections are
removed, then a piece of equipment can be left powered-up but
ungrounded, with obvious danger in case of an electrical fault.
Pretty well all domestic Hi-Fi has Class II insulation. So even if metal
cased doesn't need a safety earth. Indicated (in the UK) by only having
a two wire power cable.
Indeed. In practice most items like signal 'sources' these days seem to
have two-wire mains leads with class 2 insulation. So the user has this
decision made for them.
I'd agree that grounding each unit is desirable for safety reasons *where
the units are in metal boxes or have external metal parts.*. But in
practice you may find that some units which have a ground line to mains
cause hum in some arrangements, and so a ground lift may be needed as the
only alternative is to replace the unit, or some other item in the system.
This isn't a practice I've ever found to actually be dangerous in use. But
you need to apply it with care, and remember what you have done. In some
cases I have deliberately used the red-coloured two-wire mains cable (sold
for garden use) when doing this as a continual visual prompt that a given
unit has no mains ground link of its own.
Actually, the closest case to this being dangerous I've encountered was
when we used to have to float almost all the gear on a telescope at Mauna
Kea (Hawai'i) to get rid of loops. The result being that the entire
telescope tended to drift up in potential over time, and would give you a
belt if touched after 10-20 mins. Not something I've encountered with
decent domestic audio. But I would be very cautious of some older TV sets.
I much prefer the practice of having seperate audio and safety
grounds, grounding every safety ground and joining the audio ground to
safety ground at one point only in an installation.
So do I. The snag is that in some cases it leads to hum, and become
unusuable as an arrangement. You then either have to give up or make some
change which is less than ideal. Alas, our world is not ideal.
This is pretty much standard practice for Broadcast equipment, sadly
not yet for consumer stuff. With increasing numbers of "high-end"
equipments offering balanced connections, it would be fairly easy to
offer seperate audio and safety grounds.
Yes. However simply having balanced interconnections isn't in itself a
guarantee that you won't sometime need to lift a ground line, or make some
change to reduce hum.
While balancing is ideal for the long runs and hostile environment pro
gear oft finds itself in I'm not sure about the validity for domestic
use. Much domestic gear is internally unbalanced so providing a balanced
output might mean adding unnecessary extra electronics.
Plus which it might not actually mean that you never find that lifting a
ground on some unit doesn't help. :-)
The roots of the problem here are partly in the ways in which domestic
equipment are designed/made. Partly in domestic mains power arrangements.
In practice, the use of class 2 and twin-cable mains avoids many problems,
at the expense of the maker having to ensure class 2 compliance.
Slainte,
Jim
--
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc
http://www.audiomisc.co.uk/index.html
Armstrong Audio
http://www.audiomisc.co.uk/Armstrong/armstrong.html
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