In article , Dave Plowman (News)
wrote:
In article , Jim Lesurf
wrote:
No - the screen is not connected at the destination end - that's
what I meant by 'left open'.

Can't see why it would make any difference.

The nominal reason is that interference or hum can be introduced by
more than mechanism.

Yes - but in modern mains isolated domestic equipment with unbalanced
connections, one side of the signal is provided by the ground of the
interconnect. Most of these are co-axial, so it is simply the screen.

Agreed.


But some use balanced cable which is twin and screen. And with this,
I've not found any difference between a) linking both conductors to
'hot' and using screen for ground. b) using one conductor for 'hot' and
one for ground, with the screen connected at one end. c) using one
conductor for hot and one for ground with the screen connected to it at
both ends.

In practice, with many domestic systems, I would not expect the above to
make a lot of difference one way or the other. The reason being that the
interconnect lengths are generally 'short', the equipment is well designed
to avoid common ground impedance problems, etc.

Nor would I expect to with the normal low impedance outs and high
impedance ins - over the metre or so in most installations where there
is no external RF problems.

The most commonly given advice is w.r.t. avoiding 'loops' . These act
as loop antennas or transformer turns and allow an external magnetic
field to induce currents 'around the loop'. This is the reason people
tend to be advised to only ground via one point in a system.

Well, if you remove the ground from most domestic interconnects, you'll
end up with rather more problems than just hum. ;-)

Not quite sure what you mean. There is a distinction between 'most' and
'all'... :-)

I generally find that only having *one* item (usually the pre/control amp)
in the system connected to mains ground, and linking all other equipment
grounds to it via their ground reference/screen on the signal leads tends
to minimise hum, etc. Connecting any other items to mains ground can
produce a rise in hum due to loop formation. However this may be small
enough not to matter in many cases for the reasons outlined above.

Indeed, in my experience it has been common for some time for source
components to be double-insulated and not have an earth wire to mains.

Slainte,

Jim

--
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Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

If connecting unbal into bal, then 1 conductor core and 1 braided/lapped
screen is the least and most you'll need to convey the audio signal.
Connect screen at bal receiving end to "cold" and also to shield.
If all you have is a scrnd, twisted 2-core cable, then parallel the cores
both ends - and follow the above.
Or, if you don't want to do that, diss the screen at sending end but use
core1 for unbal hot and core2 for unbal return, but wire the receiving end
as 3-pole, ensuring cable screen is made only at power amp input receptacle
or metalwork.

The rule has always been in my pro audio wiring days to connect line- or
variable-level device's screen at the bal receiving end but to *diss* it at
the sending end of a bal scrnd 2-core cct (except of course when routing
Source and Destin via pair-normalled jackfields).
This explains why some portable gear has a thought-out Earth-Lift switch
supplied at an o/p.
Bal Mic ccts need the screen and the shield to go the actual mic socket, but
any shield tag to be left unterminated at wallbox end where the
usually-metal panel is earthed locally.
Unbal AF wiring still causes headaches where a technical earth is not
well-installed.
Jim

In article ,
Jim Lesurf wrote:
Well, if you remove the ground from most domestic interconnects, you'll
end up with rather more problems than just hum. ;-)

Not quite sure what you mean. There is a distinction between 'most' and
'all'... :-)

I mean the ground of the signal connection.

I generally find that only having *one* item (usually the pre/control
amp) in the system connected to mains ground, and linking all other
equipment grounds to it via their ground reference/screen on the signal
leads tends to minimise hum, etc. Connecting any other items to mains
ground can produce a rise in hum due to loop formation. However this may
be small enough not to matter in many cases for the reasons outlined
above.

IMHO, having both a mains earth to everything and also the signal ground
connected *will* cause hum. As was the case with early far east makers who
provided a mains earth on everything.

Indeed, in my experience it has been common for some time for source
components to be double-insulated and not have an earth wire to mains.

Yup - my point. And disconnect the signal ground on unbalanced
interconnects *will* produce problems.

--
*42.7% of statistics are made up. Sorry, that should read 47.2% *

Dave Plowman London SW
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