On 05/26/2014 11:51 AM, Dave Plowman (News) wrote:

I'd say it was unusual to have a CRT set with a mains lead earth
connection to chassis.

Sony must be the odd man out. Either that or they have some 3 core
cable that looks exactly like everyone else's 2 core.

In article ,
Peter Chant wrote:
On 05/26/2014 11:51 AM, Dave Plowman (News) wrote:

I'd say it was unusual to have a CRT set with a mains lead earth
connection to chassis.

Sony must be the odd man out. Either that or they have some 3 core
cable that looks exactly like everyone else's 2 core.

Must be - but then I've never liked Sony TVs. None of the assorted CRT
sets I've had over the ages had 3 core mains leads.

--
*A conclusion is the place where you got tired of thinking *

Dave Plowman London SW
To e-mail, change noise into sound.

On 05/26/2014 03:00 PM, Dave Plowman (News) wrote:
In article ,
Peter Chant wrote:
On 05/26/2014 11:51 AM, Dave Plowman (News) wrote:

I'd say it was unusual to have a CRT set with a mains lead earth
connection to chassis.

Sony must be the odd man out. Either that or they have some 3 core
cable that looks exactly like everyone else's 2 core.

Must be - but then I've never liked Sony TVs. None of the assorted CRT
sets I've had over the ages had 3 core mains leads.

Just realised that I'd misread the post I replied to. Therefore my post
does not make much sense. My Sony is not earthed.

Bought mine second-hand, tempted to replace it just because it is a
nuisance because it is a two man lift.

On 05/26/2014 09:50 AM, Jim Lesurf wrote:


When we added a DTTV set-top-box I isolated the audio from the TV
completely. The audio now comes via spdif (coax from the DTTV box, or
optical from a DVD recorder and a player) into a DAC Magic. The output from
that is clean and sounds fine. Or at least is limited by the source
material.

Again, my experience is with stereo, not 5.1. So I can't comment on that
aspect. But I'd prefer something like an optical link or USB into what the
audio trade now call a 'galvanically isolated' USB input to avoid the
garbage you describe. That's likely to give much better results than
analogue audio transformers unless you spend a *lot* of cash on carefully
chosen transformers.


Agreed. Isolation on the digital side is preferable. Unfortunately I
can't use it with my TV nor cable box as neither have SPDIF out -
however laptop doc and DVD player do.


If you have a coax digital link, then either a back-to back pair of
coax-optical convertors inline or a *digital* transformer might be better.


Yep, know that. Pulse transformer is a way different beast to an audio
transformer. Also cheaper. Fewer needed.

BTW you can also get USB isolators. But as yet I've never tried one. Been
tempted because some USB audio devices are sensitive to their 5V power feed
from USB. But that's another story... :-)

Given the price best avoided unless essential. I was surprised how much
they cost. Seems that there is much more going on than some opto
isolators, psu and line drivers.

In article ,
Peter Chant wrote:
Must be - but then I've never liked Sony TVs. None of the assorted CRT
sets I've had over the ages had 3 core mains leads.

Just realised that I'd misread the post I replied to. Therefore my post
does not make much sense. My Sony is not earthed.

It wouldn't much have surprised me, especially with older stuff. I get the
impression some was bodged for UK regs. I well remember some quite
expensive Pioneer separates where each one had a three core mains cable
and fitted 13 amp plug. Connect it all up with the supplied normal phono
leads and you had a ground loop.

--
*Preserve wildlife - Go pickle a squirrel*

Dave Plowman London SW
To e-mail, change noise into sound.

On 05/26/2014 05:01 PM, Dave Plowman (News) wrote:

Just realised that I'd misread the post I replied to. Therefore my post
does not make much sense. My Sony is not earthed.

It wouldn't much have surprised me, especially with older stuff. I get the
impression some was bodged for UK regs. I well remember some quite
expensive Pioneer separates where each one had a three core mains cable
and fitted 13 amp plug. Connect it all up with the supplied normal phono
leads and you had a ground loop.


This set is likely 10-12 years old. 32 in CRT TV from when they were
fashionable and expensive. Suspect it is considered double insulated -
therefore no earth.

Going slightly off topic, but still on the subject of earth loops I have
a Cambridge pre-amp and two power amps. All earthed. Slight hum
audible when connected together, not audible from across the room but it
is noticeable when you get nearer the speakers. Hum goes if you
disconnect the power amps from the pre-amp. However, lifting the
various earths does nothing. If I were keen enough I'd probably try
adding a balanced input stage to see if that cured it.

Pete

In article , Peter Chant
wrote:

Going slightly off topic, but still on the subject of earth loops I have
a Cambridge pre-amp and two power amps. All earthed. Slight hum
audible when connected together, not audible from across the room but it
is noticeable when you get nearer the speakers. Hum goes if you
disconnect the power amps from the pre-amp. However, lifting the
various earths does nothing. If I were keen enough I'd probably try
adding a balanced input stage to see if that cured it.

The usual first reaction is to have the pre-amp grounded to the mains earth
but leave and the power amps not connected via their mains leads to the
mains earth. Instead to rely on them being grounded via the coax outers
back to the pre-amp. If that doesn't cure the hum then it isn't a
conventional loop. Alas a number of mechanisms can cause hum.

I don't know what kind of PSUs you have in the power amps. But if a
traditional 'transformer' type rather than Switch-Mode it might be worth
experimenting with swapping over live and neutral as they might be the
wrong way around on one or more units.

However all the above said, please note that any experiments with earthing,
etc, come with a big 'take care' for safety reasons!

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In article ,
Jim Lesurf wrote:
I don't know what kind of PSUs you have in the power amps. But if a
traditional 'transformer' type rather than Switch-Mode it might be worth
experimenting with swapping over live and neutral as they might be the
wrong way around on one or more units.

Can you explain the theory behind this? If it's the normal 'isolating'
mains transformer? I'm curious as I'm having slight problems with a lash
up here. ;-)

--
*Procrastination is the art of keeping up with yesterday.

Dave Plowman London SW
To e-mail, change noise into sound.

In article , Dave Plowman (News)
wrote:
In article , Jim Lesurf
wrote:
I don't know what kind of PSUs you have in the power amps. But if a
traditional 'transformer' type rather than Switch-Mode it might be
worth experimenting with swapping over live and neutral as they might
be the wrong way around on one or more units.

Can you explain the theory behind this? If it's the normal 'isolating'
mains transformer? I'm curious as I'm having slight problems with a lash
up here. ;-)

The problems it might affect are practical effects due to transformer
imperfections. e.g the amount of electric field 'leaked' by the mains
transformer may depend on which end of the primary windings is 'live'.

People tend to focus on the magnetic fields, but the electric fields can
also matter. Similarly, there may be an electrostatic screen between
windings (or not!) but it may also not be perfect. And mains power is not
'balanced mode'. Come to that, it isn't really unbalanced either as the
neutral will probably wag up and down. 8-]

Having to float the *entire* system to avoid hum may imply that it isn't a
loop. Its some kind of field or potential variation due to a problem like
the above. Then getting into the signal paths. This is also why some low
noise kit may have a 'float' resistor between the signal 0V and earth.
Shifts the created hum voltage to that resistor and limits the current.
Alas, may also be a safety problem if other measures aren't in place.

Its ages ago now, but when I last did some tests on decent power rated
mains power toroids some of them also had external fields that weren't
either 'nil' or uniform around the transformer. e.g. more field at the
places where the leads joined the transformer. And things can become even
more complex if the peak currents taken at each cycle peak are large
because the system may become nonlinear. So in some cases even rotating the
transformer can alter the measured hum on signal lines.

FWIW I never really understood all the detailed theory, etc, of
transformers. Just ended up being impressed by the good (mains) ones
working as well as they did! :-)

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html