Dave Plowman wrote in message ...
In article ,
N. Thornton wrote:

Err, WTF do you think the purpose of a telephone line transformer is?

Load matching and isolation, or in some cases use as a hybrid.

A hybrid transformer is used to convert a two wire into a four wire
circuit. It's a totally different device.

Its a transformer wound specifically for use as a hybrid. I excpect we
can agree there really.

The *prime* reason that BT
insist (or used to) on a transformer between external equipment and their
line is to prevent mains getting onto it.

Yup. There's a difference in consequences between mains on a phoneline
and mains thru your head. Consequently there is a difference in what
level of rish is acceptable, i.e. to what voltage the insulation
should be rated, and what other precautions should be taken.


I also dont believe you can count on such devices to have sufficient
tested breakdown voltage for this app. I haven't changed my mind any so
far.

Well, if you can't trust a device made specifically for this purpose,

wrong purpose.

then I'd suggest you only ever use a battery operated self contained
headphone system - because there's just as much chance - or more - of the
mains transformer inside an isolated chassis set breaking down and putting
mains where it shouldn't.

A fair point to consider, but I'm going to disagree. Mains stereos are
either Class I, in which case the headphone socket is safety earthed,
or theyre class II, in which case a) the insulation is very highly
rated, and b) potentially dangerous wires are tied so that if one
comes loose it still cant touch anything and electrocute.

With a live chassis TV and a phone transformer,
a) you dont have good enough insulation rating between mains and head
b) you dont have any of those backups inherent in class I or class II
either.
c) with a percentage of home installs you'll also have inadequate
precaution against EHT arcing to the socket.
The result is a much higher level of danger, too high.

Sorry, but when you're talking about connecting mains to your head you
dont say 'well, it should be alright'. Headphones aren't insulated to
withstand mains voltage, so cant be counted on to insulate. You may
well be OK, might not tho, and thats not good enough in this case.

Then, as I say, you'd never risk using them on any mains powered equipment.


Regards, NT