I did this to a small portable TV 15 years ago - I used a 5 terminal
socket ( stereo & it disconnected 2 terminals when a plug was
inserted). As other people have warned, one or more of these contacts
could be at live potential, I didn't check, because I was using an
audio isolating transformer (I think it was from Tandys).
I had to move the loudspeaker connections to the other side of the
transformer (so the cut-out would work correctly). Trouble was the
transformer lost some power, so you had the turn the volume up louder
than before, which caused it to distort if you wanted it loud (but
normal viewing was ok).
TVs have been running transformerless for decades. There is more than
one approach to it.
you dont need any transformer. There are stacks of silicon based TVs
around running mostly on HT. I have 1 right here. The few bits that
can only be LT dont eat much power, and either HT circuits produce low
voltage at various points to run them, or a low voltage loptf wind is
used to supply them.
Another option is to use a self oscillating lop stage running on HT.
That starts up with no LT and the lop produces the LT needed for the
rest of the set. The LT circuits then pull the lop into sync. Flywheel
sync has been standard for a long old time. There are many approaches.
does that mean all TVs are isolated? Maybe you're overlooking the
consequences of 240v to the head.
those kind normally are, tho again exceptions exist. If 'most' is good
enough for you to apply 240v to the head in 5% of cases, I guess
that's your funeral.
I'd say the level of advice in this thread is just plain dangerous.
wrote:> > > I'd say the ideal cheap transformer would be a telephone line isolating
Load matching and isolation, or in some cases use as a hybrid. 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.
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.
wrote:> > > Err, WTF do you think the purpose of a telephone line transformer is?
Its a transformer wound specifically for use as a hybrid. I excpect we
can agree there really.
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.
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
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.
1. A telephone is satisfactorily insulated from the head, it is
designed to safely take not just ringing voltage but also voltage
spikes resulting from switching and inductance present in phone
systems. Headphones have no such rating.
2. The telephone system is designed to protect the phone user from
lightning strikes. This was introduced in 1930 in the UK due to deaths
from lighning strikes while subscribers were using their phones.
Quite a difference.
If you showed you were familiar with and understood all the issues,
I'd consider it.
True, only for live chassis ones. But antique radios with live chassis
are both rare and easily recognised, whereas live chassis TVs are a)
not recognisable, and b) common. Plus there is the EHT in TVs too,
circa 20,000 volts.
yup, as far as their external ports are concerned. The big problem
comes when you start home modifying equipment that is not designed to
be so connected.
Agreed, but there are also many that aren't suitable. Which would the
OP end up with? Who knows.
Class I and II kit has an isolating transformer in them, in the power
supply, AND those additional safety measures. An iso alone, as you
propose, is not comparable at all.
EHT arcing is a routine fault in TVs. Maybe you haven't done a lot of
work on TVs. It is not a rare event, it is a routine fault.
If you put a headphone iso in a TV it needs to be well away from the
EHT, or screened from it. Unknowing persons could end up installing it
next to the EHT lead. Cringe!
If you might get bitten via your finger, its no disaster, but if you
might get 20kV thru your head, its a whole different matter.
Far from it, but I do take it seriously when someone discusses leading
the mains to someones head via a series of totally inadequate
insulation and safety precautions. In most other matters I wouldn't be
nearly so concerned.
If you look into medical equipment you'll find the legally required
minimum safety standards are higher than those I've been discussing
In all honesty I dont think you really appreciate the issues. I am
qualified in this area. You have a lot to contribute, and I will be
glad to take your advice on matters you know well, just not on this
And you've tested all available headphones, or have such data to hand? Or
are only guessing? And guessing what type of headphones the OP is going to
More straws. Perhaps you'd tell us all what precautions are taken with a
Hi-Fi system provided with a headphone jack where that system can be
connected to an aerial which might also be hit by lightning?
As oppose to thinking up the most outlandish situations possible? Do you
also think things should be proof to an invasion from Mars?
If it's done incorrectly. Hence the recommendation to use a suitable
If he buys a line isolating transformer *as such* it will be fine. How
many times do you have to be told this, FFS? Not a valve filament
transformer, or an LOPT, but a line isolating transformer. It's its whole
purpose in life...
Yes it is, as an interface. This is how you make safe a piece of untested
mains equipment - by using a mains isolating transformer.
Arcing to the audio output? You must have some very strange faults to deal
with. And you're saying that life threatening currents could be present
when this happens?
Sounds like you know all about this.
Just what has this got to do with it? Or are you suggesting that a TV
supplied by the makers with a headphone outlet is built to medical
equipment standards of safety?
I'm not going to bandy qualifications with you. but I've been around
taking audio feeds off unknown equipment for many a year with a prime
regard to the safety of those using or handling it. And if your ideas on
safety were implemented, no electrical instrument would ever be used in
broadcasting or recording.
*Someday, we'll look back on this, laugh nervously and change the subject
Dave Plowman firstname.lastname@example.org London SW 12
Plenty of loudspeaker isolating transformers rated 1-2Kv between
If you really think that discretion is the better part of valour,
why don't you get a second-hand stereo VCR, plug it into your hi-fi
and plug your 'phones into that?
In my early twenties (long, long ago, while I still knew everything) I
added a headphone jack to a mains/battery radio, and used it for some
Then my father had a heart attack, and I gave him the radio so that he
could listen in hospital.
He had a second heart attack while listening to a rugby game, and died.
No suggestion that the radio had anything to do with it, but after some
time (and maybe suffering from what they now call 'survivor guilt') I
belatedly checked the insulation between the metal headband and the
headphone plug. It was quite low; long ago now, but I think around 40K.
I've never really been sure........
It's not something anyone would want to wonder about.
Roy Millar, snipped-for-privacy@Millstream.ednet.co.uk Use m o u l i n e t @
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