Adding Headphone socket to TV

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On 5 Sep 2003 06:12:39 -0700, m_ snipped-for-privacy@hotmail.com (Michael Murray) wrote:

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).
    Tony.
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I'd say the ideal cheap transformer would be a telephone line isolating type if quality isn't that important. A good quality audio isolating transformer won't be cheap.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Hi
several people write:

right. and millions dont.

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.

Regards, NT
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wrote:

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.
Regards, NT
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wrote:

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.

wrong purpose.

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.

Regards, NT
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wrote:

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 here.

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 one.
Regards, NT
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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 use?

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?

So what?

If it's done incorrectly. Hence the recommendation to use a suitable isolation transformer...

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.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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*I don't work here. I'm a consultant

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Michael Murray wrote:

Wimp! :-)
Plenty of loudspeaker isolating transformers rated 1-2Kv between windings.
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?
Roger.
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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 time.
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.
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Roy Millar, snipped-for-privacy@Millstream.ednet.co.uk Use m o u l i n e t @

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