Getting matching transformer from telephone



http://www.physicsforums.com/archive/index.php/t-79645.html
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"Lostgallifreyan"
** FUCK OFF -
YOU RIDICULOUS DICKWAD !!
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Wasn't aimed at you. I was thinking earlier that it had a higher signal to noise ratio than you though. True, no? You're all noise now.
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"Lostgallifreyan"
** FUCK OFF -
YOU RIDICULOUS LYING DICKWAD !!
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Posted after last post in a subthread does not mean aimed at last poser, when the new post is a useful read for all posting. Got that? Read it, it's a lot more interesting than you are right now.
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"Lostgallifreyan"

** Oxygen thieves like this anencephalic turd need shooting.
... Phil
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Caps to wit ratio improved, but nope, still noisy..
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Phil Allison wrote:

I have to agree.
Plus think of the advantage in reduction of methane emissions !
Graham
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Phil Allison wrote:

I think he was having a laugh !
USA and back via either underground cable or satellite can (did) do some weird stuff, but even that has gone now with IP telephony which is becoming near universal now. I can make certain calls to the USA from the UK for LESS than the cost of certain types of UK call.
In fact a US call for me from the UK to a landline OR CELLPHONE costs 7.5c to connect and 1.5c / minute.
Graham
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Eeyore wrote:

I have had severe echo on as little as ten miles of underground telephone cable on an old mechanical exchange. The phone company had run new lines out to rural areas, then connected some downtown businesses to the trunk a block from the C.O. We had one of the first digital alarm monitoring services, and had trouble decoding some numbers because of the echo. Ohio Bell refused to correct their problem stating it would be done on schedule in six years so we had to move the business to another town that had recently undergone a full rebuild. The line ran about ten miles from the C.O. to the end.
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Bill Janssen wrote:

Correct. It is in fact the classic 'characteristic impedance'.

Only on long circuits which were treated differently.

Only ever seen that in a fax machine I helped develop for Xerox (RXEG) and the hybrid part was done with differential amps, NOT the transformer. MUCH cheaper.

I have never seen a transformer in ANY phone including the carbon mic type which also 'draws power from the line' to power the mic as you say.
How do the clowns think they determine ON and OFF HOOK ?
Graham
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Eeyore wrote:

You really are stupid.
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Oh dear, you need educating.
http://www.telephonesuk.co.uk/images/232_cct.jpg
What do you think "Coil,Induction No20" is?
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
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Stuart wrote:

I'm afraid I wasn't around in 1935. 73 yr old technology is somewhat irrelevant.
These days a phone is a chip and a few passives (that *aren't* big inductors).
Graham
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Not irrelavant when we were discussing the history of why 600 ohms became the standard impedance for audio distribution.

Modern phones don't use carbon mics either yet you claim

Perhaps you'd prefer the inside of the ubiquitous BT 746 which was introduced in 1967 and was around for a very long time - we didn't replace ours till well into the 90s
http://www.telephonesuk.co.uk/images/746_cct.jpg
This is a 1978 version.
In 1985 it was fitted with the new style plug and became the 8746 but still had the "Induction coil"
However, regardless of what you know or don't know, this thread was started by someone who wanted to know whether he could re-use the transformers in telephones for another purpose so clearly he has some!
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
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"Stuart"

** That is entirely false.
What the OP *actually asked* was this:
" If I strip down some landline phones I 've got here, then will there be a matching transformer in each one? "
Chances are high there are no iron core transformers ( hybrid or other) in his phones at all - cos they are too modern.
Secondly, his purpose requires a transformer with high voltage safety isolation from the phone line - which the common 600:600 ohms phone line types all have and others do not.
Thirdly, his purpose requires a transformer with very high CMRR - cos that is why he is getting humming noises at the moment when making recordings with his passive ( ie non-transformer ) adaptor device.
Have you ever done this ??
I can assure you it ain't as simple as you suspect to get a hum free and distortion free result.
Not the least of the problems is that portable voice recorders have high sensitivity mic inputs with automatic gain control circuits - which cannot be defeated by the user. This results in annoying gain pumping and regular bursts of overload distortion PLUS increasing background noise & hum whenever there is a pause in the conversation.
The trick is to use just the right amount of resistive divider attenuation before and after the 1:1 transformer to eliminate this - plus cap couple the input side to stop pulling the line low.
..... Phil
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BS. It is *very* simple. Every simple (or complex) telephone for a hundred years or so has been doing the exact same thing!

And *you* don't know how to set levels properly to handle that?
Most people do know...

Why not use a properly designed pad, with the correct impedance? "Pulling the line low"???? Giggle snort, cough and laugh...
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"Floyd L. Davidson" wrote:

A: OFF HOOK you twit.
B: the DC resistance of the transformer primary may cause the phone to stop working by lowering the line voltage too much.
Graham
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Off hook is a loop *current* condition.

Typically a split winding is used, with a mid-point capacitor to block DC current.
Even you should have known that.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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"Floyd L. Davidson" wrote:

WOW. I'd never have guessed !

What a load of Bollocks !
Transformers are neither needed nor used any more.
Graham
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