Getting matching transformer from telephone

Cable type Impedance at 1000 Hz

19NLS 470 19NLP 262 19H50P 675 16B22 806 19H245S 1882

So have you.

Not true, as has been discussed elsewhere.

Not true, as has been shown conclusively in other posts.

That is absolutely untrue. Loading coils were not uncommon on 1/2 mile loops too! Basically whatever the design plan was for outside plant, it would be used for

*all* loops.

Well, there you go! You haven't got enough exposure to how telcom systems function to be telling anyone anything. You should be asking for help!

Then just exactly how do you think the transmitter got power in those old phones?

Do you have a clue? Do you know what the difference between "ground start" and "loop start" is?

Interestingly, the "line card" that supplies loop current, and detects hook supervision, also has a transformer. In the days of mechanical switching systems, it was for isolation only. With modern digital systems, it too is a hybrid.

In article , Eeyore scribeth thus

Course there are misunderstandings and some terminology shift in broadcast/telecoms and all related to the way various operators used to and now do things etc at different times..

But its all good for a slanging match;)...

On 30 Dec 2008 11:54:01 +0200, Tomi Holger Engdahl put finger to keyboard and composed:

I'd just like to contribute a couple of references which I found useful in the past.

This [old] document talks about complex impedances (see page 37):

International Digital Access Arrangements:

- Franc Zabkar

Interesting thread (not the childish abuse, the technical stuff!). I didn't know that loading coils were so common in the US, for example.

My twopenn'orth, as someone who has designed line interface circuits in the past:

This is true - someone (not sure who, sorry) commented earlier that this doesn't matter because the speed of light is so high, and that's true for local calls but not for long distance ones. These days there is packet delay to worry about as well. There are echo cancellers but they're not perfect. Mismatches also affect loudness.

It's true that isolation is not important for a well-insulated telephone. It's also true that differential amps are a cheap alternative to transformers for the hybrid part. But I have definitely seen phones with transformers in them in the past (the 80s). And I have designed interfaces myself (for modems) that used transformers in preference to op amps because of their superior common mode rejection, isolation and (electrical) robustness.

I have, from the days before transistors - they were specially constructed to tolerate DC current without saturating the core or overheating.

"Phil McKerracher"

** So you post more tech BS & childish stuff - great . ** Make sure you get attributitions correct - pal.

- someone (not sure who, sorry) commented earlier that this

** The comment was:

" Audible echoes on a few miles of twisted pair ??? "

Which excludes all longer lines, of course.

** But *audible echoes* on a line are not caused by transmission line behaviour.

They are caused by problems with the hybrid to line impedance match so that signals get retransmitted back to the exchange PLUS there has to be a significant time delay caused by a very long link - ie thousands of miles.

** What you claim you saw without proof is irrelevant

- seen any Martians lately ?

Exceptional cases are also irrelevant to the original matter.

** Modems are not the topic.

Different animals to ordinary phones.

..... Phil

Actually EC's damned near are perfect!

Years ago, when Data Set Termination cards first started using echo cancellation we had a field technician install one and then have a fit trying to test it! The loopback unit on the customer side of the DST appeared to go into loopback, but the testboard technician did not see a tone coming back. So out came the test equipment, and sure enough the tone was looped back, but it didn't get through this DST. Yet when a tone generator was applied, that tone worked fine. Upon investigating the new type of DST it was discovered that it had EC... and it was literally reducing the level of the looped tone to the point were the test equipment at the testboard didn't even see it.

Whatever, mismatches don't affect loudness so much as the frequency response and transient response. It will make a call sound "tinny", and that is perceived at needing more loudness to understand.

It's true that hearing the echo as an echo requires significant time delay, but hearing the echo as an annoying distortion requires very little delay.

Isolation on a tel line is not related to insulating the user from the device.

Long after the transistor became ubiquitous telephones virtually all used transformers. It wasn't until extremely good, extremely cheap op amps were developed that *any* telephone design went without a transformer. Even today, a lot of telephones have a transformer though, but it is encapsulated and not easily recognized.

Indeed, the difference between other transformers and the hybrid network in a telephone is being able to handle at least 120 mA of current without saturating (I've never heard of one overheating...).

A number of cheap substitutes at various times have tried to get by with less able designs, and many modems have been marketed with transformers that saturate at something far less than is required. It often results in poor data error performance when used on lines with more than 30 mA or so of loop current (which do happen to be very common).

What kind of screwy cables are those ? 1kHz has NOTHING to do with their characteristic impedance.

Here we use CW1308.

And why the heck do you think ADSL is based around an assumption of typical 100 ohm charactistic cable / line impedance ?

CRETIN !

Graham

Try running ADSL down a line with loading coils ! There's no need for them anymore. These days you just adjust the line gain.

Graham

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

OLD (and no longer used) is all you seem to know about.

Which go via optical fibre or microwave link, NOT cable any more.

Graham

Some classsic audio howlers in there !

High-fidelity Microphone

" Common uses include matching the relatively low 2K ohm output impedance of a microphone to an amplifier?s much higher line input impedance of

10K ohms. Studios commonly use the three terminal ?XLR? type of connector which is a balanced connection method with a terminal for a center tap. A separate ground terminal, tied to the XLR connector?s case is almost always present. The center tap may be used to phantom-feed a small amount of current for powering a pre-amp or active ?condenser? microphone."

LMAO !

The rest looks useful though.

Graham

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

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"

** 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

You mean you didn't know that either?

Ok, a typical dynamic microphone is often nearer 150-300 ohms output impedence and Ribbons can be as low as 50 Ohm but otherwise......

"Stuart"

** The above para is full of technical howlers - as are many other paras in the article

You have only spotted one of them.

** The author has first confused the typical and recommended *load impedance* of a mic input with the actual source Z of professional mics. Then he confuses the line input Z of an amplifier with the input Z of a transistor preamplifier circuit as found in ( old fashioned) mic inputs.

What he is * really * alluding to is that his company sells 2,000 ohm to

10,000 ohms transformers ( ie 1:5 ) for mic input use.

The stuff about studios using 3 pin XLRs with " centre tap" connections is an absolute pig's breakfast !!

What the confused author seems to be alluding to is the ( now obsolete) practice of using the centre tap on the 2,000 ohms winding of his company's mic input transformers as a source point for 48 volt phantom power.

Fact is - XLRs used for mic inputs ALL have pin 1 as ground & cable shield and use pins 2 and 3 for mic signal and phantom power.

..... Phil

Speaking of cretins!

The chart is from Bell System documentation (which I cited in another article). If you think the frequency has nothing to do with the characteristic impedance, argue with Bell Labs.

ADSL is based on 100 Ohm impedance for very good reasons. One of them being the frequencies used...

Did you see where it said above... "Telephone cable". Not all telephone cable can run ADSL. (And no you don't "just adjust the line gain".)