Re: Getting matching transformer from telephone

[Eeyore]

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.

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

Graham

 

[Eeyore]

A 10kV arc from a 2MV lightning strike *COULD* make it all the way into
the handset, and OUT of the perforations in the handset, through the
earpiece or mouthpiece, and hit the user. The microphone and the
earpiece transducer both use metal cans, making the distance to the user
a mere 1/4" through air. Not good.

This is ONE of the many reasons that isolation elements are
incorporated at VARIOUS locations in the system. One of which is at the
CABLE connection to the phone itself, which is why isolation elements can
be found at these positions.

In the UK the 'master socket' contains the 'spark gap'.
http://www.buzzhost.co.uk/nte5.php

Graham

 

[Eeyore]

This [old] document talks about complex impedances (see page 37):
http://web.archive.org/web/20001001070243/http://www.midcom-inc.com/pdf/TN69.pdf

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

 

[Phil Allison]

"Stuart"

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!

** 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

 

[Phil Allison]

"Stuart"

Eeysore




You mean you didn't know that either?

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

You have only spotted one of them.

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

** 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

 

[Archimedes' Lever]

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? "

Which, to anyone with even half a brain, obviously means that he wishes
to utilize it as well, you stupid fucktard!

 

[Phil Allison]

"Stuart"

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!

** 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

 

[Eeyore]

An 'audio expert' would put a 600 ohm resistor across the secondary
to provide proper loading but no expects you to know that.

Which would doubly load the line silly.

What is required here is what was typically once called a bridging transformer. Audio
doesn't use signal transformers much these days except the tube nuts. Or, come to that,
an active 'balun' with AC coupling to keep the line DC out. 2 caps, 4 resistors, a
single op-amp and maybe even battery powered. Plus some protection diodes for good
measure. 48V is a bit iffy even after a cap.

Graham

 

[Eeyore]

Not all telephone cable can run ADSL.

In civilised parts of the world with proper infrastructure it can.

Graham

 

[Eeyore]

The definition of on/off hook supervision has not changed, and that
makes it exceedingly relevant.


Not necessarily. Some do have transformers.

Such as ?

Graham

 

[Eeyore]

Not irrelavant when we were discussing the history of why 600 ohms became
the standard impedance for audio distribution.

It isn't. Not for about 45 years.

Graham

 

[Archimedes' Lever]

In civilised parts of the world with proper infrastructure it can.

Graham

In the right situation, someone might actually consider you as credible
too. Just not in the reality that we all live in. We call it the real
world. In it, you and credibility do not homogenize together. That is
one water your oily ass won't mix with.

 

[Eeyore]

plus cap couple the input side to stop pulling the line low.

Damn right ! Not ONE of these 'phone experts' has mentioned that.

Graham

 

[Archimedes' Lever]

Such as ?

Is it really so goddamned hard to accept the fact that such items do
exist?

Probably just as hard it is to accept the fact that you are a self
denying, US hating retard.

 

[Eeyore]

Which, to anyone with even half a brain, obviously means that he wishes
to utilize it as well, you stupid fucktard!

He can only utilise it if it contains one OBVIOUSLY !

Graham

 

[Eeyore]

"Pulling the line low"???? Giggle snort,
cough and laugh...

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

 

[Archimedes' Lever]

He can only utilise it if it contains one OBVIOUSLY !

Graham

That was not the argument, you retarded non-human dumbfuck.

The word is UTILIZE.

 

[Eeyore]

You falsely claimed it was otherwise. Seems *you* are
the one with a lack of "know" about this topic.

What ?

Look, I've designed line interfaces FFS. For Xerox. I've already
mentioned it once.

Graham

 

[Eeyore]

You mean you didn't know that either?

Ok, a typical dynamic microphone is often nearer 150-300 ohms output

Rather different to 2k I think you'll agree.

impedence and Ribbons can be as low as 50 Ohm but otherwise......

Ribbons are FAR lower. Maybe 50 ohms AFTER the transformer !

And Pin 1 on an XLR is ALWAYS the cable shield. It may also be GROUND - but
that's another story. It certainly isn't used to provide phantom power under
ANY circumstances. And the last time I came across a centre tapped transformer
in pro-audio was 35 years ago. It caused a bloody nuisance too. Modern
balanced audio circuits are 'floating'. The centre tap provides ZERO
advantage.

I now expect you to make an idiot of yourself trying to refute that one.

Graham

 

[Phil Allison]

"Floyd LYING FUCKWIT Davidson"

Tell us about how different a 600:600 transformer is
from a 10k:10k transformer, eh?

** Err -

could it be the optimum source and load impedances are 16.7 times higher in
the latter case ????

Nah - electronics is always much trickier than that .




...... Phil