Getting matching transformer from telephone

[Archimedes' Lever]

** More pedantic, pommy fuckwit nonsense.

Screening works even if there is no connection to AC supply ground or
actual ground.

That's a pretty broad statement, you retarded ****.

The validity of it is only true in a couple of circumstances.

The problem is that you are not a pretty broad.

I'll bet your ass is pretty fucking broad though. Likely four feet
wide! Bwuahahahahahaha!

Nice job of getting it wrong again, though, PhilTard, you sub-human
piece of shit.

 

[Archimedes' Lever]

At least we don't kill thousands of our fellow citizens each year with GUNS.

Idiot. A person that is committing a murder is not, nor does he have
"fellow citizens". A criminal is NOT a citizen of a civil society, but I
would not expect a retarded Ally hating dumb fucking sub-human retard
like you to have any grasp of civil behavior.

 

[Eeyore]

Umm... beg to differ .. they are used for 2 and 4 wire analogue or
analogue over digital voice band private circuits...

I thought we were talking POTS ?

Graham

 

[Eeyore]

You prefer car bombs.

And when do you we last had one of those ?

 

[tony sayer]

I thought we were talking POTS ?

Graham

Yeah .. sounds abaht right;-))...

 

[tony sayer]

Because I didn't spend 20 or more years of my life measuring and testing
audio systems, equipment and GPO lines that were all designed for 600 ohm
characteristic impedance (among other things)?

Because until the 1980s (I think it was) The GPO/BT didn't have a monopoly
on telecommunications,

Kingston telecoms anyone..

....alright not that much of the UK!....

 

[tony sayer]

You left out the isolation transformer in the phone.

Also, the tube will have a return path, and quell any HV down to
non-arcing voltages.

Don't think old Jove's bolts will notice that much..

 

[krw]

That is a sign of dementia.

Or rabies.

 

[krw]

Don't confuse him with facts. He has trouble breathing and typing at
the same time.

Oh he can do that, albeit through the mouth. Because Dumb Donkey is a
mouth breather, he drools as he types (loses control over other bodily
functions, as well). The result is what you see here.

 

[Jamie]

Stuart wrote:




Minority use even then.

Try and think WHY.

Graham

Really?
Minorities were allowed to use them back then?

http://webpages.charter.net/jamie_5"

 

[Archimedes' Lever]

I think he is referring to a part of the world that is about to adopt
sharia law..

They don't even have phones. Only limb chopping sabers, commissioned
by their saber wielding fucktard they think became a god.

 

[Ross Herbert]

On Fri, 02 Jan 2009 22:54:04 +0000, Eeyore

:
:
:Ross Herbert wrote:
:
:> Since your PC is mains powered and it may not have the required isolation
:> between the mains side and the sound card input you can do your own thing
using
:> an approved 600:600 transformer with 3kV isolation rating to interface the
:> telephone line to the sound card input.
:
:He DOES NOT need a 600 ohm transformer since the input impedance of the sound
card
:is not 600 ohms <sigh> !

Since the application is merely detecting signal "voltage" it hardly matters
that the secondary impedance of the transformer is 600 ohms and the input
impedance of the sound card is more like 10Kohms. The only reason one tries to
match impedances is where one needs to maximise "power transfer" and that
doesn't apply in this case.


:
:It's more likely to be in the tens of kilohms.
:
:What he really wants is something like a 10k:10k 'line bridging' transformer.
In
ractice, using a '600 ohm' transformer will probably be ok, but being
incorrectly
:loaded will degrade the sound quality (freq resonse may be peaky etc).
:
:Graham
:

Not true!

 

[Phil Allison]

"Ross Herbert"

Since the application is merely detecting signal "voltage" it hardly
matters
that the secondary impedance of the transformer is 600 ohms and the input
impedance of the sound card is more like 10Kohms.

** Fact is, it will be an advantage as the load across the phone line
needs to be high as possible so the level there is not affected.

The only reason one tries to
match impedances is where one needs to maximise "power transfer" and that
doesn't apply in this case.

** Nonsense.

Audio and other wide band transformers have rated ( ie optimum) source and
load impedances.

This relates *directly* to them both operating efficiently AND exhibiting
their specified frequency response curve.

Eg, a 600:600 ohm type operating into 10 kohms may well have a large
response peak at high frequencies while a 10k:10k type operating into 600
ohms will likely have serious roll off from a few kHz upwards.

With transformers that have 1:10 or 1:20 step-up ratios for mic-input to a
valve stage, both load and source impedances become quite critical just to
stay within a +/- 2dB corridor across the audio band.



...... Phil

 

[PCPaul]

Gun control is for morons and cowards. England is the perfect example
of a nation governed by cowards who are afraid of armed citizens.

Afraid of armed Yanks, definitely.

24% of the US casualties in the first Gulf War were down to 'friendly
fire'.

24%.

 

[John Livingston]

I now have some more information on this.

To quote:

"Studio Engineering for Sound Broadcasting" Illife 1955.
Chapter 7 "Programme circuits on Post Office Lines" G Stannard, Bsc
A.M.I.E.E - Lines department.

p142. "The distribution of BBC Light Programme to Burghead at the time of
writing contains 693 miles of 16mH/1.136m. and the estimated delay
distortion relative to 1kc/s is:
50 c/s 50m.sec
100 c/s 9.5m.sec
7 kc/s 7.3 m.sec"

16mH/1.136m describes a loaded line and m, in this case, would be miles.
Elsewhere a table gives the following information for this type of line:

Weight of conductor 40lb/mile, Approx characteristic impedance 490 ohms,
Cut-off frequency 9.3kc/s, maximum useable frequency 7.44kc/s.

The book further goes on to discuss carrier circuits

p131 (Same chapter)

"In 1938 the Post Office began a big expansion in their communication
network by laying 12-channel carrier cables. These are low capacitance
cables specifically designed for the transmission of frequencies up to 60
kc/s and subsequently up to 120kc/s"

Two schemes are discussed for using carrier circuit lines but they are
described (p155) as inferior to circuits obtained by more conventional
methods.

Thanks Stuart - this fits exactly with my memories of the time. It was a
while back though ......
The BBC now has its own digital audio distribution systems, and no
longer requires the widespread use of BT programme circuits.

To reiterate for those who have generated all the heat and the
fabulously inaccurate rantings - The programme circuits were BASEBAND
AUDIO all the way - amplified en route. As Stuart quotes - carrier is
possible, but provides worse noise and distortion standards.

Carrier was not generally used for main UK programme distribution (sorry
Floyd - you may be right about American practice, but not UK).

John

 

[SoothSayer]

Thanks Stuart - this fits exactly with my memories of the time. It was a
while back though ......
The BBC now has its own digital audio distribution systems, and no
longer requires the widespread use of BT programme circuits.

To reiterate for those who have generated all the heat and the
fabulously inaccurate rantings - The programme circuits were BASEBAND
AUDIO all the way - amplified en route. As Stuart quotes - carrier is
possible, but provides worse noise and distortion standards.

Carrier was not generally used for main UK programme distribution (sorry
Floyd - you may be right about American practice, but not UK).

John

Now that was the right, properly worded response to Floyd. Good job.

I wonder where this places the retarded donkey's assertions, if he ever
actually made any, other than his rantings. He did claim to have a
fairly deep knowledge... that he had a schematic.... somewhere.

It would be nice to see your document posted up in
alt.binaries.schematics.electronic, if, that is, you have access to it.

 

[ItsASecretDummy]

I do the exact opposite - if I get an *interesting* PDF,
then I print it and store the hard copy.

Fucking environmental retard! Do you do the stupid twit act of
printing out your e-mails as well, you stupid ****?

 

[ItsASecretDummy]

I just cannot cope with trying to read complex documents on
a screen, and the pink highlighter on the screen does not
scroll with the page, either
)

Indicative of piss poor grasp of modern computer technology, not to
mention quite lame document handling capacity. You don't know how to add
a highlight to a doc on screen? Perhaps you need a mini NASCAR scrolling
bug screen you can scroll with your document, dipshit?

Bwuahahahahahah!

 

[Salmon Egg]

Indicative of piss poor grasp of modern computer technology, not to
mention quite lame document handling capacity. You don't know how to add
a highlight to a doc on screen? Perhaps you need a mini NASCAR scrolling
bug screen you can scroll with your document, dipshit?

Bwuahahahahahah!

It is difficult to writer complex documents for publication. Most, but
not all, postings are not written well. I am guilty of contributing a
few myself. It is not the medium; it is the writer.

Bill

 

[Paul B]

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
[/QUOTE]


Hi Phil and others. I am the original poster. Thank you to everyone
who has kindly contributed to this discussion.

Please don't overlook some details I posted at:
http://groups.google.com/group/uk.telecom/msg/4513d4edcdc47c3e

You are quite right about high sensitivity mic inputs and automatic
gain controls. As you can imagine, when the phone's handset is
picked up and used, the mic input is faced with something approaching
line levels and I get severe overload of my recorder. Then when it
goes quiet, the AGC kicks in. It's as you describe.

I have tried to "listen through" any such automatic gain as best as I
could and I feel there is still detect much more hum than I would
expect on the line.

In truth, I really need a more suitable flash recorder but they are
harder to find than I expected.

Currently I use the Olympus WS-331M.
<http://www.olympus.co.uk/consumer/2581_WS-331M.htm>

Cost is important to me so I don't want to fork out for a digital
field recorder like this low-end one especially as some reviews have
not been kind to it. <http://www.dv247.com/invt/41901/>.

Prices in the flash-memory recorder market are tumbling such that now
personal video recorders (like the Archos AV400) are about the same
price and offer some additional very useful function. Although i
don't know if that sort of model has suitable input and output
sockets.

On the other hand, I don't want some MP3 player with a recorder
thrown in as an afterthought because ones I have seen can have
surprisingly limited bit rates and low quality. Anyway, many
probably don't have a line level input, so I will once again most
likely get problems with a mic input and AGC.

I take your (and other posters') points about a phone probably not
having a suitable transformer. My headache from trying to make sense
of some of the more technical aspects in this thread suggests it
might be easier in the end to go and buy some 600:600 transformers.

I'm not too clear about this "CMRR" you mention. I know next to
nothing about "common mode rejection ratio" although I did come
across the concept when looking at balanced inputs with chunky XLR
connectors. I googled it and got the following PDF but in the end I
couldn't really understand anything in it about CMRR. Do I really
need to consider CMRR if I am going to be making a recording from a
phone line?

<http://www.jensentransformers.com/an/Audio Transformers%
20Chapter.pdf>

One new line of thinking I would like to bounce off you and the other
kind contributors to this thread is whether it is possible in my case
to use a simple mixer (like those a pop group might use). If I can
avoid those 3-wire balanced inputs on such a mixer and use just the
2-wire inputs then would the mixer's op amps and other electronics
have a suitable isolating effect? The price of such a mixer seems
very competitive. I am thinking of a unit similar to this one:

Behringer XENYX 502 (£32) http://www.dv247.com/invt/31558/

In turn, that leads me to consider if I should attempt to get a
separate a analogue-to-digital converter which goes into the PC via
the USB. That way I hope to eliminate earth loop hum and other
spurious noises which I mentioned in my first post.

Maybe something like the Behringer UCA202 for £20
http://www.dv247.com/invt/32730/? To be honest I have no idea if
this would work but it seems to be good value.

I'm sorry to be darting about with various options but I didn't know
it was going to be so complicated!

Any views and useful info from anyone would be most welcome.

Andy.