Re: Getting matching transformer from telephone

[Eeyore]

WRONG!!!!!

RIGHT !

Do the damn sums.

Graham

 

[Eeyore]

Impedance for an un-loaded pair varies with frequency. So at DSL
frequencies the impedance is 100 to 110 Ohms.

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

But for telephone use They preferred to use loaded
pairs and they were designed to be 900 Ohms. And the phone should be a
reasonable match to the line to minimize refections which bother the users as
echoes

Only on long circuits which were treated differently.

The transformers in some sets was not an isolation transformer but a
hybrid and matching transformer.

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.

The carbon mic. used in the old phones
was powered from the line. And many of the newer phones derive power from the
line. So isolation can't be used.

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

 

[Eeyore]

"Bill Janssen"

** Huh ??

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

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

 

[Eeyore]

"Lostgallifreyan"

** Oxygen thieves like this anencephalic turd need shooting.

I have to agree.

Plus think of the advantage in reduction of methane emissions !

Graham

 

[tony sayer]

Unfortunately has has NO CLUE about the characteristic impedance of twisted pair
cable as used for telecoms.

http://en.wikipedia.org/wiki/Twisted_pair#Unshielded_twisted_pair_.28UTP.29

"UTP is also finding increasing use in video applications, primarily in security
cameras. Many middle to high-end cameras include a UTP output with setscrew
terminals. This is made possible by the fact that UTP cable bandwidth has
improved to match the baseband of television signals. While the video recorder
most likely still has unbalanced BNC connectors for standard coaxial cable, a
balun is used to convert from 100-ohm balanced UTP to 75-ohm unbalanced."

*** 100-ohm balanced UTP ***

Graham

I seem to remember from another time that Old Floyd worked for some
Alaskan phone company..

Anyways what was the original question .. seems to have fallen off the
news server?...

 

[tony sayer]

Quite so !



Absolutely not. About 100 ohms.



Via 'Post Office Telecommunications'.



He has made that evidently apparent.

Graham

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...

 

[Eeyore]

I seem to remember from another time that Old Floyd worked for some
Alaskan phone company..

Anyways what was the original question .. seems to have fallen off the
news server?...

A slightly obscure question as to effectively (not sure if the OP realised) whether
impedance or voltage matching was important. Needless to say, many IDIOTS think
everything audio HAS to be 600 ohms which is an irrelevant ancient standard anyway
but lingers on in the minds of the long brain-dead.

Here's the original.

" I am looking for some 1-to-1 matching transformers to connect varioua
audio devices to my PC. I usually get noises and hum.

These line matching transformers are not so cheap at about £6 or 7
each.

Telephones seem to suppress line noise and hum rather well so I
figure the components they use are probably of half-decent quality.

If I strip down some landline phones I 've got here, then will there
be a matching transformer in each one? Or is their technology
different now? "

Graham

 

[tony sayer]

A slightly obscure question as to effectively (not sure if the OP realised)
whether
impedance or voltage matching was important. Needless to say, many IDIOTS think
everything audio HAS to be 600 ohms which is an irrelevant ancient standard
anyway
but lingers on in the minds of the long brain-dead.

Here's the original.

" I am looking for some 1-to-1 matching transformers to connect varioua
audio devices to my PC. I usually get noises and hum.

These line matching transformers are not so cheap at about £6 or 7
each.

Telephones seem to suppress line noise and hum rather well so I
figure the components they use are probably of half-decent quality.

If I strip down some landline phones I 've got here, then will there
be a matching transformer in each one? Or is their technology
different now? "

Graham

Right..

Wonder what's he actually trying to do .. in practice?..

If he's still there?...

 

[Franc Zabkar]

In european specifications (for Finland etc..) I have seen this
that complex reference impedance Z = 270 + (750 //150 nF)

750 ohm
_____
270 ohm +--|_____|--+
_____ | |
--|_____|---+ +-----
| || |
+----||-----+
||
150 nF

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):
http://web.archive.org/web/20001001070243/http://www.midcom-inc.com/pdf/TN69.pdf

International Digital Access Arrangements:
http://groups.google.com/group/comp.dcom.modems/msg/232247c06425cc1d?dmode=source

- Franc Zabkar

 

[Archimedes' Lever]

I'm waiting to see your guesses to *both* questions

Hint: both answers relay on the fact that the lines are straight analogue
from end to end.

Freudian shlip? 'relay'? rely

 

[Roy]

**** Off already you Deadend Drifters..
All the OP needs is a preamp with audio controls between the PC and
whatever you cheap canabalistic fly by night wannabe engineers....

Roy Q.T.
[have tools, will travel]

 

[Archimedes' Lever]

**** Off already you Deadend Drifters..
All the OP needs is a preamp with audio controls between the PC and
whatever you cheap canabalistic fly by night wannabe engineers....

Roy Q.T.
[have tools, will travel]

Siad the totally retarded **** that doesn't even know what a ground
loop is.

 

[Archimedes' Lever]

I want to minimise any ground loop to reduce hum and other spuriae so
perhaps I should have said "isolating" transformer.

According to Roy, you want a ground loop.

He is such an electrical genius!


NOT!

bwuahahahahahahahahahahahahaha!

 

[Phil Allison]

"Paul B"

My aim is to take voice recordings made on various equipment and save
them to a PC. Some of the voice recordings are of telephone
conversations made onto tape. I would prefer to have fed the phone
signal direct to the PC but I get a lot of noise.

** Hey pal.

YOU already know what to do ( ie use a proper 600:600 ohms ISOLATION
transformer or a device incorporating same) ) and are just objecting to a
price of a few pounds or dollars.

Make you a fucking PITA wanker.

**** OFF.



...... Phil

 

[Archimedes' Lever]

It's true that isolation is not important for a well-insulated telephone.

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. This is a standard element of device design
where human contact is present, and has nothing to do with it being in a
plastic case. It isn't your Dad's AC fed two wire drill motor with an
un-phased power cord and metal case. It is, however, in close
(electrical) proximity (potentially) with lightning events, and that is
why arresting elements have been incorporated.

Most incorporations are overkill, as it were, but I am happy that our
scientists and engineers of decades past were concerned about such
things.

Idiots today seem to think everything is low voltage and harmless.

 

[Phil Allison]

"Phil McKerracher"

Interesting thread (not the childish abuse, the technical stuff!).

** So you post more tech BS & childish stuff - great .

This is true ...

** Make sure you get attributitions correct - pal.


- 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.

** The comment was:

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

Which excludes all longer lines, of course.

These days there is packet delay to worry about as well. There are echo
cancellers but they're not perfect. Mismatches also affect loudness.

** 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.

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).

** What you claim you saw without proof is irrelevant

- seen any Martians lately ?

Exceptional cases are also irrelevant to the original matter.

And I have designed interfaces myself (for modems) that used transformers
..

** Modems are not the topic.

Different animals to ordinary phones.



...... Phil

 

[Phil Allison]

"Archimedes' Wanker = total Fuckwit "

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.

** Plus it would jump across the insulation barrier in a 600 ohm line
isolation tranny too !!

FUCKWIT !

This is ONE of the many reasons that isolation elements are
incorporated at VARIOUS locations in the system.

** Surge voltage clamping is not "isolation" - FUCKWIT !!

It isn't your Dad's AC fed two wire drill motor with an
un-phased power cord and metal case. It is, however, in close
(electrical) proximity (potentially) with lightning events, and that is
why arresting elements have been incorporated.

** Surge voltage clamping is not " isolation " - FUCKWIT !!

Most incorporations are overkill,

** Shame one has not killed you then - FUCKHEAD.




....... Phil

 

[Archimedes' Lever]

** Plus it would jump across the insulation barrier in a 600 ohm line
isolation tranny too !!

FUCKWIT !

Except that they ALSO incorporate yet another arrestor at the service
entry. Nice try though... nice sig too... Fits you to a tee.

 

[Ross Herbert]

On Thu, 01 Jan 2009 18:45:36 -0800, Archimedes' Lever

:On Fri, 2 Jan 2009 02:25:48 -0000, "Phil McKerracher"
:
:>
:>It's true that isolation is not important for a well-insulated telephone.
:
:
: 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.

Let's get one thing straight about the old POTS telephone. It has never been
designed to include an "isolation transformer" - where "isolation" means
galvanic isolation....

A plain old POTS telephone is basically sacrosanct in terms of its lack of
requirement to include galvanic isolation to anything. All it has to withstand
is the normal voltage potentials found on the line itself.

However, ANY Customer Equipment (CE) which is powered from or has any connection
to the mains supply, and which interfaces to a telephone line, MUST have
galvanic isolation. The POTS telephone itself DOES NOT.

CE which is connected to the mains supply MUST have an approved mains
transformer (or SMPS) with the required galvanic isolation, and separation of
the telephone line side from the mains powered side using an approved line
interface transformer (600:600 usually). Also the spacing between any conductors
on the mains powered side and the telephone line side of the circuitry must be
at least 6mm (iirc). This includes any opto isolators used to provide signal
interface from the mains side through to the telephone line side. These devices
must have physically wide separation between terminals on the the input and
output sides, and they must a high voltage breakdown rating (eg. 7.5kV). In this
way 3 levels of galvanic isolation are incorporated in mains powered CE to pride
galvanic isolation "from the mains powered side".


:
: 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. This is a standard element of device design
:where human contact is present, and has nothing to do with it being in a
lastic case. It isn't your Dad's AC fed two wire drill motor with an
:un-phased power cord and metal case. It is, however, in close
electrical) proximity (potentially) with lightning events, and that is
:why arresting elements have been incorporated.

In the old days of magneto, central battery manual and even central battery auto
equipment there was no isolation in the terms you mention above. Yes, they did
have "protection" devices on the telephone line to limit voltage surges and high
current ingress onto the line, but the telephone line today is essentially a
straight connection from the line interface equipment (which these days may have
VDR's and that's it) through to the telephone at the other end. The types of
protection which were once used on the POTS line were; fuses to isolate the line
in the event of a heavy current surge, heat coils to break the circuit if an
aerial line came into contact with a foreign potential which did not produce
sufficient current to rupture the line fuses, and lightning arresters (spark gap
or gas discharge). These devices were provided at both the exchange end (on the
MDF) and the customer end of the line (eg. protector #1). After the 1960's these
items were rapidly disappearing from the telephone environment due to the
majority of line plant being underground where it had almost zero chance of
coming into contact with foreign potentials.

In modern days where most cabling is underground, these devices are dispensed
with - no heat coils (which were only really required for pole mounted telephone
lines), no fuses and no lightning arresters. Even today a 3 stage surge
protector (gas discharge tubes, fusible resistors and VDR's) is only required to
be connected to a telephone line at the either the customer end or the exchange
end where part of the construction is aerial, and/or the local area is prone to
constant lightning activity. Due to the requirement for customer premises
cabling and CE to conform to standards which provide for adequate separation
from foreign voltage sources there should be little chance of a breakdown in
galvanic isolation from these sources to the telephone line.

 

[Roy]

From: [email protected] (ArCharlesmedes' Lever)
On Fri, 2 Jan 2009 02:25:48 -0000, "Phil McKerracher"
It's true that isolation is not important for a well-insulated
telephone.
    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. This is a standard element of device
design where human contact is present, and has nothing to do with it
being in a plastic case. It isn't your Dad's AC fed two wire drill motor
with an un-phased power cord and metal case. It is, however, in close
(electrical) proximity (potentially) with lightning events, and that is
why arresting elements have been incorporated.
    Most incorporations are overkill, as it were, but I am happy
that our scientists and engineers of decades past were concerned about
such things.
    Idiots today seem to think everything is low voltage and
harmless.
=--------------------------------=
You're Happy ? Rwahahahahaha Scientists & Engineers what ? HAHAHA you
rednecks are just too much.

You're the only idiot that thinks like that in here, fuckwad....
Everyone knows that Voltage doesn't kill you moron., which reminds me,
you currently are the top of the line idiot in here.I never even
mentioned ground loops, but I do recall you being a fruit loop.

How many times do we have to tell this guy what he needs to do? - use a
preamp - change tape players hahaha that was so hard Archschemedes'
got a nose bleed over the topic...

The numbskull must be one of your made up trick nics to salvage the
group which is dying like the other electrical groups because you keep
trying to cunTroll it by scaring and kicking people out., so now you
make up stupid dipshit questions to keep it going..it's pathetic.Suck it
up and be yourself, Chasquito.

Roy Q.T.
[have tools, will travel]