Getting matching transformer from telephone

[Ross Herbert]

On Thu, 01 Jan 2009 18:45:36 -0800, Archimedes' Lever
<OneBigLever@InfiniteSeries.Org> wrote:

:On Fri, 2 Jan 2009 02:25:48 -0000, "Phil McKerracher"
:<usenet@mckerracher.net> wrote:
:
:>
:>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.

 

[Ross Herbert]

On Fri, 02 Jan 2009 00:32:00 GMT, Paul B <mail@nomail.invalid> wrote:


:
:Hello Tony and everyone, I am the OP.
:
:I've been away for a few days and I see there's so many posts that
:now I'm trying to get through them all!
:
:
:MY OBJECTIVE
:
: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.
:
:I want to preserve as much quality as possible because it will
robably be necessary for a third party to identify the person
:speaking.
:
:------
:
:Secondly and quite separately from the above....
:
:I didn't raise this problem in my first post. I am getting hum and
:noise when I record using a purpose build connector (Retell model 156
:~ see link below) to a hand-held battery-powered flash-memory
:recorder even when the phone is on hook. I can't see where the hum
:is coming from unless it is on the phone line because there can't be
:a ground loop this time.
:
:http://www.telephonerecorder.co.uk/recording/connectors/156.htm
:
:I do know my landlines don't have all the hum and noise so they must
:be doing something which I want for my recorder! I thought may be a
:transformer to better terminate the Virgin Media phone line might
:help but I am out of my depth here and line termination may be the
:wrong idea altogther.
:
:
EFINITIONS
:
:I guess my use of the word "matching" is not a very good electrical
:description. I'm not seeking to match impedances and I get the
:feeling that in electrical engineering, "matching" is often shorthand
:for impedence matching. So apologies for any confusion I have
:caused.
:
:I want to minimise any ground loop to reduce hum and other spuriae so
erhaps I should have said "isolating" transformer.
:
:Retell have a model (the 157) which connects direct to a PC and I
:believe it is identical to the 156 except it has the additional
:transformer I am asking about.

Get an approved line isolation unit such as this
http://www.dallasdelta.com/pdfs/liu_3kv.pdf

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.

 

[Ross Herbert]

Actually, the Delta LIU is a bit of an overkill for what you want to do so here
is a much cheaper solution http://www.accesscomms.com.au/Products/K2400.htm

Basically this will just be a 600:600 line transformer without the DC signal
pass-thru capability in the Delta unit.

 

[Archimedes' Lever]

On Fri, 2 Jan 2009 15:12:56 +1100, "Phil Allison"
<philallison@tpg.com.au> wrote:

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

I didn't write that, idiot, you did.

 

[Archimedes' Lever]

On Fri, 2 Jan 2009 15:12:56 +1100, "Phil Allison"
<philallison@tpg.com.au> wrote:

you fucking oxygen thief.

No, Phil... it appears that is your job.

 

[Phil Allison]

"Archimedes' LUNATIC"

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

I didn't write that, idiot, you did.

** ROTFL !!

See the " ** " - eh ???????????????

Wanked yourself blind ????

FUCKWIT




...... Phil

 

[Ross Herbert]

On Fri, 02 Jan 2009 04:56:03 GMT, Ross Herbert <rherber1@bigpond.net.au> wrote:

:Actually, the Delta LIU is a bit of an overkill for what you want to do so here
:is a much cheaper solution http://www.accesscomms.com.au/Products/K2400.htm
:
:Basically this will just be a 600:600 line transformer without the DC signal
ass-thru capability in the Delta unit.


If you have access to Farnell then here's a suitably rated 600:600 line
isolation transformer
http://sg.farnell.com/177497/power-products/product.us0?sku=oep-oxford-electrical-products-oep1200

 

[Archimedes' Lever]

On Fri, 2 Jan 2009 16:16:23 +1100, "Phil Allison"
<philallison@tpg.com.au> wrote:

"Archimedes' LUNATIC"


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

I didn't write that, idiot, you did.


** ROTFL !!

See the " ** " - eh ???????????????

Wanked yourself blind ????

FUCKWIT

** doesn't mean a goddamned thing in English or Usenet, you retarded
fuck.

The fuckwit sig is nice though, Phil. Fits you perfect. You should
keep using it because you are such a fuckwit, of course.

 

[Phil Allison]

"Archimedes' Wanker = LUNATIC"

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

I didn't write that, idiot, you did.

** ROTFL !!

See the " ** " - eh ???????????????

Wanked yourself blind ????


FUCK - WIT





...... Phil

 

[Floyd L. Davidson]

"Phil McKerracher" <usenet@mckerracher.net> wrote:

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:

Bill Janssen wrote:

Eeyore wrote:
...the phone should be a
reasonable match to the line to minimize refections
which bother the users as
echoes...

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.

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.

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.

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

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

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 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.

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

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).

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com

 

[Floyd L. Davidson]

Paul B <mail@nomail.invalid> wrote:

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

Noises that sound like a hot frying pan with sizzling
oil in it, are caused by leakage to ground on one side
of the telephone line. Noises that are 50-60Hz hum are
caused by an imbalance on the telephone line. You can
easily have a combination of both too.

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.

That is almost guaranteed to be a problem with balance.
I'd expect your PC input is an unbalanced circuit, while
the telephone is a balanced circuit. Connecting them
together causes the tel line to be unbalanced, which
picks up noise.

Secondly and quite separately from the above....

I didn't raise this problem in my first post. I am getting hum and
noise when I record using a purpose build connector (Retell model 156
~ see link below) to a hand-held battery-powered flash-memory
recorder even when the phone is on hook. I can't see where the hum
is coming from unless it is on the phone line because there can't be
a ground loop this time.

http://www.telephonerecorder.co.uk/recording/connectors/156.htm

Probably as above, mixing balanced and unbalanced
circuits. Given that they don't exactly tell us what is
in any of those products, it isn't possible to be sure
when and where any of them can be used.

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

I doubt you have a ground loop, and an isolation
transformer probably isn't going to help (though there
is a remote chance that it would... but it is very
remote).

Retell have a model (the 157) which connects direct to a PC and I
believe it is identical to the 156 except it has the additional
transformer I am asking about.

That appears to be true.

Consider some of the characteristics you need in your
equipment. You'll want a device that "bridges" the
line. That will prevent it from loading the line down
and reducing the volume to the person using the
telephone set. Typically a bridging device has about
10x the impedance of the circuit being bridged. (That
will cause only 0.3 dB reduction in volume due to the
extra load. However, it also cause about a 30 dB
reduction in signal to the bridging device! Probably
not significant for recording, but it does mean that
transmitting a recording back to the telephone line
requires significant power output.)

You also want the device to have no DC connection, so
that it will not interfere with the supervision of the
telephone line (on/off hook status). Use a
non-polarized capacitor, somewhere from 1 to 4 mfd, to
provide that isolation. Ideally that would be placed at
the mid-point of a split winding transformer primary; or
one capacitor on each side in series; but could be just
one capacitor in series on one side.

If you connect such a device directly across the
telephone line, you will get all the clicks from the
hook switch, you will get the ring current full force,
and you will hear the local user very loud in comparison
to the distant user. Hence you probably would want to
connect it internally in the telephone set, across the
receiver (where the telephone design will have reduced
the effects listed above).

The secondary of the transformer should be as appropriate
for what ever device you are connecting it to. That may
or may not be high or low impedance and may or may not
be balance or unbalanced. The balance issue, however,
doesn't actually make any difference because of the
isolation provided by the transformer. The impedance is
a bit tricky, because if the recording device is high
impedance then you'll want a 1:1 transformer, but if it
is 600 Ohms you'll want one with a 1:10 impedance ratio.

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com

 

[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> !

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
practice, 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

 

[Eeyore]

"Floyd L. Davidson" wrote:

Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:
Twisted pair cables are not 600 Ohms.

Absolutely not. About 100 ohms.

Cable type Impedance at 1000 Hz

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

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 ?
http://www.st.com/stonline/products/literature/an/7205.pdf

CRETIN !

Graham

 

[Eeyore]

"Floyd L. Davidson" wrote:

Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:
"Floyd L. Davidson" wrote:

Telephone cable is twisted pair

Which is why it's 100 ohms or so just like Cat 5 etc.

Except of course when it has loading coils in it, and
then it is something much higher.

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

 

[Eeyore]

Stuart wrote:

Eeyore <rabbitsfriendsandrelations@hotmail.com> wrote:

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

Oh dear, you need educating.

http://www.telephonesuk.co.uk/images/232_cct.jpg

What do you think "Coil,Induction No20" is?

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

 

[Eeyore]

"Floyd L. Davidson" wrote:

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

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

 

[Eeyore]

Phil McKerracher wrote:

Bill Janssen wrote:

...the phone should be a reasonable match to the line to minimize
refections which bother the
users as echoes...

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]

Archimedes' Lever wrote:

"Phil McKerracher" <usenet@mckerracher.net> wrote:

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.

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

Graham

 

[Michael A. Terrell]

Eeyore wrote:

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> !

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
practice, using a '600 ohm' transformer will probably be ok, but being incorrectly
loaded will degrade the sound quality (freq resonse may be peaky etc).

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


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Eeyore]

Franc Zabkar wrote:

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