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Re: Getting matching transformer from telephone

  • Thread starter Tomi Holger Engdahl
  • Start date
T

Tomi Holger Engdahl

Jan 1, 1970
0
Paul B said:
I am looking for some 1-to-1 matching transformers to connect varioua
audio devices to my PC. I usually get noises and hum.

Here are some of my experiences on making mu own such devices:
http://www.epanorama.net/documents/groundloop/audio_isolator_building.html
These line matching transformers are not so cheap at about £6 or 7
each.

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

600 ohms 1-to-1 matching transformers are quite rare in telephones.
Modern normal telephones are normally "floating" line powered
devices where electronics connect directly to line. The whole
small device is "floating" isolted from everythign else
so that gives good balance.

You can find 600 ohms 1-to-1 matching transformers most often
on modems. And those are also in some telephones that use
external power...
If I strip down some landline phones I 've got here, then will there
be a matching transformer in each one?

Propably not any transformer in a modern phone at all.
And in older ones where there was a transformer that is most
propably not a type of transformer you are looking for
(for details on transformers used at beginning of
http://www.epanorama.net/documents/telecom/teleinterface.html
document).
Or is their technology different now?

Modern normal telephones are normally "floating" line powered
devices where electronics connect directly to line.
 
P

Phil Allison

Jan 1, 1970
0
"Salmon Egg"
I know of no situation where something specified as say a 150 ohms
1-to-1 matching transformer would perform significantly different than
something specified as a 600 ohms 1-to-1 matching transformer. This
assumes that they both can support the same voltage over the same
(telephone audio) bandwidth. Am I missing something?


** Errr - yep.

Impedance ratings matter very much in relation to the drive and load
impedances that are bests for a particular transformer.

Get them wrong like that and the overall frequency response will not be flat
across the audio band.



....... Phil
 
E

Eeyore

Jan 1, 1970
0
Salmon said:
I know of no situation where something specified as say a 150 ohms
1-to-1 matching transformer would perform significantly different than
something specified as a 600 ohms 1-to-1 matching transformer. This
assumes that they both can support the same voltage over the same
(telephone audio) bandwidth. Am I missing something?

Twisted pair cable as used for telecoms has a nominal 100-110 ohm
inpedance. See the ADSL specs.

600 ohms is an irrelevant historical nonsense from the days when they used
telegraph wires for phone circuits.

Graham
 
B

Bill Janssen

Jan 1, 1970
0
Eeyore said:
Salmon Egg wrote:



Twisted pair cable as used for telecoms has a nominal 100-110 ohm
inpedance. See the ADSL specs.

600 ohms is an irrelevant historical nonsense from the days when they used
telegraph wires for phone circuits.

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

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

Bill K7NOM
 
P

Phil Allison

Jan 1, 1970
0
"Bill Janssen"
Impedance for an un-loaded pair varies with frequency. So at DSL
frequencies the impedance
is 100 to 110 Ohms. 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


** Huh ??

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

The speed of electrical signals must be a * hell of a lot * slower than C
where you live - pal.


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


** At least you do know what an "isolation " transformer is.

Unlike some here.


...... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Lostgallifreyan"


** **** OFF -

YOU RIDICULOUS DICKWAD !!
 
P

Phil Allison

Jan 1, 1970
0
"Lostgallifreyan"


** **** OFF -

YOU RIDICULOUS LYING DICKWAD !!
 
P

Phil Allison

Jan 1, 1970
0
"Lostgallifreyan"
"Phil Allison"

Posted after last post in a subthread does not mean aimed at last poser,
when
the new post is a useful read for all posting. Got that? Read it, it's a
lot
more interesting than you are right now.


** Oxygen thieves like this anencephalic turd need shooting.




.... Phil
 
T

Tomi Holger Engdahl

Jan 1, 1970
0
Eeyore said:
Twisted pair cable as used for telecoms has a nominal 100-110 ohm
inpedance. See the ADSL specs.

The twisted pair cable as used for telecoms has a nominal 100-120 ohm
impedance for high frequecies the ADSL system uses. The ADSL
system uses frequencies between 138 kHz and 1.1 MHz for downstream
data (and 25 kHz to 138 kHz ofr upstream). This 100-120 ohm
impedance holds pretty well for those frequencies above 100 kHz.

For lower frequencies the the impedance of the telephone cable
is not anymore that 100-120 ohm, but something else.
For voice frequencies used on on normal telephone (300-3400 Hz)
the impedance is normally considerable higher than 120 ohms.

Normal telephone subscriber lines in USA (0.4-0,6mm subscriber PE
insulated vaseline filled cable) are 770 ohm resistor (with 2uf series
capacitor) and 47nF parallel capacity.


2 uF
||
----+-----||--------+
| || |
| | |
--- | | 770 ohms
--- 47 nF | |
| |
----+---------------+

This diagram is referred to 800Hz, but impedance is rather complex,
and varies from high value at low frequency and drops to ca. 150 ohm
on 10kHz and 120-125 ohm above 100kHz.

Some telephone lines can have higher impedance (typically 1100 ohms in
lines with loading coils or telephone air cables).

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

Typical cable used in for subscriber lines has following
characteristics: 0.5 mm diameter wire, loop
resistance 182 ohm/km and pair capacitance 39 nf/km.


TRANSMISSION SYSTEMS FOR COMMUNICATIONS, revised 4th edition, Bell
Telephone Laboratories (1971) gives the followign information on
typical cable characteristics:

"The primary constants of twisted pair cables are subject to
manufacturing deviations, and change with the physical environment
such as temperature, moisture, and mechanical stress. The inductance,
L, is of the order 1 mH/mile for low frequencies and the capacitance,
C, has two standard values of 0.066 and 0.083 uF per mile although
lower capacitance cables are under development.

Of the primary constants, only C is relatively independent of
frequency; L decreases to about 70 percent of its initial value as
frequency increases from 50 kHZ to 1 MHz and is stable beyond; G is
very small for PIC (polyethylene insulated cables) and roughly
proportional to frequency for pulp insulation; and R, approximately
constant over the voiceband, is proportional to the square root of
frequency at higher frequencies where skin effect and proximity effect
dominate."

600 ohms is an irrelevant historical nonsense from the days when they used
telegraph wires for phone circuits.

600 Ohms is somewhat of a compromise between different real-life
impedances that could be seen. Normal telephone line connections are
theoreticallydesigned to be 600 ohm resistive impedance. This 600
ohm is kept as international reference for designing telephone line
equipment (typically the signal powers are measured to 600 ohm load).
In practice the telephone line does lot look like pure 600 ohm
resistance.

Telephone equipment which is designed to operate with 600 ohm loads
will operate with those real-life lines, but it's performance is worse
than in ideal situation. Typically the modems are designed for 600 ohm
reference impedance because they can handle the sidetone.

The return loss of the terminal equipment must be greater than 10 dB
when compared to 600 ohm reference. This measurement applies to
telephones, modems and other terminal equipments. NET4 technical specs
are European specs and they are used in many European countries (NET4
is actually a collection of different specs in use in different countries).

For best performance the telephones are designed to the exact line
impedance. Matching the hybrid circuit to the real line impedance
(instead of 600 ohm) will improve the feedback typically by
3-6dB. 20dB sidetone is easy to achieve, but 30dB is also not too
difficult provided you can measure the line impedance and take steps
to build a correct balancing network.
 
A

Archimedes' Lever

Jan 1, 1970
0
Which has nothing to do with the information I am giving here.

I have come across your attitude in other groups where americans don't
seem to understand that their way of doing things isn't universal
throughout the rest of the world.

Hey! Floyd doesn't have an attitude, and is pretty darned good at
accepting other standards and practices, once they are brought to his
attention. I am sure he will concur with you once he realizes from your
post, that the geography and era were different than that he had his
mindset in.
 
R

Roy

Jan 1, 1970
0
Floyd and You, Archneedsmeds' ?
No Shit' Sherlock...gotcha beat don't it? One day, soon, I'll look back
on this and I'll dump all the arrogant self deluded trolls and
filerkillers in the crap block.
®
 
J

John Livingston

Jan 1, 1970
0
Floyd said:
Stuart said:
You also apparently don't know what "repeater stations"
are when dealing with analog carrier systems.

Repeater Stations were not limited to FDM carrier systems. "Repeater"
refers primarily to amplification. Plenty baseband amplified circuits as
well (2 wire and 4 wire).

Correct. I've worked on circuit equalisation at Burghead end. It was
baseband audio - not carrier.
You sent them that distance via FDM carrier systems, not
via landlines, even in the 1930's.

Very rarely - most carrier channels were 300-3400Hz. The noise and
distortion figures came nowhere near the requirements for broadcast
audio. ISTR there were attempts to use carrier, but with all sorts of
problems, not the least being that the CCITT carrier frequency plans
(Groups-Supergroups-Hypergroups) are based on 4KHz channel spacing - not
10KHz. Single Sideband translations are also problematic with music
(broadcast) circuits, as the frequency stability requirements are MUCH
higher than telephone-quality speech. Master oscillators DID go off
frequency ....
Twisted pair cables are not 600 Ohms.

WRONG!!!!!

The characteristic impedance of a transmission line(Zo)varies by
frequency. At audio the influence of series resistance and parallel
conductance outweighs the ratio of inductive to capacitive reactance,
and the Zo rises as the frequency falls. A twisted pair will be
typically 600 ohms at 800Hz.
At RF the reactances become much more significant than the
conductance/resistance, and the Zo will level out at the figure usually
quoted as Characteristic Impedance. The same twisted pair will be about
140 ohms at RF.
Open wire might be though... ;-)

See above.
"rep-coils"???? WTF ??

Known as "Transformers" in ever repeater station I ever worked in .....

By and large? And 75 Ohms??? (Please don't try buzz words...)

Been there. Done it.

John
 
A

Archimedes' Lever

Jan 1, 1970
0
Floyd and You, Archneedsmeds' ?
No Shit' Sherlock...gotcha beat don't it? One day, soon, I'll look back
on this and I'll dump all the arrogant self deluded trolls and
filerkillers in the crap block.
®


**** off, Roy. There is not a damned thing happening in this group
that has a fucking thing to do with you.

Grow the **** up, punk.
 
R

Roy

Jan 1, 1970
0
From: Archimedes' Lover
On Tue, 30 Dec 2008 08:35:24 -0500, [email protected] (Roy) wrote:
Floyd and You, Archneedsmeds' ?
No Shit' Sherlock...gotcha beat don't it? One day, soon, I'll look back
on this and I'll dump all the arrogant self deluded trolls and
filerkillers in the crap block.
®
    **** off, Roy. There is not a damned thing happening in this
group that has a fucking thing to do with you.
    Grow the **** up, punk.
--------------------------------
**** you, that's what's eating you all up inside, that I am wall to wall
in here.
Give it up already, you're headed for self destruction, Faggot!

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

Eeyore

Jan 1, 1970
0
Floyd L. Davidson said:
For some cables. Not for others.

WRONG ! For all practical cables used on the 'local run' for telecoms it's
100-100 ohms.

It might, for example, be as high as a couple thousand Ohms too.

Utter tripe.

Not at all true. 600 Ohms is somewhat of a compromise,
between the low impedance of an unloaded cable pair and
the higher impedance if loading coils are used.

You haven't a FUCKING CLUE what you're talking about.

Graham
 
E

Eeyore

Jan 1, 1970
0
Stuart said:
600 ohms was the theoretical impedance of an infinite length of GPO
telephone cable. In the early days of broadcast radio (1930s), when
program was sent to the transmitters by GPO cable, assuming a line
impedance of 600 ohms and matching to it, was found to give the best
overall results. When you're sending program 600 miles from London to a
transmitter in Scotland, it matters!

They weren't using something that resembles a single pair of Cat 5 back then.

As I said, it's HISTORICAL only and of no practical value.

Graham
 
E

Eeyore

Jan 1, 1970
0
Archimedes' Lever said:
Hey! Floyd doesn't have an attitude, and is pretty darned good at
accepting other standards and practices, once they are brought to his
attention. I am sure he will concur with you once he realizes from your
post, that the geography and era were different than that he had his
mindset in.

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
 
E

Eeyore

Jan 1, 1970
0
Floyd L. Davidson said:
It has everything to do with the information we are
discussing.

You don't know the difference between telegraph wire and
telephone wire, just for starters.

Quite so !

If that was in reference to the previous paragraph of
yours, then you should have put the information there.
In that case I would have jumped all over your claim
that it makes a difference for the 600 mile distance
mentioned when in fact it is not 600 miles *per* *section*.

You also apparently don't know what "repeater stations"
are when dealing with analog carrier systems.


You sent them that distance via FDM carrier systems, not
via landlines, even in the 1930's.


Twisted pair cables are not 600 Ohms.

Absolutely not. About 100 ohms.

Open wire might be though... ;-)


By and large? And 75 Ohms??? (Please don't try buzz words...)


Thank you. (I'm familiar with BT, which evolved from
the General Post Office.)

Via 'Post Office Telecommunications'.

Your problem is that you just don't actually understand
the telephone system, at all.

He has made that evidently apparent.

Graham
 
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