Getting matching transformer from telephone

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?
Reply to
Paul B
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On Sat, 27 Dec 2008 22:03:10 GMT, Paul B put finger to keyboard and composed:
FWIW, you may find some "600 ohm" transformers in old modems, ie those with a "non-silicon" DAA.
- Franc Zabkar
Reply to
Franc Zabkar
"Paul B"
** You can get a stereo pair of "audio line isolation transformer s" from places that supply car audio gear - these come with male and female RCA plugs which you can change to mini-jacks at one end.
** Nonsense.
Generally phones have no such transformer inside ( no need exists as a phone is not grounded like your PC is ) and in any case they are not suitable for hi-fi audio.
..... Phil
Reply to
Phil Allison
D Do you meanj isolation transformer or matching transformer. There is no impedance matching (transformation) using a 1-to-1 transformer.
Bill
Reply to
Salmon Egg
Even if there is (they used to use hybrids but I don't know these days) the quality will be poor. They are only intended for voice. For music you require a much larger bandwidth and lower distortion characteristics. Expect to pay at least £30-50 for something decent by Sowter or similar. Ideally, you will also need to know the impendence of your sound card input to match it properly, or assume it is high (it probably is) and resistively terminate the transformer secondary.
Reply to
Stuart
Places like Hosfelt, All Electronics and Jameco get these 600:600 transformers in as surplus now and then and sell them for around $1 USD, I got a hand full a while ago for connecting up a PC/TV and audio amp cludge. There may be some degradation but I can't hear it.
Reply to
gfretwell
Historically they *all* had such a transformer, and even today many of them do (it's cheap).
That is quite true. The transformer in a telephone is usually referred to as a "network", and it probably includes components other than just the transformer, plus it is a balanced hybrid transformer (and the network has an imbalance built in to provide sidetone) designed to carry at least 120mA of loop current. Not exactly what one would go for in a hi-fi system!
Reply to
Floyd L. Davidson
Here are some of my experiences on making mu own such devices:
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Good quality transformers seem to cost considerable amoutn of money.
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...
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
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.
Modern normal telephones are normally "floating" line powered devices where electronics connect directly to line.
Reply to
Tomi Holger Engdahl
True.
The transformers used on 56k modems and such perform considerably better than the old telephone transformers in both available badwidth and distortion characteristics
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Line level audio input connectors on PC sound cards are high impedance inputs, typically around 10-47 kohm.
Depending on the selected transformer a terminating resistor on transformer output might be needed or not.
Reply to
Tomi Holger Engdahl
Not really. It's because the telephone system uses balanced (or differential) audio signals and your PC doesn't.
Graham
Reply to
Eeyore
Transformers are not cheap and I haven't ever seen a phone with one in, even going back 30+ years. There's simply no need.
Graham
Reply to
Eeyore
Cheaper to buy a decent sound card with balanced ins and outs (plus not on those GHASTLY 3.5mm jacks) and learn how to use them properly.
Terratec do a moderately inexpensive one IIRC.
Graham
Reply to
Eeyore
Get a diagram of any given telset you wish that works without active components (amplifiers), and you *will* find a transformer.
And trust me, they *are* cheap!
Going back 30+ years, they *all* had transformers.
Reply to
Floyd L. Davidson
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?
Bill
Reply to
Salmon Egg
You really know how to use the language of Shakespeare.
Bill
Reply to
Salmon Egg
Trollschemedes' }:) you slay me Roy.
Cuwahahahhahahahahaha !
coqu=EE coqu=EE
Reply to
Mr.Peckering Soundz
pleeeez Trollschemedes' stop trying to tell roy anythin about faith and the soul or even life, you are not among da living - its stuff you don't kno anything about.....
Troll dont you get it? eres un Evil Phantasm.
coqu=EE coqu=EE
Reply to
Mr.Peckering Soundz
"Floyd L. Davidson"
** But not the 1:1 matching/ isolating kind the OP asked about - dickhead.
The " hybrid " circuit couples the earphone and mic to the line but is generally not even isolating.
...... Phil
Reply to
Phil Allison
"Salmon Egg"
** 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
Reply to
Phil Allison
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
Reply to
Eeyore

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