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?
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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.
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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.
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Stuart Winsor

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True.

The transformers used on 56k modems and such perform considerably better than the old telephone transformers in both available badwidth and distortion characteristics http://www.epanorama.net/documents/groundloop/audio_isolator_building.html

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.
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Tomi Engdahl (http://www.iki.fi/then /)
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You really know how to use the language of Shakespeare.
Bill
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Stuart wrote:

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
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On Mon 29 Dec 15:36, Eeyore

Graham, I must say a better sound card is attractive. I checked out Terratec which you mentioned. Their web site lists the Aureon 5.1 PCI card but it has those potentially noisy 3.5 mm jack sockets.
http://www.terratec.net/en/products/Aureon_5.1_PCI_1988.html
I assume the noise would be from the build quality of the 3.5 mm jack plug as it seems to usually have a rivetted centre core that could and turn. The contacts in the sockets are also probably very basic.
To see what I would need to do for the balanced cicruit you were recommending I Googled Epanorama and saw this: <http://www.epanorama.net/circuits/teleinterface.html In the section called "Interference in the telephone line signal" it refers to a heavy 1 kg choke or transformer! That seems a bit of a non-starter.
I also saw the "DMX 6Fire USB" at Terratec but it seems too spartan and is not cheap at over 150. http://www.terratec.net/en/products/DMX_6Fire_USB_2084.html
So I have also checked out a few alternatives to the Terratec and I mention them at the end of my post to Phil Allison. They are:
(a) a Behringer Xenyx 502 mixer (32) <http://www.dv247.com/invt/31558/
(b) an analogue to digital interface Behringer UCA202 for 20 <http://www.dv247.com/invt/32730/
(C) or something which combines the two.
My post about this is at:
Google Groups: http://preview.tinyurl.com/ay6hew
-------=======------
Phew! That's a lot of researching!
Is there a simple circuit you know to attach the phone to the PC in a a balanced way?
Any constructive comments would be welcome.
Thank you.
PB
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D Do you meanj isolation transformer or matching transformer. There is no impedance matching (transformation) using a 1-to-1 transformer.
Bill
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"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
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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!
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"Floyd L. Davidson" wrote:

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
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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.
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Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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"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
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I'm sorry, but the nickname your mother uses for you is not appropriate in reference to other people.
Please not that I have not said it is a matching transformer, nor have I said that it is necessarily appropriate for the OP. All I did was properly describe what it actually is.

Wrong again. It provides isolation. As I mentioned in another post, a POTS loop needs to isolate the VF signal from the DC signal, and also to isolate the transmit signal from the receive signal.
It's primary purpose is isolation. It does not provide impedance matching (in typical telsets, though I assure you there are special ones that do have exactly that function built into the hybrid network).
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Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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That is not true. You need to use Google and learn something.
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Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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Hey, look at that... it *still* isn't true! Amazing, eh?

Your cited source does not support you statement. It merely says that is one type of "isolation transformer". (Regardless, Wikipedia is not a credible source.)

Again, that is not a definitive source, but in fact it does not support your claim anyway.
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Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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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
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Here are some of my experiences on making mu own such devices: http://www.epanorama.net/documents/groundloop/audio_isolator_building.html

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 http://www.epanorama.net/documents/telecom/teleinterface.html document).

Modern normal telephones are normally "floating" line powered devices where electronics connect directly to line.
--
Tomi Engdahl (http://www.iki.fi/then /)
Take a look at my electronics web links and documents at
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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
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"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
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