Getting matching transformer from telephone

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

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.

It isn't a matching transformer. It is for isolation purposes, and yes, there is one in all phones that attach to Ma Bell.

Actually it *is* a matching transformer (check out the impedance of a telset transmitter). It also provides isolation. It is also a "hybrid" transformer.

Pretty typical multiple use design from Bell Labs at the height of the good ol' days.

Here are some of my experiences on making mu own such devices:

Good quality transformers seem to cost considerable amoutn of money.

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

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

A real hybrid uses 2 transformers to get the 2-4 wire. All sorts of fiddle designs around 1 generally low quality tansformer in a phone. I have spent years breaking derelict BT plant I have found for their quality transformers wheich were made to a spec rather than a budget. As another poster said Sowters are good as are Partridge and Jensen although I always hark back to the late Dr Sowters designs. Sowter are the makers of the transformers in the RS range..

For decent phone signal for broadcast or even generl audio use I tend to pull a Sonifex out. For other use I tend to use A Telex/RTS 2-4 wire box.

Regards

Al .

True.

The transformers used on 56k modems and such perform considerably better than the old telephone transformers in both available badwidth and distortion characteristics

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.

Not really. It's because the telephone system uses balanced (or differential) audio signals and your PC doesn't.

Graham

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

Backward Americans as usual ! ;~)

Graham

Or a couple of op-amps suitably configured.

Graham

Lundahl are the best audio transformers around IMHO.

Graham

Or one transformer with three windings. Or, for that matter, a network of resistors can make a nice hybrid too.

An ignorant statement, to say the least.

As far as it being a matching transformer, the line impedance varies typically from perhaps 100 Ohms all the way up to perhaps 2000 Ohms... but you will not find anything in a telset to adjust it to match. That's because nobody cares if it is even close to matching the line impedance.

Eeyore is a bitter British, anti America idiot who claims to be the world's top audio equipment designer. Just ask him, and he'll inform you of his 154 IQ, and the equipment he designed is for sale on Ebay. A place that would shame Ali-baba & his 40 thieves.

Which is why I stated its primary purpose as being that of isolation

IT IS

What exactly needs isolating from what ? Have you noticed they tend to be made of plastic ?

Graham

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

Graham