Repeat Coils or 1:1 Transformers
In the telephone world a transformer that has a 1 to 1 impedance is called a repeat coil. They are also in 1 to 1.5, Usually there are four windings and the transformer is about the size of pound of butter. This is the old world.
In the new world they are still 1 to 1 but the size is considerably smaller, typically about the size of two to four sugar cubes. In my designs I have used two different types, both with about the same results. Performance is about equal, the more expensive version has a higher breakdown voltage (for safety) and is rated to handle line current.
For cheap and dirty I use the Xicon TU-016 –RC ($2.50 ea) and have never had a problem. For “professional use” I use the Triad Magnetics TY-305P ($12 ea). Source for these is Mouser Electronics (www.mouser.com).
The only real difference is that the TU-015 is center tapped and the TY-305P is not.
Any Electronic Engineer will tell you that Impedance matching is of the utmost importance. Yes that is true, but for the circuits I have on here, close and not too far seem to work quite well.
Type Ratio Impedance Current Voltage Breakdown
TU-016-RC 1:1 600CT/600CT None 100 Volt Test
TY-305P 1:1 600/600 100 Ma 1000 Volt Test
Detailed data sheets are available on the Mouser site. Frequency response of these transformers is in the voice range.
Some surplus dealers, All Electronics and MPJA, sell "modem transformers" these are close to TY-305P in design and close to 1:1. Cost is about $2 each. The are getting harder to find. Radio Shack sells a SMALL 1:1 transformer but it will not take much DC current, like from a phone line.
For the circuits I publish on here you can use the TU-016-RC transformer. If your are doing it for commercial work or professional use, switch to the TY-305P.
A repeat coil transfers audio from one circuit to the other without loss (well just a little). Generally speaking the large old style ones had less loss and a better frequency response compared to the new ones that are smaller. More iron means less loss at loss at lower frequencies. A few older repeat coils were "ring through" meaning that they would also transfer the 20 Hz ringing from one circuit to the other, but these were not common.
The most sought after repeat coil is the Western Electric 111C, this was used in broadcast work and the frequency response for music was it's purpose in life.
Read "Principles of Electricity Applied to Telephone and Telephone Work" published by AT&T and available on E Bay for a few $$.
Three coils are shown above. (1) The large Western Electric Type 120 Repeat Coil, (2) The small 1:1 Transformer (similar to the TU-016), and (3) A round Loading Coil, which is a 150 Ohm/150 Ohm Repeat Coil, but not designed as one. A House Key is shown for size comparison.
For information on repeat coils, from days of old, visit this site http://www.strombergcarlsontelephone.com/kellogg/kellogg_main.htm
and look at the booklets/publications on repeat coils. Also take a look at the Jameco Catalog, they are now selling a small 600/600 ohm transformer, it is rated at only 15 ma of current but it should work fine on a normal telephone line.