111C Coils: A Golden Find


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So while you were cleaning the storage area at the transmitter site, you found these things with solder terminals on top and a painted label that says 111C repeating coil (or something to that effect). What is it, you may ask? Don't throw them out. After all, they officially belong to the telephone company, although they'll never be claimed of course.

What you have is a valuable nugget.

Figure 1.

Figure 1.


In the old days (before digital phone line circuits) if you needed to send audio to the transmitter site (as an STL) or somewhere else, you would order 8kHz or 15kHz lines from the telephone company. When the installer showed up at your facility, he stuck these transformers on the wall in your telecom closet. The line side windings were wired in series, and the drop side windings were wired in parallel. See Figure 1. This presented a 600 ohm termination to the equipment and sent the audio toward the nearest central office (CO) with a 150 ohms source impedance. This is because each winding will present a 300 ohms load (assuming each winding is terminated in 300 ohms).

These transformers are great little problem solvers to keep around. Here are just a few things that they can do.

Source isolation. I used to do a lot of remotes with stereo phone lines (in the days before ISDN) and I had a set of 111C coils mounted to a piece of pine with terminal blocks on the ins and outs. If I needed to isolate the source (like a front-of-house system) from the remote broadcast equipment, I would insert these in the lines from the stage. Nothing works better at killing ground loops than 111C coils. Both sides would be wired up for 600 ohms (Figure 2).

Figure 2.

Figure 2.


- continued on page 2



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