111C Coils: A Golden Find

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Program distribution. You can use each winding separately of course; if you were to feed one winding with a 300 ohms source you would see that program repeated on each of the other three windings. It's a good idea to make sure each winding is terminated in 300 ohms (Figure 3), otherwise the frequency response of the system may be aberrant. You could call this a poor man's distribution amp (DA), but the fact of the matter is it works great, and it's passive.

Figure 3.

Figure 3.

Mono-Summing. You can mono sum with two of the windings, but make sure that you have the source impedance correct. Each winding needs to be fed with a 300 ohms source impedance. You may need to add build-out resistors in series with any DA or other output amps, otherwise the windings could compromise the peak output level of said output amps, creating a headroom problem. In addition, you need to make sure the phase relationship is correct between the left and right channel (unless maybe you want an L-R source?). See Figure 4. The two remaining windings will have L+R on them.

Figure 4.

Figure 4.

RF isolation. One of the neatest things about the 111C coils is the built-in faraday shield between the line side and the drop side (see Figure 1 again). If you have a long line bringing analog audio to some location, say near an AM transmitter site, you can isolate that line from any amplifiers connected to the line side by connecting the S terminal to your local ground.

In this day and age of balanced AES digital circuits, Ethernet and fiber, the functionality given by the old trustworthy 111C coil isn't nearly as important as it once was; however, one day it may save your bacon. Keep a couple on hand just in case.

Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.

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