A Matter Of Balance
Balanced lines are characterized by the fact that there are two center conductors for the signal, surrounded by a shield. (Figure 2)

This shield is connected to ground like unbalanced lines but it is not required as one of the signal conductors. Its sole purpose is to provide its line of defense against unwanted interference.

A benefit of this configuration is that the shield only needs to be connected to ground at one end of the cable in order for it to work.

Having this ground disconnected or “lifted” at one end can eliminate the ground loop problem discussed in the previous section on unbalanced lines.

Exception: The ground must be connected at both ends when transmitting phantom power. Phantom power will not work if the ground is lifted at either end.

Figure 2. Click to enlarge.

The two center conductors of a balanced line act as the sole conduit for the signal and operate in a “push-pull” manner.

That is, as the voltage on one conductor becomes positive, the voltage on the other conductor becomes negative by the same amount and at the same time (and vice-versa).

So at any point in time, both conductors are equal in voltage but opposite in polarity. The receiving circuit that processes this balanced signal is called a differential amplifier and this opposing polarity of the voltages on the conductors is essential for its operation.

Now, if any unwanted electrical interference manages to penetrate the outside shield, it will interact with both center conductors equally but with the same polarity.

The effect in the differential amplifier is that these same polarity voltages aren’t processed and effectively cancel each other out - the noise disappears.

This ability of balanced lines to reject noise and interference makes them popular when it is necessary to send signals over long distances.

Thanks to Whirlwind for supplying this article.

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