The EM Poll

Tech Basics Without Tears

Mar 14, 2008 5:29 PM, By Jon Chappell With Steve Oppenheimer

           

A PAIN-FREE EXPLANATION OF FIVE CONCEPTS ALL MUSICIANS SHOULD UNDERSTAND

Keeping Your Balance

Any electrical circuit contains at least two components: a signal and a ground. In an audio connection, this basic 2-conductor arrangement is termed unbalanced, and it's generally used for instrument cables, patch cords, and speaker cables.

Unfortunately, audio cables are subject to picking up noise in the form of hum from nearby power cables, radio broadcasts, and assorted electrical grunge that degrades your signal. If the signal level is hot enough, as with the cable from a power amp to a speaker, or the cable run is less than 20 or 25 feet, as with an instrument cable or patch cord, the noise will generally not be noticeable, and a regular unbalanced line will be fine.

But if you have a low-level signal or a long cable run, as with a microphone cable, the noise induced into an unbalanced line can be relatively loud, and when amplified along with the signal, it would be very noticeable indeed. Therefore, you need to get rid of the noise while retaining the original signal. That's when a balanced line comes to the rescue.

With a balanced audio connection, before the signal leaves the source, it is split into two paths. The polarity of one copy of the signal is inverted with respect to the original — that is, positive becomes negative and negative becomes positive. When the polarity is inverted, the signal is said to be 180 degrees out of phase with respect to the original signal. The connection is called balanced because it carries two opposite-polarity versions of the same signal.

The original and the inverted signal travel to the destination device on separate wires in a 3-conductor cable. (The third conductor is ground.) Along the way, both conductors pick up approximately the same noise. Since the noise didn't originate in the source device, its phase is the same in both conductors.

To summarize: in a balanced line, the first conductor carries the original signal and noise. The second conductor carries the inverted signal and practically identical noise. The third conductor is ground. Got it? Good!

Now, at the input of the destination device, the polarity of the already-inverted signal on conductor 2 is flipped back to its original state. But the noise in conductor 2 also gets flipped, so it is now inverted with respect to the noise in conductor 1.

The status at this point: path 1 carries the original signal and noise. Path 2 carries the original signal and out-of-phase noise.

Next, the two signals are summed. When you add together two identical signals with the same polarity, they reinforce each other. That's what happens with our original signal, which is good because it gives us a strong signal. In contrast, when you add together two identical signals with opposite polarities — as is now the case with the noise in our example — you are adding positive to negative, so they cancel each other out. Result: a strong signal and no (or significantly reduced) noise! The simple step of inverting the signal while it travels down the line and then reinverting it at the destination is the ingenious idea behind balanced lines.

Although other connectors are sometimes employed, balanced audio lines usually use XLR connectors like those found on low-impedance mics, or 3-conductor, ¼-inch phone connectors. The connection is wired with one signal going to the tip, one to the ring, and the ground to the sleeve of the phone plug, hence the term TRS. The TRS phone connector is the same connector used in stereo wiring (such as in headphones), but it carries the balanced signals instead of the left and right channels (and ground).