Balancing Act

August 15, 2008 at 3:01 pm

Circuit vocabulary is important in our industry, to say the least. I often use terms which I know are correct, but don’t fully understand. So today is a learning day for me, and I thought I’d bring you along for the ride. We’ll start with some audio circuit basics and I’ll try to define them the way I understand them. Then with the help of our service and cabling guys, the harder concepts will hopefully become clear.

Balanced Cable- A balanced circuit has 3 conductors: one hot (positive), one cold (negative), and a shield. The shield is grounded (generally to the chassis of the gear, if not to the earth) and wraps around both hot and cold in the cable. This protects the circuit from RF and DC noise. Positive and negative indicate that audio signal is duplicated on each leg. However the negative side is 180 degrees out of polarity. This is important for noise cancellation to work. Here’s how… Both the positive and the negative legs of the circuit pick up the same noise. The audio on the negative side is 180 degrees out of polarity, but the noise is not. It is identical to the noise on the positive side. Once the signals reach the gear a process takes place. The negative leg goes through a polarity flip and is combined with the positive leg. Now the noise from the negative leg is in opposite polarity than the noise from the positive leg and cancels it out. Check out the videos below.

Clip #1

Clip #2

The first image/clip is with the hot and cold legs of an audio signal out of polarity. Notice how the waves are opposites, while the noise is identical on both hot and cold. The second image/clip inverses the polarity on both the cold and cold noise. This in turn reinforces the hot audio and cancels the noise just as a balanced cable naturally feeds a balanced input.

Unbalanced Cable- Instead of a dedicated shield the cold and the shield are tied together or the cold doesn’t exist at all. Because the audio path is referenced to the system ground the power source of the gear may add noise the the circuit. Short cable runs are rarely effected in a mixer bag, but balanced circuits reduce the risk of noise.

You can’t tell if a cable is balanced just by looking at the connector. For instance, in a typical XLR connector Pin 1 is ground, Pin 2 is hot, and Pin 3 is cold. However a small touch of wire and solder between Pin 1 and Pin 3, unbalances the signal. Looks CAN Be deceiving.

In the world of location audio gear, there is rarely room to create a 100% balanced circuit. This would require duplicate circuits for hot and cold, on each input, and each output. In the effort to conserve space, electrical engineers use several methods. The inputs are generally balanced, the interior audio processing unbalanced and a combination of balanced and unbalanced outputs.

Impedance Balanced- Usually begins as an unbalanced signal. The audio is on the hot. The cold and ground are tied together. However a matching resistor is added to the cold/ground leg in order to “balance” out the resistance created by the audio on the hot leg. Resistance/Impedance is measured in Ohms. For example. If the audio signal is running through 470 Ohms of electrical resistance, then a 470 Ohm resistor is added to the cold leg, filtering hum, RF, etc and allowing the hot audio to pass.

Active Balanced- Each leg of the circuit both cold and hot are pushed by individual amplifiers. This creates steady constant flow down long cable runs.

Transformer Balanced- This is a basic form of balanced circuitry. It is commonly used when physical space is available in the gear. Transformers take much more space than Impedance or Active circuits. Here’s how it works. A transformer in its most basic form is little more than 2 coils of stranded conductor wrapped around a magnetic metal such as iron. These two coils are set very close together, not touching. As the first side takes the audio signal (can be either balanced or unbalanced on the input) into the transformer, the audio (AC signal) creates a magnetic pulse called flux. The flux from the the primary side induces a current in the secondary coil, which theoretically is an identical copy of the original. The gap between the two magnets helps remove noise from the circuit as only the audio signal is passed.

These are MY understandings of how these circuits work. If every electrical engineer, gear manufacturer, cable maker, etc. agreed, then troubleshooting compatibility wouldn’t be necessary.

For deeper understandings in the world of electrical engineering and circuit basics, read up on Ohm’s Law, and Faraday’s Law.

If you can fully grasp these concepts my hat is off to you. I understand them, every time I relearn them, then I forget them. Someday, I hope the cycle will end! I’m sure many of you will want to correct my understandings. Feel free below.

Skylor Morgan