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Unbalanced or Balanced...HUH?

Blake Engel, All Church Sound


What's all the talk about things being balanced and unbalanced? Who cares if two cables will balance on a teeter-totter or not? Seriously, what's the difference and why does it matter?

First, a little background on cable construction. All low signal level audio cables are shielded--that is, the inner conductor(s) have a shield of wrapped or braided wire, or foil wrapped around the outside of the center conductor(s). This shield blocks RFI (radio frequency interference) from entering the cable. Besides just picking up noise, if an audio cable isn't shielded well (or at all), as the length of the cable approaches the length of a transmitted radio wave, the cable turns into an antenna. This antenna then picks up the transmitted radio wave very clearly!

A consumer tape deck, CD player, and VCR all have unbalanced audio outputs. An unbalanced output is one where the audio signal is conducted through two wires to provide a simple circuit. An unbalanced cable has two wires--a center conductor and the shield. The shield serves to drain the RFI as well as acting as a conductor to complete the circuit. The trouble is that if two pieces of interconnected equipment are both ground referenced (if they have a grounded AC plug or are otherwise grounded), a ground loop can be formed between the two units. To fix this problem, the ground wire at the receiving end of the audio cable can be disconnected, but then the audio circuit path includes the signal going through the grounding system, which introduces ground noise.

A second problem is that where connections are longer than a few feet, the possibility of EMI (electromagnetic interference) from power cables increases. Every wire carrying power has a magnetic field around it. This includes AC power wires, speaker level cables (from amplifiers), and even other microphone or line level cables. EMI noise shows up as humming, buzzing, and static. The problem may be constant, or it may be intermittent (if the cables producing the EMI are powering a device that's not used constantly).

The diagram below shows the input stage of a mixer. An unbalanced input passes both the audio signal and noise to the output. OK, now that we have a basic understanding of how an unbalanced connection works, what about balanced? A balanced connection uses a total of three wires--the ground/shield, and two wires that both carry the audio signal (but opposite polarity of each other). As you can see from the balanced circuit diagram, the two signal wires both carry the audio signal (curved sine wave), but the signal is opposite on the negative wire. Noise that's induced onto the two wires is induced equally on both wires. When the signal gets to the equipment input, the negative (opposite) signal is inverted (reversed) and added to the positive input signal. The audio signals are then in correct polarity with each other, and add up nicely. On the other hand, any noise carried by the wires cancels out because the one signal has been inverted. Notice that the output contains only the clean audio signal and no noise.

Balanced inputs on equipment have a CMRR (common mode rejection ratio) number that tells you how well the input can reject a signal (such as noise) that's common to both inputs. The higher the number, the better it is. (CMRR decreases with an increase in frequency because it's difficult to maintain the same capacitance and inductance to ground in each leg of the wiring and circuitry.)

Many, many churches have key cable runs (mixer to EQ to amplifier) connected unbalanced, and they wonder why the system is noisy. It's not uncommon to run new cable, make new connections (all balanced), and have the hum and noise totally disappear.

Balanced circuits are always better than unbalanced, but there are many factors which determine how good a balanced input or output is. This can affect overall performance.

Balanced connections are made using XLR connectors, 1/4" TRS (tip-ring-sleeve (stereo)) connectors, or just bare wires (using 3 wires). Unbalanced connections use 1/4" TS (tip-sleeve (mono)) connectors, RCA connectors, and bare wire terminations (where only 2 wires are used). Even if a connection uses an XLR or 1/4" TRS connector, all 3 wires may not be used (and so it may still be unbalanced)! Short runs (less than 5 feet) of unbalanced cable to/from tape decks, CD players, or VCR's isn't as much of a problem.

Check your sytem for unbalanced connections--and have them changed to balanced connections as soon as possible! Give us a call for help in solving your noise problems.