Physics of Balanced Audio: XLR vs. MIC Lines

[Rockazella]

I don't know if anyone here is familiar with audio/video recording equipment, or if anyone has ever heard of the 'XLR' standard...But While I was lookin over some specs on on it, I noticed that XLR systems/cables are referred to as balanced while your common MIC lines(used for headphones n such) are not. They say XLR being balanced tends to cut back on picking up noise when you have long runs of cable.

So anyway...reading all this sparked my interest in the basic physics of things, and now I want to know what makes these XLR hook ups 'balanced' while MIC lines are 'unblanced' and also how does being balanced help with maintaining signal integrity?

 

[Jimmy]

A balanced XLR line has three conductors: one ground and two signal wires. The two signal wires, referred to as hot and cold, carry the same signal from the source but the cold is inverted. The receiving end of the balanced line inverts the cold signal and adds it to the hot. This is done to eliminate common mode signals A common mode signal is a signal which appears equally on both conductors of a two wire line. Usually this appears as noise. Since the cold signal is inverted at the source, the common mode signal on the cold, when inverted at the receiving end, gets canceled when signals on the hot and cold wires are added together.

Common Mode Signals

Common mode signal rejection is not possible with a two conductor line. Typically, 1/4 microphone cables are unbalanced. As far as Microphones go, the Sure Beta 58 is an excellent, nicely priced balanced microphone which uses the XLR standard. Many good PA amplifiers, mixing boards, cross-overs and other components use balanced, XLR style connectors. Some also use a 1/4 inch, 3 conductor balanced cable. They look just like a 1/4 stereo cable or headphone cable.

I use balanced equipment and cables whenever possible. They are much better than the 1/4, 2 conductor cables. They cut down on a lot of noise. Especially if you have a complex system with lots of components and long cables.