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How an XLR mic cable rejects interference

With all the interference from all sorts of radio waves, you might wonder why you never get any in a microphone inputs. A mic level signal is very small, and needs some help to make it from the mic to your board. It does this by becoming balanced.

There are 3 pins on an XLR connector. Pin 1 is ground. Pin 2 is the audio signal as it comes off the microphone. Pin 3, however, is the mirror image of the signal on pin 2. The microphone accomplishes this by putting the signal off the mic capsule through a transformer. A transformer is two different coils of wire, both wrapped around the same metal core. The input is electrically isolated from the output. Transformers are used all over the place. Virtually everything electronic you use has a transformer inside it, or supplied as a "wall wart" power supply. These convert he 120 volts that comes out of the electrical outlet into something lower for use in the device. In fact the wiring coming into your house is an excellent example of a source of balanced audio, 120 volts of 60hz audio. Half of the breakers in your house are on the positive wire, and the other half are on the negative wire. Higher consuming devices like an electric dryer or stove use both phases, one wire from each side to get 240 volts.

Back to audio now. Since the output has no electrical connection to the input, you can put a connection in the middle of the coil called a 'center tap' and attach that to your mixers ground. Now one side of the transformer is positive (pin 2) and the other negative (pin 3). Much like the dryer example above, the input on your mixers measures the diffence between the two signals, and outputs this. Most mixers do this electronically and not with a transformer because of cost. The transformer balanced input is actually better as it gets you some gain without any additional noise, and they were found on older Shure M67, M268 & similar AV mixers.

We go through all of this because when interference strikes the mic cable, it strikes both audio signals equally, and when the mixer input measures the difference, the interference vanishes. Neat trick.

The cable used makes a difference too, because shielding helps reduce interference as well. Cheap cables have some bare wire wrapped about the audio carrying wires. Lots of holes there. A braided shield uses interwoven wires to get more coverage. It's a pain in the butt to work with though. An aluminum foil shield provides 100% shielding, but adds rigidity, and in time the foil can break. You find foil shielding in mic snakes & permanent installs. Conductive plastic is plastic that has the conductive properties of metal, but remains flexible.

Balanced audio exists at line level as well, especially for making long runs like from the mixer to an amp rack. Even some 1/4" jacks are balanced, called a T/R/S connection. T=tip=positive signal, R=a band in the middle of the connector=negative signal, Sleeve=ground=case of the connector. In this configuration if it was plugged into a regular 1/4" jack it would still work, but it just wouldn't be balanced or have any of the rejection associated with it.


About the Owner
photo of Lonnie Bedell of AVlifesavers.com My name is Lonnie Bedell, and I'm the guy behind AVLifesavers. Unlike big companies, I've been doing actual live sound work since 1995, and live recordings since 1985.
It's that kind of real world experience that I feel makes AVLifesavers stand out. It's one thing to examine problems theoretically, it's a whole different thing to deal with frantic last minute changes from an inexperienced client who wants it done yesterday.

Been there, done that, so I've designed products to solve live sound problem fast.

All products are assembled right here in the USA. Living Wages to American workers is and will always be part of the fabric here, despite the temptations.

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