What does "Audio Common" mean on a balanced connection?

FAQ #645 Updated August 03, 2017

Question:

Several times in reading about audio cables and wires I have heard the term "audio common" used. Could you explain to me what the audio common is? Sometimes it seems as if they are talking about the ground wire, but I am not sure.

Answer:

A balanced microphone cable requires three wires: a shield (which carries no audio information) and is connected to XLR pin 1; inner conductor "A" (which carries the "hot" audio signal) and is connected to XLR pin 2; inner conductor "B" ( which carries the "cold" or "common" audio signal) and is connected to XLR pin 3.

The signal on pin 3 is the mirror image of the signal on pin 2. In technical terms, it is 180 degrees out of polarity with pin 2. A balanced input on mixing board only responds to the signal difference between pins 2 and 3. This is also called a differential input.

On a professional microphone, pin 2 provides a positive voltage and pin 3 provides a negative voltage when the microphone diaphragm receives a positive pressure wave that pushes the diaphragm inward. Thus, pin 2 is "hot"; pin 3 is "cold" or "common".

To read more about balanced line, please refer to page 11 of: Microphone Techniques for Music - Sound Reinforcement

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