APPENDIX: Line connections: unbalanced and balanced
21
There are two basic ways to transport an audio signal with microphone or line level:
Unbalanced line: Utilising a two conductor cable, it transports the signal as the voltage between them.
Electromagnetic interference can get added to the signal as undesired noise. Connectors that carry
unbalanced signals have two pins, such as RCA (Phono) and ¼” (6.35mm, often referred to as jack) mono. 3
pin connector such as XLR (Cannon) may also carry unbalanced signals if one of the pins is unused.
Balanced line: Utilising a three conductor cable, one of them acts as a shield against electromagnetic
noise and is the ground conductor. The other two have the same voltage with respect to the ground
conductor but with opposite signs. The noise that cannot be rejected by the shield affects both signal
conductors in the same way. At the device’s input the two signals get summed with opposite sign, so that
noise is cancelled out while the programme signal doubles in level. Most professional audio devices use
balanced inputs and outputs. Connectors that can carry balanced signal have three pins, such as XLR
(Cannon) and ¼” (6.35mm) stereo.
The graphs that follow show the recommended connection with different types of connectors to balanced
processor or amplifier inputs. The connectors on the left-hand side come from a signal source, and the ones
on the right hand side go to the inputs of the processor or amplifier. Note that on the unbalanced connectors
on the left-hand side, two terminals are joined in side the connector. If hum occurs with balanced to balanced
connections, try disconnecting the sleeve (ground) on the input connector. Note that the illustrations show
what should be connected to what, but that pin locations on an actual XLR connector are different. Also, pin
2 hot is assumed on XLR connectors.
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