Noise is, for the most part, a common mode phenomenon, which is another way of saying that the noise is mostly equally shared between the two balanced voltage outputs and will dropout of the signal when processed through a balanced device, such as a transformer or preamp. Since many professional audio environments, such as recording halls, auditoriums,  and nightclubs are chock full of wiring, lights, air-conditioning, and electromechanical gear, they are also filled with electromagnetic pollution. For this reason professional audio gear is usually balanced, or at the least, balanced at the beginning of the chain where noise is more problematic, i.e. the microphone and its amplifier.

Pro Audio in the Home?
    Every year we read one or two articles proclaiming how balanced audio will become mainstream. Don't bet on it, as the recent resurrection of single-ended amplifiers does not bode well for an inherently push-pull technology.
    Still, there are some situations where an existing piece of equipment in your system may boast a balanced output that could be used profitably. (Because balanced outputs are used in professional applications, the design requirements are more stringent, as the loads are more severe, i.e. 600 ohms or lower. Consequently, the balanced output is usual beefier than the average unbalance; even the connectors used are much more robust and tight than any RCA jack, all of which can equal better sound.) Or maybe noise is a real problem in your home and the use of critically placed balanced links in the chain could only help. Or maybe you would like to forego the phase splitter in your push-pull amplifier and feed a push-pull signal from the beginning.
From Unbalanced to Balanced

Balanced VS Unbalanced
    Most consumer audio gear is unbalanced in design. For example, the phono cartridge has one hot and one ground pin per channel. Likewise, the phono preamp has one input RCA jack per channel, which comprises a ground sleeve and a hot input socket. The output of this preamp also sports just one RCA jack per channel. Unbalanced from beginning to end. This means that the input and output are of but a single voltage relative to ground. In a balanced design the input has two voltages, two phases relative to ground; so too the output, two voltages, two phases relative to ground. For example, a single mono microphone has three connection pins: one negative, one positive, and one ground. And the microphone preamp has three input connections to match the microphone.
     In the balanced setup, the two input and output voltages are in anti-phase to each other, but are equal in amplitude to each other. In other words, if the two signals are summed together, the result would be silence. On the other hand, if the difference between these voltages were amplified, the sum of the two output signals would be twice the amplitude of the same signal level coming from an unbalanced source.
    Here is the lever behind a balanced design: amplifying the difference between anti-phase signals does more than just double the gain, it ignores what is common to both signals in phase and voltage. Balanced designs are engineered to null what is common and amplify what differs. This makes for the single great advantage that balanced designs hold over the more conventional unbalanced designs: much greater noise suppression.

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