Balanced

Overview

The term "Balanced" is used in audio to describe a form of signal transmission that has two signal conductors which carry signals which are both: a.) identical except for being of opposite polarity and b.) have a signal amplitude voltage range equally above and below signal "ground" (a positive and negative voltage).

History

In order to minimize noise pick-up by long cables carrying low level signal such as those output by a microphone; a system of transmission was developed using audio "signal" transformers. One of the advantages of an audio transformer is that there is no physical connection between the input circuit ("primary coil") and the output circuit ("secondary").

Before audio electronics were developed that worked on power supplies with both "plus" and "minus" voltages; some method was needed to isolate the DC voltages required for operation of audio circuits operating on "single-ended" DC power from different stages of the circuitry and from devices connected to the input and output. In many cases; "coupling capacitors" were used to allow the AC (audio signal) to pass while "blocking" the DC. Transformers could also serve this function; and provided other useful functions such as impedance and level matching.

A microphone can have a transformer in its output with a "floating" secondary (output). This means simply that neither of the two signal conductors is referenced to ground. At the input of the microphone preamp; there was a second transformer that contain a primary with a "center-tap" which referenced the center of the winding to ground. As a result; the signals in the two signal conductors would be of the "same" except that they were of opposite polarity. The ground reference provided by the center-tap of the micpre input transformer caused the signal voltages to be "centered" on ground (of equal positive and negative voltage range).

The advantage of this approach is that the audio signal was passed from the primary to the secondary for further amplification; but interference signals would appear of equal voltage and the SAME polarity, resulting in them effectively "canceling" each other in the transformer by generating no signal in the secondary. Thus the low-level audio signal was transmitted and the noise was not.