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.
Basics
There are basically two types of analog audio interconnects: Balanced and "Unbalanced."
The differences are: 1.) The Unbalanced connection uses the outer "shield" conductor as the audio signal "return" conductor. A Balanced connection has a separate shield and two signal conductors. 2.) In Unbalanced connections; one of the signal conductors is connected directly to ground. In Balanced connections; neither signal conductor is connected to ground.
Unbalanced connections are simpler and OK for relatively short connections between equipment with relatively high level signals. In the vast majority of cases; internal connections and circuitry of audio equipment is "unbalanced" and the signal is only converter to balanced at the output and converted form balanced at the input.
The advantages of Balanced connections are excellent noise rejection on long cables; even with low level signals, and the possibility of ground isolation between equipment. In most cases; the ground isolation is a matter of degree as there must be some form of ground reference between the connected equipment. In many instances, it is not a matter of the ground connection existing so much as how and where the ground of the two pieces are connected. The most common way a ground issue manifests itself is as a "hum" in the audio; and this is often due to the presence of a "ground loop" caused by multiple ground paths that do not take the same physical path between the two pieces of equipment.
The other form of interference is "noise" induced by electromagnetic or electrostatic interference on the audio cable's shield. In the case of an Unbalanced connection; the shield must be connected directly to the audio input of the receiving device to provide the "signal return" path for the audio signal on the center conductor. Balanced connections allow the shield to be connected to a ground that is isolated from the input audio signal path; or in more extreme cases involving ground loops; to not be connected at all to the input device as the other ground connection provides the necessary reference.