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Technically Speaking
Balanced and Unbalanced Lines
Recording and broadcast studios use balanced connections exclusively because of
their inherent ability to reject noise and hum, thus assuring the best sound. Certain high
quality preamplifiers and surround controllers built for residential use utilize balanced
connections with XLR jacks for the same reasons. All Parasound Halo series power
amplifiers have balanced inputs with XLR jacks so you can take full advantage of their
inherent noise reduction capability and superior sound quality.
Unbalanced connections with RCA jacks are found on all home audio equipment. RCA
jacks and two-conductor wires are less costly than the additional circuitry, higher priced XLR
connectors and three-conductor wiring required for balanced connections.
In an unbalanced line, the positive audio signal appears at the center pin of the RCA
jack and the negative signal on the outer shield wire, which also functions as the ground
connection. Unbalanced interconnect cables are vulnerable to hum from an AC line, or other
noise, such as RF (Radio Frequency), which can be reproduced through your loudspeakers.
Since the unbalanced line’s ground also carries the audio signal, there is no way for the
connected amplifier or preamplifier to distinguish between the audio signals you want and
unwanted noise emanating from external sources.
Balanced lines are superior because they utilize separate conductors for audio and
ground: two inner conductors carry the positive and negative audio signal, and a third outer
wire connects the grounds and also shields the two signal conductors. When the positive and
negative signals appear at the component receiving the signal they are equal, but 180
degrees out of phase with each other with respect to ground. To send and receive balanced
signals requires special differential circuitry.
A differential input circuit amplifies only the difference between the positive and
negative signals. For example, when a 1 Volt signal arrives at a balanced input stage, the
differential input “sees” a positive 1 Volt minus a negative 1 Volt, or 2 Volts total. External
hum and noise that somehow gets into a balanced line is common to both its positive and
negative conductors with respect to ground. Therefore, it is canceled or rejected by the
differential input circuit.
This phenomenon of rejecting noise signals common to both positive and negative
conductors is called common mode rejection. Differential inputs are specified according to
how well they reject signals common to both conductors. This is measured in dB and is called
the common mode rejection ratio or CMRR.
Ground Loops - Eliminating Hum and Buzz
Audible hum and buzzing noises in a system are usually related to issues with the
component grounds. Ground (sometimes called common) is a point of reference for voltages
in virtually all audio and video components. Ground is supposed to remain at zero volts while
the audio signal swings positive (voltage above ground) and negative (voltage below ground).
If ground isn’t at zero, there can be an audible 60 Hz hum (or 50 Hz hum in regions with 50
Hz AC). The harmonics of these frequencies (120 Hz, 240 Hz, 480 Hz or 100 Hz, 200 Hz,
400 Hz) may add buzz in addition to the hum.
The ideal of zero voltage ground for all the components in a system is practically
impossible, because some resistance between the ground points of different components is
inevitable. By keeping components close together with their power cords plugged into a
common AC outlet or power strip, you’ll avoid the problems created by resistance in the
house’s wiring.
Hum and buzz is also caused when unwanted voltage flows through multiple
component ground points called ground loops. Here are three tips to avoid ground loops: