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
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:
13
