Parasound Halo JC 5, Руководство пользователя

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13
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 differential balanced circuitry required for balanced connections.
In an unbalanced line, the positive audio signal is conducted through the center pin of an RCA plug and
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 broadcast 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 the audio and the 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 po1 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.
Note: Balanced XLR connections result in a 6 dB higher gain, compared with the
unbalanced RCA input jacks. In other words, the sound will be noticeably louder when
you use balanced instead of unbalanced connections.
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 voltage 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 a raspy “buzzing” noise to the hum. The most common cause of a
ground loop in an audio system is from cable TV. If disconnecting the cable line stops the hum then you
might need a cable TV ground loop isolator.
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:
1 . Your Cable TV or Satellite receiver box might require a Cable TV Ground Isolator.
2 . Use balanced input cables with your Parasound JC 5. (See Balanced & Unbalanced Lines in this section).
3 . When rack mounting, always use the insulated “shoulder” washers on both the front and rear sides of the