Parasound A 21 - Manual (Page 19)

TECHNICALLY SPEAKING continued 16
Choosing Interconnect Cables and Speaker Wire
We are often asked to recommend specific
interconnect cables and speaker wire. It’s true that
with some amplifiers, sound quality will vary greatly
according to interconnect cables and speaker wires.
However, Parasound amplifiers use robust circuitry
that sounds superb regardless of interconnects and
speaker wires. Therefore, we feel that choosing a
brand of cable for Parasound amplifiers is purely a
matter of personal taste.
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:
1. Use a balanced input line with your Parasound A 21.
(See Balanced and Unbalanced Lines in this section).
2. When rack mounting, always use the included
insulated “shoulder” washers. These break the ground
loops caused by metal-to-metal contact between the
rack, the units, and their rack-mount bolts. Extras
are available from rack manufacturer Middle Atlantic
Products, www.middleatlantic.com.
3. Use the Ground switch on the A 21 to eliminate most
ground loops. This separates the A 21’s signal input
ground from its chassis ground to isolate unwanted
voltage in the ground shield of the unbalanced (RCA)
input cable. Be sure your system installation is finished
before you try moving the Ground switch from its
Normal to its Lift position.
Loop Out Jacks
The Right and Left Loop Out jacks are convenient
connections to route the same input signals from your
preamplifier or surround controller to an additional
power amplifier. These jacks eliminate the need for
“Y” connectors to split the source signal between
your A 21 and another amplifier. The Loop Out jacks
work with either the Balanced or the Unbalanced
Input jacks. Here are some examples of where you
could use the Loop Out jacks:
1. When multiple speakers and amplifiers are to be
played simultaneously, you can route the output of
your preamplifier or controller to the first A 21 then
connect its Loop Out jacks to the Unbalanced Input
jacks a second A 21 (or other amplifier) to power
additional speakers for the same channel.
2. Some high-end speaker manufacturers recommend
bi-amplifying or “bi-amping” without using an
electronic crossover. When bi-amping this way, one
A 21 drives only the woofers and another A 21 drives
only the tweeters. For example, you could connect
the left and right channel outputs of the preamplifier
to the inputs of the first A 21 to power the woofers,
and then connect its Loop Out jacks to the second
A 21 to power the tweeters.
Gain Controls
When the gain knobs are turned fully clockwise, the
actual gain controls are essentially out of the circuit.
The left and right channels are perfectly matched,
and your A 21 can reach its fullest sonic potential.
In some cases, you’ll need to reduce the gain settings.
For example, if you find it difficult to make precise
volume adjustments at low listening levels, your
preamplifier or surround controller probably has a
very high output (or gain) that you can match by
reducing the A 21’s gain. Conversely, if you need to
advance your preamplifier or surround controller’s
volume to nearly the top of its range for average
listening levels, you should increase the gain settings
on the A 21. The A 21 gain adjustment that optimizes
the range of motion on the preamplifier’s volume
control is sometimes called “gain staging”.