Audio Trigger Sensitivity Adjustment
The Audio Sensitivity Control sets the threshold
of the audio trigger signal. You can adjust this
level from a maximum sensitivity of 50 mV (fully
counterclockwise) to a minimum sensitivity of
200 mV (fully clockwise). If you set this control to
50 mV, the A 21 might be falsely triggered on by
non-musical or noisy signals that can appear in the
system, such as when you switch preamp inputs
at high volume levels. If you set this control to
200 mV, the A 21 might not turn on during quiet
musical passages. The detented position (click stop)
at 12 o’clock corresponds to 100 mV. It’s a good
starting point and will be suitable for most systems.
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
TECHNICALLY SPEAKING
15
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.
Bare Speaker Wire Ends
If you plan to use connections with bare wire ends,
use a wire stripper to remove just enough insulation
to expose a 1/2” (13 mm) length of bare wire. You
can insert the stripped wire into the hole that goes
sideways through the terminal's metal post. Before
inserting the wire, twist its bare strands to prevent
any of the strands from making contact across the
two speaker terminals. If you have a soldering iron,
you can “tin” (apply a small amount of molten
solder) to each stripped bare wire to prevent it from
unraveling, fraying and oxidizing.
Bridged Mono Operation
In normal operation, the left and right amplifier
channels each amplify both the positive and negative
halves of the musical signal. In bridged mono
operation, the A 21’s entire left channel drives only
the positive half of the musical signal and its entire
right channel drives only the negative half of the
musical signal. This doubles its voltage swing.
This doubled voltage swing enables the A 21 to
deliver nearly double its 400 watts per channel 4
Ω
power, or 750 watts, into a single 8
Ω
speaker.
The audible benefit of this higher power is increased
dynamic range, or headroom, so that musical peaks
can be reproduced with less distortion. Considering
that musical peaks and crescendos require 10-100
times as much power as average listening levels,
this added headroom is a substantial advantage
for unrestrained, undistorted listening, even if your
average listening level is moderate.
Bridging allows an amplifier to deliver more power
into a single speaker because in this mode, the load
appears to the amplifier as only half of its rated
impedance. Thus, the single 8
Ω
speaker appears as
a 4
Ω
load and a 4
Ω
speaker appears as only a
2
Ω
load.
The A 21 is not capable of driving 2
Ω
for extended
periods because it will draw more current and
generate more heat than it can dissipate. We strongly
recommend against using speakers rated at less than
8
Ω
when bridging.
Remember that mono bridging requires a separate
A 21 for each speaker. You can’t simultaneously drive
two speakers in stereo and one speaker mono
bridged.
