Owner's Manual
9
Audio Input Connections
Always disconnect the AC cord to your A 21+ before making or changing any input, output or trigger
wire connections. Inserting or removing an input or output cable while the A 21+ is turned on can result in a
blast of sound that can damage your loudspeakers.
Balanced XLR Input Jacks
In most systems balanced XLR connections will give you the best sound. If your stereo preamplifier has
balanced XLR output jacks, we recommend that you connect them to the A 21+ XLR inputs. Refer to the
Balanced and Unbalanced Lines in the Technically Speaking section for additional information about why we
recommend using balanced lines.
Note: Using balanced XLR input connectors results in a 6 dB higher volume level
compared with using the RCA input jacks. This is a noticeable increase.
Unbalanced RCA Input Jacks
Use these inputs if your preamplifier doesn’t have balanced XLR output connections or if you simply prefer to
use unbalanced connections.
Balanced/Unbalanced Selector Switch
Place the switch in the position for the input type you will be using.
Note: The Balanced/Unbalanced switch is not an input selector. Its function is to optimize
the signal to noise ratio for each type of input. You should not connect both the Balanced
and Unbalanced jacks at the same time with the expectation of switching between two
different devices such as a preamp and surround processor.
Balanced XLR Pin Configuration
The A 21+ XLR jacks conform to the industry standard of:
Pin 1: Ground Pin 2: Positive (+) Pin 3: Negative (–).
Loop Output Jacks
These jacks enable the incoming audio signal to pass along or “daisy chain” from the Left and Right
Input jacks to an additional amplifier or powered subwoofer. The Loop Out jacks are not affected by
the Gain control settings. If you connect the preamp to the A 21+ with balanced XLR input jacks the
RCA Loop Out jack functions normally.
Bridged Mode
Bridged operation configures the Left and Right channels to function as a single channel higher
power amplifier to drive a single speaker. When a A 21+ is in Bridged Mode it provides far greater
power output than the sum of its left and right channels’ power output. Please note that in bridged
operation the minimum speaker impedance is 8 ohms. We do not recommend using the A 21+ with a
4 ohm speaker while set to bridged mode.
Bridged/Stereo Mode Selector Switch
Select Stereo for normal operation when the A 21+ will be powering two speakers. Only select
Bridged when the A 21+ will be powering a single 8 ohm speaker.
Note: If the Bridge Mode switch is accidentally set to Bridge while you are using the A 21+
as a stereo amplifier the sound will be very weak and thin.
Speaker Connections for Bridged Mode
When operating in bridged mono mode a A 21+ should only power a single 8 ohm speaker.
Connect the speaker’s positive (+) speaker wire to the A 21+’s right channel positive (+) speaker terminal.
Connect the speaker’s negative (-) wire to the A 21+’s left channel positive (+) speaker terminal.
Correct + and - speaker polarity for bridged mode is shown below the A 21+ speaker terminals.