ALESIS POWER AMPLIFIER RA500 - Manual (Page 19)

Chapter 2: Connections
RA150/300/500 Reference Manual 17
Isolation transformer or direct box
If your mixer or other sound source doesn't have a
balanced output, you may turn it into a balanced
signal with an external device, either active
(electronic, powered) or passive (a transformer).
These commonly available audio accessories have a
balanced line output, usually in the form of an XLR
connector, and an unbalanced line input, usually in
the form of a 1/4" phone jack.
A line transformer can also convert balanced
signals to unbalanced signals. Advantage of a
transformer: High signal carrying capacity, no power
required, generates no hiss. Disadvantages:
Inexpensive transformers may color the sound due
to frequency response irregularities and can pick up
hum due to inductive nature of transformers. Very
high-fidelity models are expensive.
Active direct boxes use an active electronic circuit
to drive a balanced line, but does not work in the
other direction. Advantages: Good frequency
response specs, no inherent hum pickup, less
expensive than transformers. Disadvantages:
Requires power, generates some noise.
To feed the same source to several
different amplifiers:
The input impedance of the Alesis amplifiers is 20
k
Ω
in balanced and 10 k
Ω
in unbalanced
configuration. Thus, the inputs of several amplifiers
may be connected in parallel if desired. The
maximum number of amplifier channels will
depend on the minimum output load impedance of
the device feeding the amplifiers. For example, if
the minimum output load impedance of the source
device is 600
Ω
, up to 30 amplifier channels may be
connected to it in parallel without overloading the
source device’s output.
However, use of a distribution amplifier is
recommended in large systems.
To use the amplifier in Bridged Mono
mode:
Connect the input signal to Channel A/Left only.
Move the recessed slide switch under the right
input connectors to the Bridged Mono position. Do
not connect any signal to the Channel B/Right
input jack.
Leave the input level control for
Channel B down. See page 20 for output wiring in
Bridged Mono configuration.