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Chapter 1: Introduction
RA150/300/500 Reference Manual 7
CHAPTER 1
ABOUT THE RA150/300/500
PRINCIPAL FEATURES
Please remember that a power amplifier
is a high-current, high-power device and
should be treated with respect and care.
Even if you are an audio veteran, we
urge you to read the entire manual to
make the best use of the RA150/300/500.
Thank you for purchasing an Alesis Reference
Series power amplifier, one of the most advanced
and highest value amplifiers available.
The Alesis models RA150, RA300, and RA500
provide extremely accurate sonic quality as well as
measured specifications, but their conservative
design and careful attention to electronic protection
means they will continue to operate at top condition
without special attention, year after year.
THE CHASSIS
The Alesis Reference Series amplifiers are
constructed on rugged, heavy gauge steel chassis.
All units feature rack mount faceplates. For
maximum reliability and low-noise operation the
amplifiers use convection cooling with massive
extruded aluminum heat-sinks for reliable
operation under all ambient and load conditions.
The design of the external chassis protects rear
panel connectors (except RA 150). All models
feature detented front panel input level controls,
and overload/protect indicators. The RA300 and
RA500 amplifiers incorporate bar graph LED power
metering.
INPUTS AND OUTPUTS
The input sections of the amplifiers utilize precision
electronically-balanced input circuitry for low noise
balanced input operation. All units have 1/4”
balanced TRS input connectors. The RA300 and
RA500 also have XLR connectors. Unbalanced RCA
(phono) style input connectors are also used. The
output connectors are industry-standard, heavy-
duty dual binding posts, which can accept large
diameter wire or 3/4" (19 mm) spaced dual banana
plugs. A recessed rear panel switch allows easy
selection of stereo or bridged monaural operating
modes. AC power entry is via a detachable,
grounded IEC connector cable.
AMPLIFIER TOPOLOGY
The amplifier sections of the Reference Series utilize
a direct coupled, fully complementary circuit
topology. The input stages are a dual differential
type with active current sources. Output from the
differential amplifiers are coupled to balanced level
shifters operating on isolated boost rails for
improved efficiency and higher performance. These
stages then drive discrete, high current, triple
darlington output stages, fully rated for continuous
4 ohm operation with 2 ohm dynamic stability
(stereo mode only). The output stages are biased for
class AB operation. Emphasis is placed on
maintaining wide bandwidth, linearity and stability
under all operating conditions. An active DC servo
circuit is employed to automatically null DC output
voltage offsets. All models feature bridged mono
operation.
PROTECTION CIRCUITRY
Protection for the amplifier consists of active
circuitry that continuously monitors the output
stage for excessive DC offset, short circuits and
thermal overload. In the event of an output fault,
the protection circuitry will activate the output relay
and disconnect the loudspeaker load. This circuitry
will also mute the amplifier during power on/off
transitions.
The power supplies of the amplifiers utilize heavy-
duty stacked steel EI laminated transformers for
maximum performance and reliability. High current
bridge rectifiers, massive secondary capacitors and
current in-rush limiters are used in all models. All
models are designed for 120/230-volt operation
with an external voltage selection switch.
All components are of high quality and mechanical
construction and are optimized for high reliability
under adverse physical and electrical conditions.