Chapter 1: Introduction

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

RA150/300/500 Reference Manual

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.

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Alesis RA150, RA300, RA500 manual Principal Features
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RA150, RA500, RA300 specifications

The Alesis RA series of amplifiers—comprising the RA300, RA500, and RA150—have earned a respected spot in the audio equipment landscape, catering to both musicians and audio professionals. With a blend of innovative technology, robust performance, and user-friendly features, these amplifiers offer solutions for various sound requirements.

Starting with the RA300, this amplifier delivers a solid output of 150 watts per channel at 4 ohms, making it ideal for smaller studio setups or personal listening environments. It is built with a lightweight yet durable design, employing a modern Class AB architecture that ensures low distortion and high audio fidelity. The RA300 features comprehensive input options, including balanced XLR and quarter-inch TRS inputs, allowing seamless integration into diverse setups. Additionally, the built-in protection circuitry prevents overheating and damage during prolonged use, ensuring longevity and reliability.

The RA500 steps up the power and performance, presenting a robust 250 watts per channel at 4 ohms, making it suitable for larger venues and demanding applications. It utilizes advanced thermal management technology, which aids in efficient heat dissipation, ensuring the amplifier operates smoothly even under heavy load. The RA500 also boasts significant dynamic range and headroom, making it a preferred choice for professional environments where audio clarity and volume are paramount. The addition of a frequency response that spans from 20Hz to 20kHz means it can faithfully reproduce a wide range of audio frequencies.

Meanwhile, the RA150 serves as a versatile option for various applications, delivering 75 watts per channel at 4 ohms. Despite its compact size, the RA150 does not compromise on sound quality or functionality. Its Class AB amplifier technology results in enhanced audio accuracy, making it popular among beginner studios and educational settings. The straightforward controls and indicators ensure that users can quickly adjust settings for optimal performance.

All three amplifiers in the Alesis RA series share a common goal: to provide high-quality sound amplification without unnecessary complexity. Their combination of solid construction, advanced technologies, and thoughtful features make them suitable for a multitude of audio environments—from home studios to professional gigs. Whether you opt for the RA300, RA500, or RA150, you are investing in a reliable tool designed to deliver exceptional audio performance, catering to the needs of discerning audiophiles and professionals alike.
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