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Parasound Halo A 51 Amplifier Review

by October 19, 2004
Contributors: Steven DellaSala
Parasound Halo A 51 Amplifier

Parasound Halo A 51 Amplifier

  • Product Name: Halo A 51 Amplifier
  • Manufacturer: Parasound
  • Performance Rating: StarStarStarStarStar
  • Value Rating: StarStarStarStar
  • Review Date: October 19, 2004 19:00
  • MSRP: $ 4000
  • Circuitry designed by legendary John Curl
  • THX Ultra 2 certified
  • High bias Class A/AB operation
  • Balanced inputs with discrete circuits and XLR connectors
  • Direct Coupled - no capacitors or inductors in signal path
  • Complementary MOSFET driver stage and JFET input stage
  • 40 beta-matched 15 amp, 60 MHz bipolar output transistors
  • 2.2 kVA encapsulated toroid power transformer with independent secondary windings for each channel; 164,000 µF power supply filter capacitance
  • DC Servo and relay protection circuits
  • AC present, channel status, high temperature indicators
  • Gold-plated RCA input jacks, loop output jacks
  • Heavy-duty 24k gold-plated 5-way speaker binding posts
  • Auto turn-on by 12v trigger or audio signal, 12v trigger out
  • Rear panel gain controls; ground lift switch
  • 4U Chassis; carry handles; rack mounting adapter included
  • Parasound's 10 years parts, 5 years labor warranty
  • Continuous Power Output: 250 watts RMS x 5, 20 Hz - 20 kHz, 8 ohms, all channels driven; 400 watts RMS x 5, 20 Hz - 20 kHz, 4 ohms, all channels driven
  • Current capacity: 60 amperes peak per channel
  • Power bandwidth: 5 Hz - 100 kHz, +0/-3 dB at 1 watt
  • Total harmonic distortion: < 0.2 % at full power
  • IM distortion: < 0.04 %
  • Slew rate: >130 V/µsecond
  • Dynamic headroom: > 1.5 dB
  • Interchannel crosstalk: > 78 dB at 1 kHz; > 63 dB at 20 kHz
  • Input sensitivity: 1 V for 28.28 V, THX Reference Level
  • Input impedance: 47 kW unbalanced; 66 kW balanced
  • S/N ratio: > 112 dB, input shorted, IHF A-weighted
  • Damping factor: > 1100 at 20 Hz
  • Voltage: 110V - 120V
  • Dimensions: 17-1/4" w x 7-5/8" h x 20" d, 7" h without feet
  • Net weight: 80 lbs


  • Bridgeable Channels
  • Balanced and Unbalanced Inputs
  • Diversified features
  • Exceptional Build Quality
  • Multiple "power on" options


  • Two amplifiers required for 7-channel audio (A 21 and A 51)
  • Increased cost associated with two amplifiers


Parasound Halo A 51 Introduction

At first look the Parasound Halo A 21 and A 51 are appealing and robust amplifiers accented by brushed metal finish and a "Halo" aqua blue light around the power button. But an amplifier is not just about cosmetics especially when considering that most Audio Enthusiasts seem more interested in sound quality and features. The A 21 and A 51 amplifiers are both loaded with usable features above and beyond most other amplifiers in today's market.

The understated look of the A 21 / A 51s subtle yet empowered front panel implies a need for a space aged protective shell that contains a powerful yet precious cargo. For example the "P" on the top of the front face glows red to indicate that the unit has AC power as does the "Halo" insignia around the power button which accents the power button with its soft blue glow. Once the power is turned on the soft blue changes to a brighter blue after all circuitry has been initialized. In the center bottom of the front face are the two indicators that verify proper functionality of each channel by glowing with a different shade of blue. On the right bottom is another indicator that glows red if the unit overheats with a redundancy portrayal from the "Halo" insignia that also switches to red.

About the author:
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Ken Stein is a contributing writer and reviewer for Audioholics and he really REALLY likes his speakers (which he should, since he spent countless hours hand-crafting them himself.) Ken is an engineer with FedEx and applies his diligent attention to detail to his speaker and electronics reviews here at Audioholics.

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