Parasound Halo A 51 Five Channel Power Amplifier

Raw Power Meets Elegant Circuit Design

Barry Willis praises the Parasound A 51 for its "harmonic rightness" in Stereophile Guide to Home Theater for October 2003. "Its midrange was totally seductive, its soundstaging and imaging abilities in the same league as some of the best gear I've ever heard at any price... With music, the A51 radiated sweetness and light. With action films, it rained thunder and lightning. There aren't many amplifiers that can so effortlessly cover both ends of the sonic spectrum."

The Jan / Feb. 2004 Top Choice Awards issue of DVD ETC. magazine praises the Halo A 51 multichannel amp and the Halo C2 surround sound processor for their "sonic performance, auto calibration, complete feature set, programmability and upgradeability."

Uncompromising Audiophile Performance

Parasound's five-channel Halo A 51 power amplifier combines raw power with elegant circuit design. Designed by legendary audio guru John Curl in collaboration with CTC Builders, the Halo brings uncompromising audiophile performance to home theater.

THX Ultra 2 certification ensures movie-house quality sonics, but the Halo surpasses this critical benchmark by delivering Parasound-quality sound derived from the ultra-high-end Parasound JC-1 400-watt mono amp. Matched JFET inputs, MOSFET drivers, and high-current bipolar outputs contribute to its exceptional performance.

Class A / AB Operation and Impressive Firepower

The A 51 boasts balanced inputs with discrete circuits and XLR connectors. Beautifully implemented high-bias Class A / AB operation makes the most of an impressive 250-watt-per-channel power output at 8 ohms and 400-watts-per-channel at 4 ohms. The 2.2 kVA encapsulated toroidal power transformer with independent secondary windings for each channel and 164,000 µF power supply filter capacitance delivers an impressive current capacity of 60 amps per channel. The direct-coupled design uses no capacitors or inductors in signal path. The complementary MOSFET driver stage relies on the firepower of 40 beta-matched 15 amp, 60 MHz bipolar output transistors and works in combination with a JFET input stage.

"Instant" Warm-up Feature

Like all the other amps in the Halo product series, the A 51 has Parasound's exclusive bias circuit, which permits the amplifier to sound perfect from the instant it's turned on, rather than having to wait for bias to gradually build up as the amp warms up over time. Very high bias allows greater Class operating range than others. Even Halo's implementation of its automatic turn-on feature is exceptional. Special relays bypass the AC "soft start" circuitry after it's done its job for the purest AC and the purest sound.

And That's Not All…

AC present, channel status, and high temperature indicators team up with DC Servo and relay protection circuits to keep this powerhouse running safely at peak performance for years of reliable service. Auto turn-on by 12-volt trigger or audio signal simplifies home theater and multi-room operation. Other features include heavy-duty 24k gold-plated 5-way speaker terminals, gold-plated RCA input jacks, and a rear-panel ground lift switch.

Product Features

Circuitry designed by 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 toroidal 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
Heavy-duty 24k gold-plated 5-way speaker terminals
Gold-plated RCA input jacks; ground lift switch
Auto turn-on by 12v trigger or audio signal
4u Chassis, carrying handles, rack mounting adapter included

Product Specs

Power Output - All Channels Driven

250 watts RMS x 5, continuous,
20 Hz - 20 kHz, 8Ω
400 watts RMS x 5, continuous,
20 Hz - 20 kHz, 4Ω

Current Capacity

60 amperes peak per channel

Slew Rate

> 130 volts per microsecond

Frequency Response

5 Hz - 100 kHz, +0/-3 dB at 1 watt

Total Harmonic Distortion

< 0.2 % at full power
< 0.03 % at typical listening levels

IM Distortion

< 0.04 %

TIM

Unmeasureable

Dynamic Headroom

> 1.5 dB

Interchannel Crosstalk

> 78 dB at 1 kHz
> 63 dB at 20 kHz

Input Impedance

Unbalanced: 47k Ω
Balanced: 94k Ω

Input Sensitivity for 28.28 V Output into 8 Ω

Unbalanced: 1 V
Balanced: 1 V per leg

S/N Ratio

> 112 dB, input shorted, IHF A-weighted
> 102 dB, input shorted, unweighted

Damping Factor

> 1100 at 20 Hz

DC Trigger Requirements

+9 Vdc to +12 Vdc, 2 mA

Audio Trigger Requirements

50 mV AC - 250 mV AC

Dimensions

Width: 171⁄2" (445 mm)
Panel height: 7" (178 mm)
Height with feet: 75⁄8" (194 mm)
Depth: 20" (508 mm)

Power Requirement

Standby: 25 Watts
Full Power into 4Ω: 3000 Watts

Net Weight

80 lb. (36.4 kg)

Shipping Weight

87 lb. (39.5 kg)

Product Warranty

Parts & Labor

5 Years

2 Years for Moving Parts, Remote Controls, Lasers, Front Panel Displays and Video Screens

Parasound Warranty Information

Product Manual

	Parasound Halo A 51 Amp Owner's Manual (PDF Format)

	2019 Parasound Halo Product Line Brochure (PDF Format)

"I expected the A51 to sound good. What I wasn't prepared for was how good it sounded right out of the box. Its midrange was totally seductive, its soundstage and imaging capabilities in the same league as some of the best gear I've ever heard at any price."

"Parasound's Halo A 51 is all the amp anyone could need. I can't imagine any audiophile -- 2-channel traditionalist or multichannel adventurer -- being unhappy with the A51."

- Barry Willis, Stereophile Guide to Home Theater, March/April, 2004

"As with the [Parasound] C2, I expected the A51 to sound good - with its John Curl design pedigree and close kinship to Curl's esteemed JC-1, I could expect no less. What I wasn't prepared for was how good the A51 sounded right out of the box. For the first two days it was in the system, I plowed through my CD collection, eagerly hunting out old favorites just to enjoy this amp's pure musicality."

"The A51 got very close to the Red Rose [Model II Silver Signature tube amp] in terms of absolute harmonic rightness - you know it when you hear it - and trounced by homemade amp in the departments of dynamic headroom and bass impact. Its midrange was totally seductive, its soundstaging and imaging abilities in the same league as some of the best gear I've ever heard at any price."

"If that 2-channel test drive had me thinking about buying the A51, the multichannel experience made me want to slap down my Visa card… With music, the A51 radiated sweetness and light. With action films, it rained thunder and lightning. There aren't many amplifiers that can so effortlessly cover both ends of the sonic spectrum."

"The A51 and C2 form an unbeatable combination - one that I intend to enjoy for a very long time. They exemplify elegance, ease of use, near-universal compatibility, and what appears to be universal reliability - backed by 10-year parts and 5-year labor warranties. Both get my highest recommendation."

- Barry Willis, Stereophile Guide to Home Theater, October 2003

"I found that this amp was able to extract the bass in even heavily compressed recordings, and helped balance the bright, smeared compressed artifacts sometimes found in the top end."

"As you can probably tell, I have been very impressed by the Halo products. ...Overall, this is a class-leading pair, and I do not say this lightly. The C2 package with it's sonic performance, automatic calibration, complete feature set, excellent remote, programmability, beautiful aesthetics, and hardware and software upgradeability is quite possibly the most complete processor package in the $4,000 area ...and rightly takes its place at the head of the pack."

"The A51 [amplifier] so thoroughly bettered the performance of my Classe' amp with the tough-to-drive KEF speakers that I cannot image going back to it."

"Parasound has totally hit the proverbial home run with the Halo C2 and A51 and I very strongly recommend them."

"Rating: 98 out of 100."

- Manoj Motwani, M.D. DVD Etc. May 2003.