Parasound Halo A 21+ Stereo Amplifier

"The Very Model of a Modern Power Amplifier"

Parasound’s big bruiser of a muscular yet always adept power amplifier gets a powerful update with a new "Plus Series" model. The Halo A 21+ improves on the already over-the-top amazing Halo A 21 via carefully thought-out performance upgrades for cleaner, ultra-low distortion sound, revised front panel cosmetics, and a revised rear panel layout offering enhanced features, too. More than ever, this beefy Parasound amp is an elegant brute, able to effortlessly deliver crashing crescendos in all their incendiary glory while also adroitly turning on a dime to reproduce ever nuance of a quietly quivering violin string.

The March 2020 issue of Stereophile magazine features a rave review by Kalman Rubinson, who says, "Listening to the A 21+ was eminently delightful and satisfying from the first note." According to Rubinson, "Over a span of weeks, the A 21+ was a constant companion and a bringer of musical enjoyment; compared with other amps on hand and in spite of a few sonic differences, the new Parasound was impossible to fault."

Rubinson concludes, "Anyone in the market for a stereo power amp would be well advised to consider the Parasound A 21+, especially those who believe they must spend more. With apologies to W.S. Gilbert, it is the very model of a modern power amplifier."

"Extremely High Performance-Per-Dollar"

"Parasound has chosen to take a staple core product that has remained an exceptional value for over fifteen years, the Halo A21 amplifier, and re-spin it into the Halo A21+," reports Michael Zisserson at positive-feedback.com. "Doing so has proved that extremely high performance-per-dollar is still possible today when a company leverages decades of experience, and is spearheaded by a legend."

Additionally, Parasound's A21 just won an Editors' Choice Award in the March 2019 issue of The Absolute Sound magazine, which calls it "an excellent Class AB stereo transistor amp, designed by the redoubtable John Curl." Conclusion: "The A21 offers a lot of power at an affordable price."

Impressive Performance Upgrades

The performance upgrades are impressive, beginning with a big 25% to 30% increase in output power compared to the previous model. The A 21+ delivers 300 watts-per-channel at 8 ohms compared to 250 watts-per-channel for the A 21, 500 watts-per-channel compared to 400 watts-per-channel at 4 ohms, and a huge 1,000 watts in bridged mono mode compared to 750 watts with the A 21.

The performance of a power amplifier is highly dependent on the quality of its power supply, and Parasound went the extra mile with the A 21+, which boasts a 10% to 15% larger main power transformer compared to the previous model A 21. Transformer size has been increased from 1.2 to 1.3kVA, and the filter capacitance increases 20-30% from 88,000 uF for the A 21 to 108,000 uF for the new A 21+. You'll hear the advantages of these upgrades as the A21+ presents lower distortion at full power. In fact, signal-to-noise measurements have been upgraded from 115 db from the A 21's 112 dB. THD at full power is now a vanishingly low 0.1% compared to 0.2% for the A 21.

Other performance upgrades include tweaked input circuits for improved crosstalk specs (70 dB at 20 kHz compared to 63 dB) and upgraded internal gold on gold audio connections, resulting in a cleaner signal path that's also more reliable.

Rear Panel Layout and Features Upgrades

The rear panel of the A 21+ has been revised in terms of layout and new features. It boasts upgraded speaker terminals, using identical terminals to the JC 5 power amp, a new jack/terminal layout with more space and more clear text/labels, and a master "Vacation" AC switch. The 2.5mm trigger jacks of the A 21+ have been replaced with 3.5mm jacks, and the audio trigger circuit has been updated for better low signal reliability.

Revised Halo Front Panel Cosmetics

The Halo series has always been known for its elegant aesthetics, and the look is even better than before. New front panel cosmetics include slim metal end caps replacing wider plastic end caps, an added gold metallic accent stripe along the end cap, gold feet now on black units, and an engraved metal "Parasound" badge replacing the red "P."

Balanced Inputs and Discrete Circuits

The A 21+ boasts balanced inputs with discrete circuits and XLR connectors. Beautifully implemented high-bias Class A / AB operation makes the most of an impressive power output. The 1.3 kVA encapsulated toroidal power transformer with independent secondary windings for each channel and 108,000 µF power supply filter capacitance delivers an impressive current capacity. The direct-coupled design uses no capacitors or inductors in signal path. The complementary MOSFET driver stage relies on the firepower of 16 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 A21 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 five-way speaker terminals, gold-plated RCA input jacks, and a rear-panel ground lift switch that can help eliminate any audible hum caused by the presence of ground loops in your hi-fi or home theater system.

A21+ Performance Upgrades

• Power up 25-30%
• Filter capacitance up 20-30%
• 10-15% Larger main power transformer
• Upgraded internal gold on gold audio connections (cleaner signal path and more reliable)
• Tweaked input circuits for Improved Crosstalk specs
• Improved power supply for lower distortion at full power
• Filter capacitance: 108,000 uF
• Transformer size: 1.3 kVA

• Slim metal end caps replace wider plastic end caps
• Added gold metallic accent stripe along end cap
• Engraved metal PARASOUND badge replaces the red P
• Gold feet now on black units

• Upgraded speaker terminals (A21+ gets JC5 terminals)
• New jack/terminal layout with more space and more clear text/labels
• 2.5mm trigger jacks replaced with 3.5mm jacks
• Audio trigger circuit has been updated for better low signal reliability
• Master AC “vacation” switch on the rear panel

Power Output – Stereo Mode

• 300 watts x 2, 8Ω both channels driven
• 500 watts x 2, 4Ω both channels driven

• 1000 watts x 1, 8Ω
• Bridged 4 Ω operation is not recommended
• Power measurement are at 120 VAC: 0.1% THD, RMS continuous power, full audio band (20 Hz - 20 kHz)

• 60 amps peak, per channel

• > 130 volts per microsecond

• 5Hz - 100kHz, +0/-3dB 20Hz - 20kHz, +0/-0.25dB

• < 0.1% at full power
• < 0.03% at typical listening levels

• < 0.04%

• Unmeasurable

• > 80dB at 1kHz
• > 70dB at 20kHz

• Unbalanced: 33kΩ
• Balanced: 66kΩ,(33kΩ per leg)

• 29dB

• Unbalanced: 1 V
• Balanced: 1V per leg

• > 115dB, input shorted, IHF A-weighted
• > 110dB, input shorted, unweighted

• > 1100 at 20 Hz

• +9 Vdc to +12 Vdc, 5 mA

• 2 mV - 10 mV

• 1 = Ground (Shield)
• 2 = Positive
• 3 = Negative (Return)

• Width: 17-5/8"
• Height without feet: 7-1/8"
• Height with feet: 7-3/4"
• Depth: 20"
• Depth with cables: 21-1/2"
• Weight: 71 lbs.

• Standby: 1 Watt
• Idle (no music playing): 160 Watts
• Typical Listening levels: 300 Watts
• Maximum: 1400 Watts

2022 Recommended Components - Stereophile

"The successor to the Parasound A 21, the new A 21+ offers 300Wpc into 8 ohms, compared with its predecessor's 250Wpc—and the new model, which operates in class-AB, can be bridged to serve as a 1000W monoblock. Other refinements include a gruntier power transformer, brawnier speaker connectors, an increase in power-supply filter capacitance, and other niceties. Balanced (XLR) and unbalanced (RCA) inputs are both provided, as are dual-mono level controls. According to KR, 'listening to the A 21+ was eminently delightful and satisfying from the first note,' and he pointed to the new amp's 'transparency without highlighting or emphasis' and the manner in which it handled dynamic challenges in orchestral music 'without stress.' Technical Editor JA found that the A 21+ exceeded its power specs, delivering a full 400Wpc into 8 ohms, and noted that the amp 'isn't fazed by impedances as low as 2 ohms.'"

- Stereophile magazine, Vol. 43 No. 3, March 2020

"Listening to the A 21+ was eminently delightful and satisfying from the first note… Over a span of weeks, the A 21+ was a constant companion and a bringer of musical enjoyment; compared with other amps on hand and in spite of a few sonic differences, the new Parasound was impossible to fault."

"Anyone in the market for a stereo power amp would be well advised to consider the Parasound A 21+, especially those who believe they must spend more. With apologies to W.S. Gilbert, it is the very model of a modern power amplifier."

- Kalman Rubinson, Stereophile magazine, March 2020

"With its high-quality chassis, thoughtful circuit design, and excellent sound quality, the A 21+ offers an impressive value proposition. Those shopping for a stereo amplifier in this price range should give the A 21+ serious consideration."

- Dylan Seeger, hometheaterreview.com, February 19, 2020

"Parasound has chosen to take a staple core product that has remained an exceptional value for over fifteen years, the Halo A21 amplifier, and re-spin it into the Halo A21+. Doing so has proved that extremely high performance-per-dollar is still possible today when a company leverages decades of experience, and is spearheaded by a legend."

- Michael Zisserson, positive-feedback.com, Issue 101, January 26, 2019

"An excellent Class AB stereo transistor amp, designed by the redoubtable John Curl… Though not the last work in solid-state amplification, the A21 offers a lot of power at an affordable price."

- Jonathan Valin, The Absolute Sound, March 2019