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Big Bruiser of a Power Amp with a Deftness for Delicacy
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 every nuance of a quietly quivering violin string.
"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 23+ 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 23 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 Output – Stereo Mode
"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
Editors' Choice Award 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 SoundMarch 2019