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Parasound A 21+ Stereo Power Amplifier (Each)

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The Parasound A 21+ Stereo Power Amplifier 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.”


  • Changes from the original A 21→A 21+:
    250 → 300 Watts (8Ω)
    400 → 500 Watts (4Ω)
    750 → 1000 Watts (bridged)
    Filter cap: 88,000 uF → 108,000 uF
    S/N: 112dB → 115 dB
    THD (at full power): 0.2% → 0.1%
    Crosstalk (20kHz): 63dB → 70dB
    Transformer size: 1.2 kVA → 1.3 kVA
    Weight: 60 LB → 71 LB
    Refined Halo cosmetic with aluminum end caps & gold highlights
    Updated rear panel with heavy duty speaker terminals from JC 5
    Upgraded internal gold on gold audio connections
    Updated auto turn on circuit
  • 300 watts x 2 @ 8 Ω RMS, both channels driven
  • 500 watts x 2 @ 4 Ω RMS, both channels driven
  • 1000 watts x 1 @ 8 Ω, in bridged mode
  • Circuitry designed by legendary John Curl
  • High bias Class A/AB operation
  • 60 amperes peak current per channel
  • Stable with speaker impedance dips to 1.5 Ω
  • Balanced inputs with discrete circuits
  • CHK Infinium, custom 5-way speaker binding posts
  • Gold-plated RCA input & loop output jacks
  • Direct-coupled signal path, no capacitors or inductors
  • DC Servo and relay protection circuits
  • AC turn-on inrush surge protection with relay bypass
  • JFET input stage, MOSFET driver stage
  • 16 beta-matched 15 ampere, 60 MHz bipolar output transistors
  • 1.3 kVA shielded toroid transformer w/ independent windings for each channel
  • Independent power supplies for input stage, driver stage and output stage
  • 108,000 µF filter capacitors for output stages
  • Full power is rated all the way down to 5Hz for unrestricted low-end bass
  • Harris hyper-fast soft-recovery bridge diode rectifiers
  • Automatic turn-on by 12 V trigger or audio sensing
  • 4 rack space chassis with rear carry handles
  • Rack mount hardware is included
  • Available in black


  • Power Output – Stereo Mode (0.1% THD): 
    300 watts x 2, 8 Ω both channels driven
    500 watts x 2, 4 Ω both channels driven
  • Power Output – Bridged Mode (0.1% THD): 
    1000 watts x 1, 8 Ω
    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)
  • Current Capacity: 
    60 amps peak, per channel
  • Slew Rate: 
    > 130 volts per microsecond
  • Frequency Response: 
    5 Hz – 100 kHz, +0/-3 dB
    20 Hz – 20 kHz, +0/-0.25 dB
  • Total Harmonic Distortion (THD): 
    < 0.1 % at full power
    < 0.03 % at typical listening levels
  • IM Distortion: 
    < 0.04 %
  • TIM: 
  • Inter-channel Crosstalk: 
    > 80 dB at 1 kHz
    > 70 dB at 20 kHz
  • Input Impedance: 
    Unbalanced: 33 kΩ
    Balanced: 66 kΩ,(33 kΩ per leg)
  • Total Gain: 
    29 dB
  • Input Sensitivity for 28.28 V Output into 8 Ω: 
    Unbalanced: 1 V
    Balanced: 1 V per leg
  • S/N Ratio: 
    > 115 dB, input shorted, IHF A-weighted
    > 110 dB, input shorted, unweighted
  • Damping Factor: 
    > 1100 at 20 Hz
  • DC Trigger Requirements: 
    +9 Vdc to +12 Vdc, 5 mA
  • Audio Trigger Requirements: 
    2 mV – 10 mV
  • XLR Pin Identification: 
    1 = Ground (Shield)
    2 = Positive
    3 = Negative (Return)
  • Dimensions: 
    Width: 17-5/8″ (448 mm)
    Height without feet: 7-1/8″ (181 mm)
    Height with feet: 7-3/4″ ” (197 mm)
    Depth: 20″ (508 mm)
    Depth with cables: 21-1/2″ (546 mm)
  • Net Weight: 
    71 lb. (32.2 kg)
  • Shipping Weight: 
    88 lb. (39.9 kg)
  • Power Requirement: 
    Standby: 1 Watt
    Idle (no music playing): 160 Watts
    Typical Listening levels: 300 Watts
    Maximum: 1400 Watts


“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.”


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.


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.


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.”


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.


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.


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.




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