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.

Changes from The Original A 21→A 21+

- 250 → 300 Watts ( at 8Ω )

- 400 → 500 Watts ( at 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

Features

- 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 silver or black

Parasound A 21 + Halo Stereo Power Amplifier User's Manual

Specifications

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: Unmeasurable

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"
Height without feet: 7^1/8"
Height with feet: 7^3/4" 
Depth: 20" 
Depth with cables: 21^1/2" 

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

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