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 
 
 
 
All 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