For those of you who are curious about the Benchmark AHB2 here is a summary of the design gaols and the unique features of the amplifier:
It was my goal to create a power amplifier that matched the performance of the DAC2 D/A converter. After all, the performance of the D/A converter is only useful if it can be delivered by the downstream power amplifier.
In many ways, the AHB2 is a radical departure from conventional class AB amplifier design:
1) Low gain (9 dB) - allows +22 dBu input at amplifier clip - essential for low noise
2) Patented feed-forward error correction - virtually eliminates crossover distortion
3) Feed-forward design makes bias currents unnecessary, and non-critical - class B operation is possible with very low distortion
4) Multiple output stages run in parallel to eliminate crossover distortion - one output stage is active while another is in the crossover region
5) Class AB output stage uses very low bias current
6) Due to low bias currents, idle power consumption is only 20 W
7) Feed-forward design makes class H or G operation possible - no rise in distortion at class H or G switch point
8 ) Class H (or G) rail switching at a 1/3 power threshold
9) Tightly regulated power supply
10) High-bandwidth control loop on switch-mode power supply responds to amplifier loading over the entire audio band, and at ultrasonic frequencies
11) Amplifier does not rely on capacitive energy storage
12) Switch-mode power supply eliminates AC line magnetic interference to levels that are not possible with a linear power
supply
13) >200 kHz bandwidth to achieve excellent inter-channel phase at 20 kHz.
14) < 0.1 Hz low frequency cutoff to minimize low-frequency phase shift
15) Feed-forward design improves damping factor
The AHB2 design was optimized for low distortion and low noise. It was not optimized for the highest possible efficiency that can be achieved with this new topology. Nevertheless the AHB2 is much more efficient than a conventional class AB design. Peak power does not vary with AC line voltage (due to regulated supply). Power drawn on one channel does not influence the power available from the other
channel.
The AHB2 is a linear amplifier, it is not a switcher. For this reason, it produces very little out-of-band noise. A-weighted noise is only 2 dB
less than noise measured over an 80 kHz bandwidth. This was an important design goal because ultrasonic noise can be folded into the audio band by non-linearities in speaker transducers.
THD is better than -120 dB (0.0001 %)
John Siau
VP
Benchmark Media Systems, Inc