AHB2
Instruction Manual Rev F
Page 38
Appendix
A Radical Approach to Power Amplification
The performance of the
AHB2
would not have been achievable without taking a radical approach to
power amplification. In many ways, the
AHB2
is a complete 180 degree departure from traditional
high-end amplifier designs.
AHB™ Ampl
ifier
–
This unique amplifier uses THX-patented AAA(TM) (Achromatic Audio
Amplifier) technology to achieve high efficiency and vanishingly low distortion. The AHB2
combines class-H, and class-AB amplifier technologies with feed-forward error correction.
Feed-Forward Error Correction -
Each channel of the
AHB2
is equipped with a main
amplifier and an ultra-clean low-power auxiliary error-correction amplifier. The main class-
AB amplifier provides the bulk of the power while the auxiliary amplifier provides a low-
power error-correction signal. These amplifiers are combined to create a near-perfect null of
all distortion components. The
AHB2's
patented feed-forward error-correction system is
much more effective than traditional feedback error-correction systems. Distortion does not
change with loading, and does not rise near maximum output. Difficult loads are driven with
near-perfect clarity. Low-level musical details are delivered without the masking effects of
crossover distortion. The
AHB2
is virtually distortion free at low levels, at maximum output,
and everywhere in between. Overall, the distortion produced by the
AHB2
is so low, that it
approaches or exceeds the measurement limits of some of the finest audio test systems.
Error-Correction Amplifier -
The error-correction amplifier is a small ultra-clean amplifier
that feeds a correction signal forward to the main output. The correction amplifier actively
drives the output while the main output devices are transitioning between push and pull
states. The correction amplifier actively suppresses distortion products that would normally
occur in a class-AB output stage. This feed-forward system is much more effective than
traditional feedback networks. Feedback systems attempt to correct errors after they
happen. In contrast, feed-forward systems can prevent errors from occurring.
Low-Bias Class-AB Main Amplifier -
Class-AB output stages always produce "crossover
distortion" whenever the push-pull output drivers change state. Precise bias control is
usually required to minimize this crossover distortion. Traditional feedback networks have
difficulty correcting the transients caused by push-pull crossover transitions. However, the
THX patented feed-forward error-correction system has the speed and precision necessary
to virtually eliminate all traces of crossover distortion. With this feed-forward system, bias
currents have no significant impact on the distortion at the output of the amplifier. This
topology provides the freedom to conserve power by setting unusually low bias currents.
The
AHB2
uses very low bias currents but achieves vanishingly low distortion - even under
heavy loading.
Class-H Tracking Rails -
Multiple power supply rails (class-G), and tracking power supply
rails (class-H), can improve the efficiency of a class-AB amplifier. The
AHB2
uses a lower
fixed rail and an upper class-H tracking rail. The improved efficiency provided by this
topology usually comes at the cost of increased distortion. For this reason, there are very
few amplifiers that use multiple supply rails. In a traditional feedback design, every rail
switch point adds another layer of crossover distortion. In contrast, the feed-forward error
correction used in the
AHB2
is so effective that multiple rails can be added without
incurring any distortion penalty.