Axiom Audio A1400-8 Design Overview
Technological progress is a wonderful thing. Over the past 40 years or so, we’ve seen
computers that filled an entire room get eclipsed in processing power by ones
that can fit in your wristwatch though I’d never admit to wearing one. Thanks to Bell Labs and the invention of the
transistor, this and more has become a reality and the reason why today we can
get such powerful and reliable amplification. By contrast, years ago 30
watts/channel meant large bulky tube
amps that generated lots of heat and often required constant upkeep. For the last couple of decades, solid-state
amplifier technology has been pushed to its technological limit. Incredible sound and reliable operation are
all hallmarks of the very best designs.
However, the problem of bulkiness remains, and they generate lots of
heat. Class D amplifiers have been
working their way into the consumer marketplace to address these issues. The promised advantage is up to 95%
efficiency (at full power), and thus less heat generation, and a smaller
footprint. Until recently, Class D amps
were used almost exclusively for powered subwoofers and car audio because they
were best suited for low-frequency and non-critical audio applications. A lot has changed since the introduction of
these amps. With the very same silicon advances we observed in the computer
industry, we are seeing a similar trend now for audio. We’ve tested numerous Class D amplifier
designs that have achieved mixed results both sonically and objectively. But Axiom claims their Class D amp is of a
different breed. What makes the Axiom
Class D amp any different and will it be able to sit aside the very best linear
amplifiers? Continue reading to find
out….
Design Overview
The Axiom A1400-8 is a Class D switching
amplifier. But it’s not your typical run
of the mill digital design. Most
companies simply repackage the B&O ICE module with a switching power supply
and call it a day. While this approach can
certainly offer respectably good amplification for low cost, it does have design
compromises that Axiom felt were too great to put their name on. The A1400-8 is Axiom’s very own proprietary
design. It switches at a much higher
frequency than most other switching amps, which in this case is between 450kHz
– 500kHz. Switching higher in frequency
like this allows the clocking transients to be far outside of the audible
range. The feedback topology on the
A1400-8 is also very different than most other designs. The A1400-8 uses a dual
feedback loop as can be seen in the block diagram: One very low for wideband
audio frequencies and one that is rather high from 2Hz to DC to minimize DC
offset dynamically. This allows the speakers to reach their full x-max or
excursion, which eliminates Doppler distortion due to the DC shifts of the
amplifier. Getting the feedback right on
these types of amplifiers is very critical. Few companies have the math skills
and/or resources to really implement this correctly. We’ve seen Class D designs with no local
feedback at all, which lead to a very high output impedance (approaching 1 ohm)
within the entire audio band causing the amp to sound drastically different
depending on the loudspeaker it was driving.
The A1400 uses state of the art power
devices from a company called International Rectifier that were specifically
designed for audio applications. The driver IC has zero over-shoot or
under-shoot at the full clock frequency. The output devices are especially designed
for very low gate capacitance that allows them to switch very accurately and
quickly follow the musical input in its most intricate nuances.
The A1400-8 chassis is die cast aluminum instead of cheap stamped steel typically found on most A/V receivers
and budget amplifiers. This rugged and strong chassis translates into a more
stable platform for the electronics, which reduces mechanical vibrations and
increases longevity while also allowing heat to pass through it more efficiently. The A1400-8 power
supply is a linear topology similar to what is used on very high power class
A/B amplifiers. Axiom chose to go with a
linear design instead of an SMPS
(switch mode) power supply to ensure greater dynamic headroom and sustainable
power levels that could be achieved with very low noise residuals. It was designed with a very high current,
high voltage specially wound toroidal transformer. The power supply capacitors
are all 105°C 100WVDC caps. Their total capacitance is over 148,000uF meaning
the reserve energy charge (Coulombs) is more than enough for any demanding
music or movie listening experience. The power supply of the A1400-8 is also
designed to act as a line conditioner as well as a line filter. This eliminates
the added expense of a separate line-conditioning box.
Editorial Note about PWM Class D Amplification & the Low Pass Filter (LPF)
The most common modulation technique used in Class-D is called PWM (Pulse Width Modulation) - a square wave is produced that has a fixed frequency. The PWM signal contains harmonics of the switching frequency along with a copy of the input signal. A power low-pass LC passive filter is used to remove the unwanted harmonic components. The output filter helps maintain efficiency and also suppresses RF interference produced by the fast output transitions. Getting this filter right is imperative for a good Class D amplifier design to ensure the amplifier will sound good regardless of what loudspeaker impedance is presented.
