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.