April 22, 2009 by Audioholics.com
Axiom Audio has redefined performance expectations of Class D amplifiers with their A1400-8. They have proven that a highly efficient space saving digital amplifier design like this can not only hang with some of the best traditional linear amplifier designs, but also surpass them in some aspects such as dynamics, bass accuracy and power delivery.
On the surface this amplifier seems pricey. However, when you think about the state of the art components that went into this baby to offer top notch fidelity at power levels unachievable in linear amplifier designs via a single power cord, the premium price tag begins to materialize. If you’re assembling an upscale home theater system with the design goals of uncompromising fidelity and dynamics, and your speakers are capable of handling its mighty power, you should definitely add the A1400-8 to your shortlist of high power multi-channel amplifiers for consideration. The digital revolution has come full circle with Axiom being one of the few pioneers truly breaking new ground with their first generation of high performance digital amplification.
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.
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