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….
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.
I don't mind the M3, I loved them at first until I listened to other speakers over the years. They are certainly not perfect. Axioms customer service is terrific. I wouldn't rate them an 8.5 out of 10 from my own subjective non-controlled listening. I find them to be too colored with vocals to give them that high of a score. That's my biggest issue with them. Also sometimes there can be some weird harshness present on some recordings. I am thinking it might be the metal cone breaking up as it lacks a proper crossover on the 6.5". If I remember correctly it is only a 1st order on the tweeter and the woofer rolls off naturally.
your assessment is dead on. I discussed this in my review of their outdoor version of the M3s which share the same crossover design and parts:
As for customer service, they have some of the best in the business IMO.
Another problem with using employees along with being able to pick out their own speaker in a blind test is the ability to bias the subjective listening scores. It would be very easy to score a speaker much better if you could tell the other speaker was clearly awful, but in the case where one is better you can always score them to be similar to avoid offending your employer. Your speakers can never lose.
Hence how the term "similarly good was born". This is how most of the companies that run DBTs do it however.
Here is an example of their DBT run by their own employees: http://www.axiomaudio.com/archives/October2010.html [axiomaudio.com]
guess who won the comparison
As for an employee taking part in a controlled test I know I would have a very difficult time being unbiased even if done blind.
Bingo! Last time I visted Axiom they put me through their "double blind test" procedure. Prior to going they sent me a pair of M60v3s for our most recent $1k floorstanding shootout [audioholics.com]. I spent over 1 month listening to those speakers. When I sat in their blind test, I identified the M60s everytime. I know the sonic signature of an Axiom speaker since I've listened to it so extensively. Listening blind doesn't remove that bias.