Axiom Audio A1400-8 Digital Multi-Channel Amplifier Review

Axiom Audio A1400-8 Introduction

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.

Axiom Audio A1400-8 Setup

 

Moving the Axiom A1400-8 amp around my test bench and home theater rack proved much easier than the typical power amplifier I review. Despite its enormous power supply and bodacious output capability, it’s a lightweight on the scale, barely hitting 60lbs. This is just the way I like electronics to be given my numerous back issues caused by years of abusive weight training and lifting heavy Class A/V amplifiers. The one unusual thing you will find about this amp is the absence of unbalanced connectors. When I first heard about this, I thought to myself, are they kidding? Nope they weren’t. In order to minimize chassis height, they went with all balanced connectors and included gold-plated straight and right angled unbalanced phono and RCA mating connectors. I’ve never seen this approach before with consumer gear but I liked it. I tested both the balanced locking connectors and the RCA mating connectors and they fit nice and snug. The speaker binding posts are a bit cramped in the chassis but luckily each adjacent-channel pair speaker connector is the ground side to prevent arcing or shorting.

  

Speaking of shorting, I’ve never seen so many warning labels slapped onto an amplifier before. There are numerous shock hazard logos and high voltage notifications placed on the amp chassis and user manual and they made it exceedingly hard to remove the top chassis cover to get a peek inside. Considering that the power supply working voltage is very close to the rated wall voltage and this sucker is capable of dumping all of that current into any given channel at a time, I certainly understand their cautionary notes and encourage all users to heed their warnings.

The power receptacle is located at the bottom of the chassis making this amp difficult to rack mount (more on this later) and the unit comes with a 6-ft 14AWG 3-prong power cord. The master switch is located on the back panel and the amp has trigger capability via a mini plug. The trigger worked correctly only when the front panel standby button was depressed as stated in step 6 of the slim but useful user manual.

I placed the A1400-8 on my esoteric Audiav Crystal Series C4 AV Rack and connected it up with Kimber 8PR speaker cable, Sonicwave balanced cables and Blue Jeans 1694A composite RCA cables when conducting unbalanced tests. I used my Denon AVP-A1HDCI and Denon AVR-5805mkII as the preamps, the Denon DVD-5910CI and Yamaha MCX-2000 as the sources. The A1400-8 was powered by my APC S20 power conditioner that had no troubles at all powering this amplifier even with its enormous inrush current draw during power up. Axiom recommends NOT plugging this amp into a power conditioner, which for 90% of the ones on the market, I’d tend to agree with, the APC S20 being one of the few exceptions. I utilized my RBH Signature T30-LSE reference series speaker system and threw in my Status Acoustics Decimo bookshelf speakers for comparative purposes.

Axiom Audio A1400-8 Listening Tests

 

All of my direct listening comparisons between the A1400-8 and my much more expensive Denon POA-A1HDCI amplifier (MSRP: $7500) was with the latter being configured into bridged mode. This put the Denon amplifier on near equal power footing with the A1400-8. The preamp gain was adjusted -4dB when listening to the Axiom amp for proper level matching. On average it took me about 20 seconds to swap the cables and level match when doing direct comparisons between the two amplifiers making it a very difficult task to point out the subjective differences I was hearing since both amplifiers exhibited exemplary fidelity. Listening comparisons between the Denon and Axiom amps were not done for multi-channel due to the complexity of changing cables and level matching and the fact that it’s easier to discern sonic differences in stereo as opposed to multi-channel. This is especially true since both amplifiers have huge power supplies and are fully capable of hitting reference levels in my theater room without sacrificing performance whether powering two or all seven speakers.

 

Critical listening tests were conducted mostly with two-channel sources such as CDs and SACDs. Multi-channel listening tests were conducted for extended periods of time at reference levels to ensure the amplifier was capable of unfaltering sustained output in my theater room.

CD: Fourplay – The Best Of Fourplay

I always like a little Fourplay before really getting into the thick of things. Track #5 “The Chant” is a song I often use to separate the men from the boys in loudspeakers. The bass energy of the kick drums will either reward you with an adrenaline surge or have you covering your ears from the horrible sound of woofer bottoming just before it plays for the very last time. Of course this isn’t a problem for my reference speakers but I wanted to see just how far the A1400-8 could push them and get their four 10” high excursion subwoofer drivers moving. The A1400-8 delivered thunderous bass response with aplomb. The transient response was lightning quick, extracting all of the decay of the kick drums, making it feel more like a live performance in my own theater room than listening to an recording. The wood percussion instruments were delicately reproduced with all of the subtle nuances preserved. In comparison to my Denon POA-A1HDCI, I felt the Axiom amp was a bit more lively and crisp but the soundstage on the Denon seemed a bit wider and more open.

 

Regardless of how loudly I played the A1400-8, it sounded effortless and unstrained revealing excellent sustain and decay of the kick drums and percussion instruments. In fact it wasn’t until I really cranked up the volume that the bass differences between the two amps became more apparent with my preference leaning more towards the Axiom’s “quicker” more “lively” rendering.

CD: Dianne Reeves Never Too Far

When I want a quick reference for gauging accuracy in bass response of loudspeakers and even amplifiers, I turn to this CD. The bass in track #2 “Never Too Far” will sound muddy or weak on an improperly set up or inadequate system. I have this song pinned into my head as to how the bass should sound so I wanted to directly compare two the amplifiers. A back and forth comparison between these amplifiers on my reference speakers revealed that the Axiom amp was a bit leaner in the bass department but much tighter and more lively. The decay of the bass drum was so detailed that it sounded stereophonic to me when listening on the A1400-8. I couldn’t help but to really punch up the volume and give my speakers the workout they’ve been waiting for. Track #3 “Come In” again revealed the Axiom’s very well mannered bass response with less apparent overhang that I was hearing on my reference amplifier. However, I did feel that the Denon was offering up a slightly smoother, more spacious top end. Ironically when I switched over to my bookshelf speakers to make the comparison, I conversely preferred the bass I was hearing from the Denon amp which seemed fuller to me. Perhaps on smaller less bass capable speakers, I was preferring the warmer sound the Denon amp was offering. It’s also possible that the Axiom amp preferred the lower impedance (dips to 2 ohms) of my tower speakers over my 8 ohm bookshelf speakers. Needless to say the sonic differences between the two amps thus far were subtle but noteworthy. The A1400-8 reminded me how far Class D amps have come and really shattered my misconception of them for full range high fidelity audio applications. Until I heard this amplifier, I was unconvinced that Class D would ever be a serious contender against good old fashioned linear designs.

 

CD: Special EFX - Collection

This is an old favorite of mine not only because of the excellent musical content, but of the pristine fidelity that really helps gauge a system’s frequency range. On track #2 “Jamaica, Jamaica”, the triangles seemed to spread out beyond the plane of the speakers on both amps with perhaps a bit more 3-dimensional depth on the Denon amp. While I thought I heard slightly better separation of the instruments on my Denon amp, the Axiom sounded more vivacious. On the A1400-8, the plucks of Chieli Minucci’s guitar seemed more vibrant while the bass was also snappier, especially at higher listening levels. In contrast, switching over to my Decimo speakers had me preferring the (dare I say?) more sluggish bass response of the Denon amp that made those little bookshelf speakers sound more grandiose.

 

Track #5 “Udu Voodoo” seems surreal as I got lost in the textures of the percussive instruments and the ping-pong effect between the front speakers that the A1400-8 conveyed. Self-restraint with the volume control became quite difficult for me as the sound was just effortless at all power levels. My T30-LSE’s simply craved the unadulterated power that the Axiom amplifier was providing. The A1400-8 was proving it was right at home with the rich and complex textures of jazz music, which dominates much of my listening preferences.

CD: Randy Johnson – Hit & Run

Opening up with track #1 “Down Time”, the A1400-8 did a fabulous job of transporting me into an intimate jazz cellar enjoying a nice Brandy snifter with my foot tapping to some funky Jazz you simply can’t find on terrestrial radio these days. The snare rolls popped right out at me providing a very bold, in-your-face experience typically only heard at live events. Track #4” The Hat Man” had me putting on my Fez in no time. This CD sounded absolutely fantastic on both amplifiers and at this point it was really about splitting hairs; at times I felt the depth of the soundstage favored the Denon amp while the Axiom got the nod for more boldness and vibrancy.

Miscellaneous CDs

I ran through a gamut of CDs and SACDs during my listening sessions and took notes on the stuff that stood out the most to me. Listening to the Lee Ritenour & Larry Carlton duo, particularly “Remembering JP” really showed off the A1400-8’s abilities to faithfully playback dynamic program material. Percussive instruments were explosive on this amp giving you that “better than being there” experience I always seek out on the very best equipment that I review. I spent a good deal of time listening to a few tracks on an Usher CD sampler that was handed to me at the CES trade show a couple of years back. In particular track #6, “Don Juan,” revealed the superb transient bass response of the A1400-8 in droves. The guitar strumming attack and decays were simply mind-boggling. I felt like the bongos had more pop to them on the A1400-8 then they did on my POA-A1HDCI amplifier but the separation between the shakers seemed a bit wider on the Denon amp.

 

I threw on some bass-heavy techno type music from the likes of Depeche Mode and Roger Powell. “Only When I Lose Myself” is one of those Depeche Mode mixes that they brilliantly spin a CD off the hit single with remix options some of which work while others don’t. The “Gus Gus Longplay” is pretty trick so I went with that. The bass in this song is extremely deep and highly distorted. On most systems it sounds like mud and will easily send wimp woofers on a trip past their excursion limitations. I was expecting the Axiom amp to walk all over my Denon amp on this song but I was instead thrown a curve ball. I preferred the looser more bass heavy presentation that the Denon was portraying. The bass seemed a bit thin on the Axiom amp but ultimately it could have also been rendered more accurately as a result. On track #2 “Fallout Shelter” of Roger Powell's Fossil Poets, the Axiom amp was my preferred amplifier of choice. It loved to just belt out the power. On the Axiom amp, the bass seemed to have better control and the sound was just insanely dynamic. I had the volume cranked so ridiculously loud that I actually saw the mids in my T30-LSE’s moving from nearly 18 feet away. In nearly three years of owning these speakers, I don’t think I’ve ever driven them this hard. I was pleased to not only see the Axiom amp’s effortless ability to do this but with my speakers’ willingness to accept the power and not suffer from audible compression or distress.

DVD: Genesis - When in Rome

Its no secret that I’m a huge Genesis fan. I take all eras but really prefer the late 70’s post-Gabriel musical period of their career. Luckily, when Genesis reuinted in 2007 they played a lot of program material from that era. Hearing the opening track of Duke in full 7.1 at reference levels was, to put it mildly, an energizing experience. The powerful cord progressions of Tony Banks' keyboards and drum rolls of Chester Thompson and Phil Collins were perfectly rendered, sounding as good as I’ve ever heard on my reference system. “In the Cage” medley is a must-listen track for all true Genesis fans despite its nearly 18-minute length. With the A1400-8 powering my system, 18 minutes never felt so good. At all power levels, the A1400-8 delivered pristine sound, a large surround envelope and just begged to flex its muscles to remind me it was on call to deliver everything the wall outlet was capable of providing.

DVD-Audio: Beatles Love

Track #1 “Because” has become one of my favorite Beatles tunes as it really exemplifies the uncanny melodic nature of the Beatles that few if any bands today can match. I was immersed in the wonderful voices of The Fab Four. It reminded me of being at the actual LOVE performance by Cirque du Soleil in Vegas only it sounded much better in my own theater room. The A1400-8 seemed like an infusion of octane boost to my system when playing back tracks #5 “I am the Walrus” and #10 “The Benefit of Mr. Kite”. The bass peaks seemed bolder than I’ve experienced on my very own reference amp while the vocals were anchored and, although a tad analytical sounding at times, extremely clear. I was quickly realizing the A1400-8 was equally comfortable driving two or seven channels at listening levels beyond my ears’ capabilities without introducing graininess or mushy sound similar to what I’ve heard from lesser-designed amplifiers.

Recommendations for Improvement

As good as the Axiom A1400-8 multi-channel amplifier is, it could stand some improvements to make it even better. My experience with this amplifier has led me to believe this is NOT a field tested design for the following reasons:

 

• It’s too wide - The A1400-8 is unfortunately about ½” too wide to fit into a 17” shelf of a standard rack mount. This was an unfortunate oversight made by Axiom so be mindful of where you plan on installing this amplifier.

• Lots of internal wires instead of short low impedance returns to the power supply increases susceptibility for ground loops via unbalanced connections and also limits the channel to channel crosstalk performance. Axiom claims they’ve since then installed a low impedance grounding bar across all the speaker returns to eliminate this problem.
• Sensitivity to heat. Due to the precision internal clocking this amplifier needs to be installed into a cool location. I found when I had it about 6 inches above my Denon linear amp, the internal fan of the A1400-8 would often come on even during idling conditions. This was slightly audible at very low listening levels about 15 feet from my rack. It’s best to keep this amp away from heat generating sources.
• It’s more susceptible to damage under negligent usage than most conventional amplifiers.
• Aesthetically this amplifier doesn’t really stand out or scream high end. Its diminutive size is partly the reason, but I think Axiom could have dressed it up a bit by perhaps adding LED backlighting to the front panel logo. Personally I always go for function over form but in this market many audiophiles chose quite the opposite when selecting their products.

 

I completely destroyed the first sample Axiom sent me when I was doing a power sweep test and forgot to attach a test load. Once my test gear reached 20kHz, the unloaded amp hit a resonant peak (caused by the unloaded output filter) and blew up the output devices and zobel networks. Luckily Axiom had the foresight to design the amplifier to self contain the fire as I was in no immediate danger, but the smell was horrific and something I hope to never experience again. Granted my scenario is highly unlikely in the real world but the lack of protection for this type of scenario was a bit unsettling to me.

 

I managed to test my second unit without incident until I was swapping cables in my sound lab. I was comparing the Axiom A1400-8 and my Denon POA-A1HDCI amp on a pair of my bookshelf reference speakers while also switching over to my reference towers. Everything was fine until I decided to bi-amp my towers and move the cables over with both amps turned on. I never recommend this but as a reviewer trying to switch out as quickly as possible for the most accurate results, I got a bit reckless and took the risks for the benefit of our readers. On my towers, I had the Axiom A1400-8 powering the subs. As I switched over my live speaker cables from my Denon amp to the mid/tweet portion of my towers, I heard a loud popping sound and saw sparks come off the speaker cable and terminal lug of my speakers. I looked over to the rack and both amps were still on and no smoke or smell was evident. My speakers were also perfectly fine, no damage to any of the drivers! However, channels 5 and 6 of the Axiom amp were now dead. I have no idea what happened here but from my experience of accidentally shorting amplifiers out when powered on, they typically shut off and protect themselves. The user manual also warns that if this amplifier is shorted when outputting 200 watts or greater, serious damage will occur.

 

In speaking with Axiom, they informed me that since this amp has an unusually high switching frequency, it’s difficult to protect it from fault conditions without compromising sound quality. They could have very easily implemented the same current limiting protection that others such as B&O utilize but the performance tradeoffs simply weren’t worth the trade-off in fidelity. The reported failure rate on this amplifier is very low and I am certainly an unusual user but hopefully the technology will soon be available to have uncompromised protection for such circumstances. In the meantime, make sure the amplifier is always turned off when swapping out cables and don’t let anyone run sweep tones through your amp with no load attached.

Axiom Audio A1400-8 Measurements and Analysis

All measurements were conducted on the new Audio Precision APx585 8 Channel HDMI Audio Analyzer. Unless otherwise noted, the internal 40kHz HPF of the APx585 was utilized to obtain more accurate test data by limiting the potential of slew-rate-induced distortion caused by switching amplifiers.

I did some quick spot-checking on the A1400-8 amplifier gain structure to ensure it could be properly driven with a wide assortment of preamps or receivers. My personal criteria for amplifier gain structure is that it should be able to hit full power when driven with 2Vrms. The A1400-8 blew this notion out of the water with an unusually high voltage gain of 31.8dB. Axiom clearly understands that many A/V receivers lack sufficient output drive and thus purposely boosted the gain of this amp to work with a wider assortment of A/V receivers and dedicated preamps. The A1400-8 reached its full power with only about 1.35Vrms.

SNR

Figure 1. SNR @ 1 watt

I measured SNR at 1 watt output power with seven channels driven using A-weighting and recorded commendably good results as indicated in the graph above. Anything above 80dB at 1 watt is pretty much dead quite. Considering the high gain structure of this amplifier, I’d say this is quite an excellent achievement and explains why this amp was very clean at low power levels.

Frequency Response

Figure 2. A1400-8 Frequency Response

I know what you’re thinking. How could there be such a big bump at 20kHz in an amplifier? The truth is all Class D amps that utilize an output filter exhibit this behavior to some varying degree. The A1400-8 showed anywhere between +2 to +2.3dB at 20kHz into an 8 ohm load and about -1.5dB when driving a 4 ohm load. This amplifier was optimized to drive low impedance loads though it will be right at home driving any load you throw at it. I do however wish Axiom had pushed the LPF response out a few kilohertz to reduce the load impedance interaction at 20kHz. As advancements in silicon progress and the switching frequency moves further out, so will the low pass filter poles, which will in turn flatten the response of these amps more akin to traditional linear amps but with the huge advantage of efficiency and power delivery.

Power vs Distortion

With the new APx585, we are now able to do a full barrage of multi-channel amplifier testing. We tested power using three methods all of which were taken at < 0.1% THD + N:

• Continuous Full Power Bandwidth (CFP-BW) from 20Hz to 20Khz into 8 and 4 ohm loads (up to two-channels)

• 1kHz Power Sweep vs Distortion (1kHz PSweep) - popularized by the print magazines, this is an instantaneous power vs distortion test at 1kHz. The problem with this test is it often masks slew related and or frequency response problems some amplifiers exhibit at the frequency extremes, and thus inflates the measured power results. It does provide an instant gratification number for consumers to argue over on the forums so we are now incorporating this test to please the masses.

• 1kHz CEA-2006 Burst Method testing. This is a dynamic power measurement adopted from the car industry similar to IHF method but a more strenuous test condition on an amplifier and more representative of real musical content.

Figures 3& 4. A1400-8 PSweep-1kHz Power Testing

    

Figures 5 & 6. A1400-8 CEA2010 Dynamic Power Testing

Figure 7. A1400-8 Continuous Power vs Frequency (4 ohms)

Table 1. A1400-8 Output Power Under Various Test Conditions

Note: I measured this amplifier to consume around 80 watts just idling which seemed a bit high to me for a Class D amp but certainly more energy efficient than other traditional multi channel amps I’ve measured.

Keep in mind most review publications test at clipping and don’t do continuous power measurements so our power numbers are usually a lot more conservative than what you typically find from other reviewers.

For more info, see: The All Channels Driven (ACD) Test

Upon my initial multi channel testing, I noted a couple of channels had a slightly reduced power level and pulled up the Oscope on the APx585 to see what was going on. The 1kHz sinewave was a bit distorted and it wasn’t until I realized that the alligator clip was shorting to the heatsink of my resistor boards that I was essentially feeding the A1400-8 a dead short and it was happily driving it at 7Vrms or nearly 50 watts. Most amps would have displayed significantly less output under this condition (if at all) but the A1400-8 was more than happy to deliver the current demand of a short circuit at this power level.

The A1400-8 is the most powerful multi-channel amplifier I’ve ever tested as indicated in the Table 1 test results. Axiom’s power claims into 8 and 4 ohm loads with one channel driven (350wpc / 700wpc, respectively) are a bit higher than I measured but they also don’t specify at what distortion level they test to. I limit my testing to 0.1% THD to be more conservative and although there seemed to be a lot more juice available, I didn’t want to risk blowing the amp out using full power bandwidth sweeps out to 20kHz.

Thanks to the A1400-8’s high efficiency (> 90%), it was able to drive two-channels at near equal power to the one channel ratings. I was unable to test all 8 channels simultaneously since I don’t have enough test loads, but running all seven channels, I achieved a whopping 226wpc at 0.1% THD + N which exceeds Axiom’s specifications of 200wpc x 7. Dynamic power testing using the 1kHz CEA-2006 method proved this amp had real headroom as claimed.  Comparing the continuous power output to the dynamic power output (two-channels driven) showed 1.4dB of dynamic headroom which is darn good.

FFT Distortion Analysis

        

 

Figures 8 & 9. FFT Distortion Analysis

I ran FFT distortion plots at various power levels to determine how clean this amplifier really is. At 1 watt, the spectral plot was squeaky clean which really surprised me considering the topology of this amplifier and all the switching residuals typically associated. At high power the residual harmonics were about -80dB lower than the 1kHz fundamental. This is exceptionally clean performance for a linear amp let alone a switching one.

Crosstalk

    

Figure 10 & 11. A1400-8 Crosstalk at 100wpc

Figure 10 depicts channel-to-channel crosstalk with only one channel as the disturber while figure 11 shows all but the particular test channel as disturbers to yield a worst-case scenario measurement. Unfortunately crosstalk performance of the A1400-8 was less than stellar when compared to other amplifiers we’ve reviewed. At 1kHz I observed anywhere between 75dB to 100dB depending on what channel pairs were under test. At 20kHz this translated between -50dB to -75dB. I’ve seen other less expensive multi-channel amplifiers utilizing a similarly single large centralized power supply produce much better figures than this (about 20-30dB better), so I feel Axiom has some room for improvement in this regard.

Upon closer inspection I found that when I set CH2 as the disturber, it polluted Channel 1 significantly worse than any other combination of channel pairs. I suspect it’s NOT a board level capacitive coupling problem since the other channel pairs measured significantly better. This is likely coupling in the wiring harness inside the amp. Thus I recommend running non-adjacent amplifier modules for the front channels to ensure you achieve the best stereo separation.

Output Impedance and Damping Factor

Figure 12. A1400-8 Amplifier Output Impedance vs Frequency

Amplifier output impedance of the A1400-8 is commendably low (especially for a Class D amp) until we see the effects of the output filter at around 10kHz where it jumps from around 100mohms to over 2 ohms. I’d again like to see the filter effects get pushed out of the audio band entirely but the highish output impedance above 10kHz didn’t seem to negatively impact my listening experience. This is likely because there is so little energy at those frequencies and most speakers exhibit rising output impedance due to the inductance of the voice coil making this more academic than a real world concern.

 

Figure 13. A1400-8 Amplifier Damping Factor vs Frequency

The damping factor of the A1400-8 was above the 50 mark we like to see for high performance amplifiers out to 10kHz. What is interesting to note is that it was virtually unchanged regardless of load impedance. Typically when you halve the load impedance we see the damping factor also go down proportionally. With the A1400-8, the amp behaved more like an ideal voltage source which may explain why it really exhibited phenomenal bass response into my 4 ohm speakers. This amp loves to be driven at low impedances and high power which are great attributes for stress free sound in all listening environments.

Axiom Audio A1400-8 Conclusions

Axiom Audio has redefined performance expectations of Class D amplifiers with their A1400-8. They have proven that a highly efficient digital amplifier topology 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 it this design to offer top notch fidelity at power levels unachievable in linear amplifier designs via a single power cord, the premium price tag becomes more understandable. Factor in that it would take a stack of bulky linear amplifiers to equal the power offered by this slimline lightweight design and you’ve got a real space saver on your hands that is also green friendly.

Though this amplifier isn’t flawless in execution, most of the issues I noted won’t present real world problems if you follow the installation guidelines in the user manual and my review. If you’re assembling an upscale home theater system with the design goals of uncompromised performance 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.

A1400-8
MSRP: $3,850

Axiom Audio
www.axiomaudio.com

Highway 60
Dwight, Ontario
Canada
P0A 1H0 

For more than two decades, Axiom Audio has made its reputation by achieving unprecedented performance quality. Founded in 1980, Axiom is counted as one of the cornerstones of the Canadian loudspeaker business. Using the world's most advanced computerized woodworking equipment, the company has become one of the world's premier manufacturers of quality loudspeakers. The company's home office and manufacturing facilities are located in the Muskoka district of Ontario.

The Score Card

The scoring below is based on each piece of equipment doing the duty it is designed for. The numbers are weighed heavily with respect to the individual cost of each unit, thus giving a rating roughly equal to:

Performance × Price Factor/Value = Rating

Audioholics.com note: The ratings indicated below are based on subjective listening and objective testing of the product in question. The rating scale is based on performance/value ratio. If you notice better performing products in future reviews that have lower numbers in certain areas, be aware that the value factor is most likely the culprit. Other Audioholics reviewers may rate products solely based on performance, and each reviewer has his/her own system for ratings.

Audioholics Rating Scale

• — Excellent
• — Very Good
• — Good
• — Fair
• — Poor

Metric	Rating
Frequency Response Linearity
SNR
Output Impedance
Measured Power (8-ohms)
Measured Power (4-ohms)
Multi-channel Audio Performance
Build Quality
Fit and Finish
Ergonomics & Usability
Features
Performance
Value