Axiom Audio EP500 and EP600 Subwoofers - First Look

Update: 4/20/14: Axiom has replaced these subs with smaller more expensive sealed versions.  See: Axiom EP500v4 and EP600v4 Subwoofer Preview for more details

When Axiom invited us out to their facility to show us their latest EP500 and EP600 subwoofers, I knew it must be big news.  Knowing Audioholics' reputation of testing manufacturers' claims (especially regarding cables) the folks at Axiom must have been pretty confident with their new designs. The email Amie Colquhoun (lovely wife of Ian) sent me regarding the SPL capabilities of these new subs left us wanting more information, pictures and specifications, and more importantly opportunities to hear them first hand.  Being the bass nuts we are, you could imagine our level of anticipation waiting for this to happen.

The massive 12-inch aluminum-cone driver with a 3-inch diameter dual voice coil was designed with dual voice coils to lower the internal impedance to 1.3 ohms, so the digital amp (pictured below) could utilize high-current drive.

Axiom's new digital subwoofer amp is designed from the ground up by industry veteran Tom Cumberland.  Unlike most digital amplifiers, this one features an analog power supply to ensure plenty of headroom for large dynamic peaks.  The EP500 gets a 500 watt version and the EP600 gets a 600 watt version.  Tom says he could have pushed the limits of the design to get 1000 watts RMS, but that would have involved redesigning the 12" driver to withstand the extra juice.  At 111dB (anechoic) SPL's I think you can give it a rest Tom. :) 

Amazingly, due to the high efficiency of this design (> 90%) I was able to touch the heat sink directly coupled to the output devices during operation without burning the skin off my finger.  To get this much power out of a conventional amplifier would require nearly double the power supplied by the wall outlet which approaches the limits of most household circuits rated at 15 amps.  This is especially true when other devices are also attached to the same circuit simultaneously.  The secret behind this amp is in its use of Pulse Width Modulation (PWM) technology.

Editorial Note on Class D (PWM) Amplifiers
Class D amplifiers utilize a technique called pulse width modulation which is sometimes combined with pulse frequency modulation. The input audio signal is converted to a sequence of digital pulses whose width at any time is proportional to the amplitude of the signal at that time. The frequency of the pulses is typically 30 or more times the highest frequency of interest of the audio signal. Unfortunately the byproduct of the output of such an amplifier contains unwanted harmonics that must be removed by a passive analog filter. The output of such amplifiers usually work best with constant impedance crossovers since they can react a bit unpredictably with highly reactive speaker loads which is why these amplifiers are best suited for subwoofer applications or specifically designed amplifier and speaker packages.

The main advantages of a class D amplifier are efficiency and space savings. Because the output pulses have a fixed amplitude, the switching elements (usually MOSFETs) are switched either on or off, rather than operated in linear mode like conventional Class A /B designs. This means that very little continuous power is dissipated by the transistors except during the very short intervals of on and off states. The wasted power is low because the instantaneous power dissipated in the transistor is the product of current and voltage, both of which are almost always close to zero.

 

Of course the Axiom engineers didn't just end it with a great digital amp design.  They went one step further and implemented Digital Signal Processing (DSP) with a custom-designed "algorithm" tailored around the specific driver to linearize the response across the usable bandwidth at all power levels. This was to automatically correct frequency response deviations.  The DSP also serves as a limiter to prevent the amplifier from exceeding its output limits and going into distortion. It also limits the subwoofer driver from over-excursion to minimize excessive distortion and cone break-up.   The system is designed to minimize frequency response deviations to +-1.5dB (in anechoic environments) within the full bandwidth of the product.   More on this later.

EP500 and EP600 - The Listening Tests

The EP600 measures about 46 x 15 x 17 inches. By combining the new driver with the application of a very long tuned port (3 feet!) in a large enclosure and the assistance of the Digital Signal Processing of the response, the desired bass extension to 16Hz in an anechoic environment was achieved.

 The slightly more manageable box size of the EP500 measures in at 20 x 15 x 20 inches and plays down flat to 20Hz with about 2-3dB less output than its bigger EP600 sibling.

Listening Tests

Ian started us out with an assortment of pop rock music to hear and feel the capabilities of the new subs.  He initially popped in a track with contained only female vocals.  I was shocked to see the EP600 woofer get such a workout while no bass energy was present.  Tom brought to my attention that many CD's have a lot of "junk" information below 20Hz.  Since these amps were prototypes, they didn't yet employ a subsonic filter to squash it.  Production units should have a High Pass Filter implemented at around 12-13Hz to eliminate this issue.  While it looks cool to see the woofer moving frantically in and out, it is unwise to have it run free like this during the absence of real program material as it eats amplifier power and increases cone excursions which in turn can increase distortion and reduce dynamic range.

Moving on, we listened to Bobby McFarren's duet album with Yo Yo Ma.  I was dumbfounded by the amount of deep articulate bass in this song.  Switching off the EP600 one could really hear the difference (or to put it mildly absence of deep bass).  While the M80ti's were no slouch, having them paired with an EP600 really did add significant and noteworthy impact and slam to the music.  Ian had the M80ti's running full range and the EP600 crossed over at 40Hz.  While this was not my set-up preference, I was very pleased with the overall sound and how well the system seamlessly integrated.

Moving on to musical references I was well familiar with, I asked Ian to pop in some Steely Dan.  We started out with the title track for Aja.   What a wonderful cut this is.  Despite the long length of this song, we all enjoyed it in its entirety.  In fact, the EP600's low extension prowess added a new dimension of excitement and enjoyment to this song.  I anxiously awaited Steve Gadd's infamous drum solo to hear what the EP600 would do.  I was pleased to hear no muddiness or "one notey" type bass response typical in many long excursion high power subwoofers.

Now was the time for the fun stuff.  We proceeded to our torture tests with bass tester CD's typically listened to by car audio enthusiasts.   Initially I heard some rattling noises and thought it was attributed to the port.  Ian politely squashed my concerns and told me to look over at his kitchen cabinets.  When I saw the cabinets flexing on the wall I realized he may have a point.  I then stuck my head (don't try this at home, kids) up near the woofer and port and confirmed there was no port noise.  I did get dizzy for a moment, but it was well worth it and all in the name of science.  What I found most impressive was the well extended bass response even at low listening levels.  The EP600 was still convincingly producing solid extension at low output levels.

While we were impressed with the EP600, I was more interested in hearing what the EP500 would do simply because its cabinet dimensions are more workable in most homes and will likely yield a higher spouse approval factor.  Ian happily switched over to the EP500 and we went through the gamut of our previous music selections to have a listen.  I personally preferred the EP500 and M80ti combo over the EP600 in the listening environment we were in.  While it didn't have quite the output or extension of the EP600, it was close enough and I felt it integrated even better.  I was pleased to hear and feel the might of the smaller and lest costly EP500.  Ultimate bass lovers with room to spare will likely opt for the EP600, but regardless both appear to be winners and should please any bassaholic.

After an extensive listening session we decided to take a dip in the 68 degree Fahrenheit lake.  While this may seem a palatable temperature to many, realize I live in Florida and swim in 90 degree water almost all year long.  I had to do it.  I had to suck it up, especially since they inferred someone might anonymously call me a wimp in our forums if I didn't!

Pictured from Left to Right: Amie, Ian, Me, Tom, Alan, Steve
and Kathy by the Dock at the Colquhoun home.

I found the shock of cold water to be invigorating and surprisingly more awakening than a shot of espresso (or 120 volts of AC for that matter).  I was particularly fascinated that we could still clearly hear the EP500 belting out the low tones in the distance and it wasn't even turned up.

EP500 and EP600 Performance Tests

While subjective commentary can give good insights as to how a product performs and how the listener/reviewer perceives its performance within the confines of their listening environment, the proof is in the pudding so to speak.  This is why it's always good to have some supplementary measurements and analysis to see if those measurements are audibly perceivable and correlate to what we are hearing.  Since frequency response can demonstrate the product bandwidth or tonality, this is a good start.  To go one step further, one must test this frequency response at various power levels to determine power bandwidth uniformity and freedom from excessive compression.  With that in mind, we ran some frequency vs SPL tests on the EP600 at various output levels to validate Axiom's claim quoted below:

"Our DSP system wants things to go smoothly all the time - no bumps in the road. Using a digital roadmap, the XLF controls every frequency from 17Hz to 100Hz, never deviating more than 1.5dB through this entire range."

EP600 SPL vs Frequency Plots

The bottom purple trace- indicates 17Hz to 100Hz +/-1.5dB @ 85.5dB SPL
Note 16Hz at 85.5dB is the threshold of audibility for such a low frequency

System linearity remains excellent as power is increased though the lower frequencies handled by the port are increasingly attenuated. In an acoustic suspension system this type of curve would appear to signal power compression. Without an audible reference with this ported system, however, it is impossible to know how the system actually sounds, that is, if extraneous noises are coming into play, as the power levels are increased.  I personally didn't hear any problems with audible compression in my listening tests.  In fact the EP500 and EP600 appeared to have enough headroom to rumble the foundation of the Colquhoun house.

It appears that the driver-port transition area is at 30Hz with the transducer being equalized approximately +4dB at 33-34Hz. The ultimate low bass capability of the system has been extended downward below 30Hz by the use of a large enclosure with a well designed port. The system is thus able to achieve a free air roll-off of ~18dB/octave which should allow better-than-average room coupling of extremely low frequencies at the cost of ultimate system (broad bass-band) output.

Plotted below are the measurements for the EP500, again done in a 4pi environment 1 meter away.

EP500 SPL vs Frequency Plots

The bottom purple trace- indicates 20Hz to 100Hz +/-1.5 dB @ 85dB SPL

As you can see the EP500 does have a few dB less output at the lower frequencies, and given the smaller box size and slightly less amplifier power, will not play quite as loudly as its bigger brother.  However make no mistake, these subs have a well extended low end with ample power.

Thus the Axiom Audio claim about maintaining +/- 1.5dB 17Hz to 100Hz for the EP600 and +/-1.5dB 20Hz to 100Hz for the EP500 through the entire range is in fact true providing that the SPL output is kept to under 92dB (anechoic) which equates to about 103dB or so in a real room.

Closing Comments

I walked away from Axiom with a new respect for the company.  Despite the fact they are solely involved in selling direct to consumers via the internet, make no mistake that they are just as serious, if not more so, than many high end loudspeaker companies with dealer based distribution.  What amazed me was how kind hearted, casual and well spirited the Axiom group was, yet when push comes to shove, extremely meticulous about their art of loudspeaker research, design and manufacturing.

Axiom has assembled a very talented engineering and support staff.  Most of the engineers currently working for Axiom have been on the research council for the NRC, Harman, and other highly respected research based companies.

Their remote support staff (equivalent to the dealer of a direct distribution loudspeaker company) includes industry veterans such as Alan Lofft whose experience and familiarity with the products he represents rivals that of most typical support dealerships.

Tom, Ian and Steve really have a couple of winners on their hands with the introduction of the EP500 ($1,150) and EP600 ($1,750) subwoofers.  DSP processing to linearize the driver response is certainly a valid method of designing a subwoofer whose goal is accurate bass reproduction.  However, the DSP processing only tells part of the story.  Axiom’s potent 12” driver, coupled into a rigid cabinet with the always clever port designs of Ian, truly sell a product where the sum of the parts exceeds the overall package.  We will reserve final judgment on these super subs until we can review them in the most important environment, our listening rooms.  But until then, they are certainly worth a try, especially given Axiom's excellent customer support and 30 day return policies (not to mention free shipping.)

Impressions of Trip and Country

Our trip to Canada was both memorable and enjoyable.  It was reminiscent of my childhood days spent in the countryside of Long Island, NY, only with an even more intimate and primitive feel to it.  In the fast paced days of our lives where McDonalds and Walmart dominate every other street corner in our towns, and Reality TV's shows are considered to be "entertainment", it is nice to get back to the basics of life and realize the important fundamentals such as nature, friendship, peace and serenity.  After spending a few days in the Canadian countryside I began to realize why Ian and the folks at Axiom were always cheerie and laughing.  The way of life and the way they run their company is one of comradeship, family, and placing quality of life and well being above status meetings, rushed reports and deadlines to ensure company stock maintains the status quo.  Their way of life also manifests itself in the products and services they deliver to their customers.  The Axiom experience was unlike any other company I have worked for or visited.  These folks actually enjoyed being at work and doing their jobs.  In fact, one of their technicians seemed a bit flustered at all of the time we spent in their lab which prevented her from completing her work.  Talk about devotion!  This was foreign in my experience and I thought only happened in movies or with fortunate soles that operate their own home theater review publications for a living ;-)

Additional Pictures and Sights