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.
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