Audioholics Axiom On Location #2: New Products

Many of the new products from Axiom Audio are under development which means they are understandably tight-lipped about them. Though they were indeed tough as nails in telling me only what they thought appropriate, I did manage to eek out snippets of information from them which I am happy to share in this report.

New Loudspeaker Products

For years we've been bugging Axiom to develop in-wall speaker products to address a part of the market they were currently missing and perhaps underestimating. Their contention always centered on concerns of performance compromises in such products, mainly because they haven't heard any in-wall products from competitors that they were entirely happy with.

Just as with most other loudspeakers, care must be taken in the design phase. Selecting the right crossover topology and response is essential to properly manage boundary effects that result from having an in-wall speaker baffle flush with a wall. Secondly, it is crucial to minimize the energy transfer from the speaker system to the wall. The finest in-wall products usually sport back boxes to deal with this problem.

In speaking with Ian, he told us Axiom's design goal when developing an in-wall / on-wall product was to suffer no performance compromises. They wanted the in-walls to deliver the same quality performance consumers have come to expect from Axiom's box speaker counterparts, but with the convenience of occupying less space in the room. That being said, they felt the best solution was to develop a speaker that is fully enclosed, and partly in the wall, with the front baffle slightly protruding from the wall. A majority of high performance in-wall products with enclosures such as the RBH Signature Series must be installed during the framing stages of new homes since their enclosures are so enormous. M ost people don't have this luxury in existing homes, so a product such as the Axiom In-wall / On-wall makes good sense. Think of this speaker as a plasma on-wall type, but with the potential of a significant performance edge.

Introducing Axiom Audio's new In-wall / On-wall loudspeaker solution

  

This prototype (left) is the in-wall equivalent to the M3ti box speaker. Note the double ports on the bottom (right). This prototype had the wires running through the ports, but actual production models will sport traditional back terminal speaker connectors. Its back enclosure allows the speaker to achieve comparable box volume to the M3ti enclosure ensuring similar low end frequency response and even more importantly sound isolation from the wall enclosure.

Axiom plans on releasing a matching center channel shortly after the M 3ti in-wall hits the marketplace. Projected launch date is sometime late November of this year.

New Electronics from a Loudspeaker Manufacturer?

You betcha! With the enormous resources, talent pool, and test equipment of Colquhoun Labs you didn't just expect them to stay put designing loudspeakers, did you? Based on the excellent digital amp which is the heart of the EP500 and EP600 subwoofers, Axiom has taken this design concept to the next level by developing one of the industry's first high-performance full-range digital amplifiers. Granted, there have been a wide assortment of full range digital amp designs preceding this, but they usually employed low-pass filters on the output stages that interact with the reactive load of most loudspeakers. As a result, they had to be used with speakers with constant-impedance crossovers in order to maximize their sound.

For more information on this topic check out - The Truth About Digital Amplifiers

The Axiom solution differs in many aspects from traditional Class D PW M amplifiers:

• Fully complementary design from input to output - very few "audiophile" (or otherwise) linear amps maintain a balanced design though the output stages
• High switching frequency (500kHz clock with 1MHz Mosfets), which allows the Low Pass Filter (LPF) to be implemented well above 20kHz thus avoiding deleterious reactive-load loudspeaker interaction
• Fully analog power supply ensures maximum dynamic range and low level linearity. As you can see in the picture of the prototype I was holding, the transformer and capacitor bank are tremendous. Carry a few of these around the lab all day and you can skip your gym workout.

In discussing this amplifer design with Tom, he informed me that the design group at Axiom carefully selected the poles of the LPF so there would be no phase or frequency response shifts in the audio passband while driving 1-ohm to 8-ohm reactive loudspeaker loads. The amplifier utilizes a single-rail balanced H bridge design which further increases efficiency over a two-rail amplifier design, since the balanced topology allows the amp to track the rails perfectly for positive and negative swings. In contrast, conventional amplifier designs using two rail supplies must deal with the higher drop-out voltage on the negative rail, and less ideally, load balancing, which compromises efficiency and to some extent dynamic range during sharp transients.

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 works best with constant impedance crossovers since they can react a bit unpredictably with highly reactive speaker loads. This 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.

The enormous efficiency advantage of this type of amplifier topology over traditional linear Class A/B is two-fold in terms of heat dissipation and power delivery to the loudspeakers. There are a wide assortment of so-called "audiophile" amplifiers boasting enormous power delivery into all seven channels while only utilizing one power cord. While we have strong viewpoints against the validity of the "all channels driven test", it is academically interesting to note that the manufacturers advocating it usually cannot meet their cherished specification. Consider the typical Class A/B linear amp with a single power cord, and actual efficiency in the ballpark of 40% (factoring in conservative transformer losses and power factor). The best case scenario power that the amplifier could deliver to the loudspeaker system under this test condition is 1800 x 0.40 = 720 watts / 7 channels = 102 wpc! This assumes ideal test conditions: line voltage held constant via a Variac (not a typical scenario in one's home), minimal transformer saturation, and not blowing any rail fuses.

In contrast, with the efficiency of Axiom Audio's new PW M amplifier in the ballpark of 96% (93% with transformer losses and power factor considerations), power delivery under the exact same test conditions will pretty much be double that of conventional amplifier designs! Finally a solution that actually can hit power specs even in the most unrealistic and uncommon test condition. Best of all, it doesn't require two dedicated 15A circuits or an amplifier housing the size of a truck.

They take it one step further by providing the ability for any channel to dump over a kilowatt of power to the speaker load at any given time. Of course this assumes the loudspeaker has a low enough impedance to source this much current and further assumes it could actually handle it, not to mention if your ears could! Most loudspeakers can't, and don't, but I would expect Axiom knows this and may be working on one that will complement this amplifier.

When I asked how they managed to create such a highly efficient design, even by digital standards, Tom informed me that they use a high-flux core toroidal power transformer which is very efficient (up to 1.2x rated current and voltage). This results in minimal core losses and I^2R losses in wire - a typical problem with poor transformer design. Axiom's massive power supply and efficient amplifier topology allows the amplifier to maintain a very high power factor of greater than 90%.

Editorial Note on Power Factor
Power Factor is the ratio between real and apparent power. It is related to the phase angle between voltage and current and is a simple way to describe how much of the current contributes to real power in the load. A power factor of one ( unity or 1.00 ) indicates that 100% of the current is contributing to power in the load while a power factor of zero indicates that none of the current contributes to power in the load. Purely resistive loads exhibit a power factor of unity, meaning the current through them is directly proportional to the voltage applied to them.

The beauty of this design is it provides limitless dynamic range and headroom allowing it to drive pretty much any conceivable loudspeaker solution to full power with minimal heat dissipation or associated losses.

No official product release date has been announced by Axiom, but we were told to keep a look out sometime early next year.