- Written by Chris Eberle
- Published on 03 June 2013
Design of the Axiom LFR1100 Omnidirectional Speakers and ADA-1500 Power Amplifier
The LFR1100 towers set a record, at least for me, of most drivers in one cabinet. Each tower contains eleven; seven in front and four more in the back. The front also has a port at the very bottom with Axiom’s special shaping applied to eliminate chuffing and other related artifacts. At the back are two more ports of the same size and shape. The driver complement is two titanium tweeters, two aluminum midranges, and three aluminum woofers in front; and two pairs of tweeters and midranges at the rear. The rear baffle is angled outwards to further reduce issues with placement near a wall. Because of this, the towers are specific to the left and right.
Fit and finish is up to Axiom’s typically excellent standards. The samples I received were black with a wood-textured vinyl coating that had a nice deep grain and was perfectly applied. Corners and edges were ever so slightly beveled and there wasn’t the slightest mark from the shipping process. A word about that: Axiom takes great effort to package their products for safe and secure delivery. The boxes are heavy-duty corrugated cardboard and stapled shut. Inside, the speakers are protected by a cloth bag and full enclosure foam. There is very little void space in the carton. Everything is shipped via Fedex Ground to the customer. Their customer service is legendary as well. In the unlikely event something arrives damaged, even in the slightest, they will replace it immediately.
Since the towers are four feet tall and relatively narrow, Axiom has included outriggers to widen their footprint. Rubber feet are included and you can specify spikes if you wish. The grills are magnetic (bravo!) and come off in three pieces in front and one in the rear. They’re always fun to remove for guests so you can watch them gasp as all those lovely aluminum drivers are revealed! The cabinets are extremely solid and sound suitably inert when subjected to the rap test. There is no doubt that they are assembled with great care and pride.
That care and pride extends to the LFR’s internal structure as well. The front and rear driver sets are isolated from each other; both physically and electronically. Each set has a passive crossover that creates linear sound power and radiation patterns from the drivers. The interaction between the driver sets is then managed by an outboard DSP that prevents phase errors and frequency cancellations. The drivers only interact with each other in an additive manner. In other words, all of the disadvantages of bi-polar speakers have been addressed and eliminated. Additionally, the mid-range drivers are each housed in a separate sealed enclosure. This tightens up their response greatly. The lower portion of each tower, which is about half the cabinet’s volume, is dedicated to the three 6.5-inch woofers.
You can see in the above photo, the LFR1100 cabinet laid out on a work table. The vinyl is already attached to the underside creating a hinge by which the four panels can simply be folded up into their final shape. The black boxes are the sealed midrange enclosures. And you can clearly see the independent crossovers for each bank of drivers. Note little details like the threaded metal receivers for the screws that secure each driver. No hardware is threaded into the wood itself. Internal cabinet bracing is quite substantial as well with seven MDF parts making contact with all sides of the tower. It doesn’t get more solid than this!
In designing their flagship ADA-1500 amplifier, Axiom took a different path than the more-common Class A/B switching topology; incorporating its own Class D modules. The ADA comes in configurations ranging from two to eight channels. Axiom’s goal here is plenty of clean power delivered as efficiently as possible. Class D does this very well as it wastes far less power in the form of heat than Class A/B designs. The ADA also allocates power dynamically so when those inevitable peaks arise in only one or two channels, the current is there to handle the demand. With multiple channels driven to maximum, there is more than enough power available for even the most difficult to drive speakers.
I asked Andrew Welker, a speaker and amp designer at Axiom, for a technical explanation of the inner workings of the ADA-1500. Here is his response:
“The ADA amplifiers differ mainly in the power supply topology used. We are not fans of switching power supplies which are hard to control from an RF radiation standpoint and are can have longevity issues. This is key when you already have a sophisticated switching system within the amplifier modules themselves and any interference here can be detrimental to performance. Our ADA amplifiers use traditional, high-current linear power supplies with over-specified toroidal transformers and large banks of filter capacitors. This allows for large instantaneous current supply to any amplifier channel that requires it. When a multichannel amplifier is being used in the context of a home theater or multichannel audio system, all channels are rarely called on to deliver full levels at the same time. With a large, shared linear power supply the entire current available can be instantly supplied to a single channel or shared among any number of channels. The Class D amplifiers themselves are our own design using the latest in switching device technology. In the ADA-1500 which you have, each amplifier channel uses two pairs of high current output Mosfets that are capable of delivering many times the current that they will ever see in use. Paired with this, by necessity, is sophisticated protection circuitry that monitors DC offset, output current, multiple power supply voltages, and heat-sink temperature in order to protect both the amplifier and the loudspeakers connected from any fault conditions that might occur.”
I received a four-channel ADA-1500 which is rated at 650 watts per channel (375 watts with all channels driven) into four Ohms. This provides plenty of headroom for the LFR-1100s which are fairly sensitive at 95 dB 1 watt/1 meter (in-room).