Burmester B25 loudspeaker
The B25 is a small, ported speaker standing about 42" tall, 8" wide, and almost 14" deep. It follows Burmester's usual three-way design of tweeter, midrange, and side-firing woofer with rear-panel bass-reflex port. All of its drivers were initially developed to be used in the Bugatti Veyron. These are claimed to meet strict tolerancesonly the best 10% of these are used, apparently. All drivers are burned in for two weeks using a 10Hz continuous sinewave for the midrange (at 10W) and woofers (at 250W), and a 7W continuous pink-noise signal for the tweeter. The drivers are then measured and matched by computer to be within ±0.5dB of their specified frequency responses. The actual performance of each driver is filed so that a B25 in the field can receive a new driver that precisely matches the one being replaced.
The B25 shares with the B100 its 30x40mm Air Motion Transformer (AMT) tweeter. The 12cm2 area of the AMT tweeter's diaphragm, made of heat-resistant Kapton foil, is four times the area of a conventional dome tweeter. The AMT uses neodymium magnets with an energy density 20 times that of comparably sized ferrite magnets. Although the tweeter's upper-range output extends out to 45kHz, a short horn had to be used at the front of the driver to extend its lower response to allow a crossover frequency of 2.7kHz.
The midrange driver has a 5.1" woven Kevlar cone. Woven Kevlar composite has the best ratio of rigidity to mass of known diaphragm materials, says Burmester, and the B25's midrange cone is claimed to be resonance-free up to 3kHz. Its die-cast aluminum-alloy chassis is rigid and nonmagnetic. The B25's woofer is a 9.1" by 6.5" oval driver with a double-vented voice-coil. The diaphragm is formed from air-dried paper , the 50mm-long voice-coil former is Kapton, and a 25mm coil allows long linear excursion. The reflex-aligned woofer vents through a large flared port on the speaker's rear; foam port plugs are supplied if the speaker's bass is found to be excessive in smaller rooms.
The split crossover is attached to the bottom of the B25's enclosure, isolated from vibration. Second-order filter slopes are used between the tweeter and midrange, third-order slopes between the midrange and woofer. Impedance corrections of the driver's resonant frequency and magnetic coil inductance are set so that theoretically correct filter behavior remains independent of changes in any one driver's impedance. Woofer connections are made using 6mm2 OCC copper wire, while 4mm2 OCC copper wire is used to connect the crossover to the tweeter and midrange drivers. Biamping or biwiring is made possible by four sizable five-way binding posts on the rear panel, equipped with large plastic wing nuts and set below the reflex port, near the floor.
The B25's enclosure has a rigid inner housing; instead of being internally damped with wool, it's filled with felt mats of various thicknesses. Burmester produces the B25 in mirror-imaged pairs: the woofers face each other from behind a series of slots cut in each speaker's inner side panel. This side placement is claimed to reduce interference between the woofer and the front-firing midrange and tweeter.
The B25 comes in three standard finishes: high-gloss Macassar, silver laminate, and Elsberry. The fit and finish of the cabinetry, hardware, and drivers are superb.
Two Burmester B25s in their cardboard shipping cartons were delivered to my house by Udo Besser himself. I was pleased that the cartons easily fit into the trunk of his rental sedan, and didn't require a truck with powered lift gate, as more massive floorstanders do. Besser easily lifted each 60-lb B25 and carried it up the six steps to my listening room.