B&W 703, HTM7, 705, ASW750 surround speaker system
To quote Longfellow, "All things come round to him who will but wait." Most speaker manufacturers use their "signature" designs as proving grounds for new technology—technology that inevitably trickles down to their less expensive models. So it is with B&W's new 700 series. I'm not going to tell you that the new model 703, the top of the 700 line, equals the performance of B&W's Signature 800—or even that it matches up with one of the slightly less ambitious designs from the company's "standard" Nautilus series. The only time I've heard five 800s in full voice was at a Sony demonstration of multichannel SACD. They sounded stunning (driven by a big Sony receiver, no less). But the 703—and the system built around it reviewed here—has nothing to apologize for. One big step down in price from B&W's Nautilus line, the new 700 models aren't anywhere near as big a step down in design sophistication or performance.
Thinking Outside the Box
The first thing that struck me about the full-range speakers in the review system—the 703, the HTM7 center channel, and the stand-mounted 705 (used here as surrounds but also designed for use as main-channel speakers in a surround or 2-channel system)—are the tweeter pods perched on top. This isn't a new idea. The CDM-NT series, which the 700 models replace, used a similar pod, and B&W first used a similar design way back in the 1970s, in the DM7. The purpose of the pods in all these models was to reduce diffraction from the cabinet.
In the 700s, as in the Nautilus and CDM-NT speakers before them, the pod holds a tweeter inspired by the design developed for the Nautilus series. According to B&W, a tube extending out from the back of the tweeter (hidden here by the design of the cabinet and pod) more efficiently absorbs the backwave from the driver than does the restricted chamber at the back of most tweeters. It also allows the tweeter to be set back from, and thus time-aligned with, the midrange driver.
After the tweeter design, the innovations are less obvious. But look closer at the 703's midrange speaker and you'll see that the cone has no visible surround. The surrounds of most conventional drivers create significant problems. They don't always flex in concert with the cone itself—in fact, they can actually move in the opposite direction at some frequencies, partially canceling the cone's output.
B&W has minimized this problem with the Fixed Suspension Transducer (FST) driver design used in the 703. They haven't completely eliminated the surround, which would create its own set of problems, not least of which is the lack of an air seal with the cabinet. But they've replaced it with a narrow ring of foamed polymer so tightly coupled to the cone that it always follows the cone's movements. This would not work with a driver designed to cover both the midrange and bass (the cone moves far less in the midrange than in the low frequencies), which is one reason you don't see it in the HTM7 center speaker or in the 705 (another reason is cost).
B&W has been using woven Kevlar cones in their speakers since 1976. They've continued to refine this synthetic aramid fiber with constant improvements in weave and geometry, and in the application techniques used to stiffen and damp this inherently flexible material. While no cone is perfect—many speaker manufacturers argue convincingly for other materials—B&W makes a good case (including photographic evidence) for the effectiveness of Kevlar and their efforts to optimize it for this application. Kevlar is used in one or more of the drivers in each of the full-range speakers in this package: in the bass-midrange drivers of the 2-way 705 and HTM7 center, and in the 703's FST midrange.
All of the bass-midrange drivers also use an open basket structure to minimize reflections from the back of the cone. The FST, in particular, uses the most open support structure I've ever seen in a speaker driver.
The cones in the 703's bass drivers are made from a mixture of paper and Kevlar fibers. The Kevlar stiffens the cones against the severe buckling forces produced by the violent movement demanded of a bass driver. The cone is further stiffened by extending the carbon-fiber voice-coil bobbin to the underside of the large dustcap to form a ring-shaped girder.
The 12-inch driver in the ASW750 subwoofer, driven by a rated 1000W of class-D amplifier power, uses the same paper-and-Kevlar cone technology. A ported-box design, it's rated down to 16Hz (–6dB), though the output level at this frequency, as with most subwoofers, is not specified. Controls include the usual level adjustment and lowpass-crossover frequency selector (which can be bypassed), a highpass filter (useful mainly in 2-channel systems that lack the filters found in most surround preamp-processors), an auto-sensing turn-on switch, and an equalization (EQ) control that selects one of two bass contours. Position A provides maximum bass extension, while position B yields a higher maximum output at the sacrifice of a few hertz of response at the bottom end. I did most of my listening in EQ position A; my comments on the subwoofer's performance reflect that setting.
The cabinets of the 703, 705, and HTM7 incorporate a number of features new to B&W. The tops slope forward, with a smoothly rounded curve into the front baffles that is said to minimize diffraction. The sides also taper inward slightly toward the rear. According to B&W, this does not eliminate resonances inside the box (a common audiophile misconception, as is the erroneous belief that angled walls eliminate standing waves in a room). Rather, as the folks from Worthing argue, it spreads these resonances across a wider range of frequencies, making them less potent and therefore less audible.