Aerial Model 20T loudspeaker Page 2
Michael Kelly is a driver man. During his time at a/d/s/ developing drive-units, he established invaluable connections with most of the world's high-quality suppliers, and he's used them in the design and construction of the drivers used in the entire Aerial line. All of the 20T's drivers are custom-made. The 9" woofers, optimized for the vented enclosure, have cast magnesium baskets made in Germany, German-made damped fiber bilaminate cones from Mueller, and long, copper-aluminum voice-coils wound on titanium bobbins. The low-Q, linear-excursion drivers are claimed to have a long "in-gap" coil movement. Aerial claims that the two 9" drivers, run in parallel, move air more effectively than a single 12" one.
Like the 10T's, the smaller 20T's bass response is claimed to be flat to 28Hz, and to roll off smoothly from there. Kelly set the Q at a "slightly overdamped" 0.65. More damped and it would be too dry for his tastes, less damped and he was afraid the bass wouldn't blend well with the fast, crisp-sounding ribbon tweeter. Still, he told me, the system's bass response runs "on the edge of warm"—a deliberate design choice, as was Kelly's decision to limit low-frequency extension. Should the buyer wish to extend response to the deepest frequencies, an optimally placed Aerial SW12 subwoofer could be added to create a quasi-four-way system.
The woofers cross over to a new 7.1" midrange driver that features a German-made carbon-pulp cone made of "randomized" 1" fibers that are stiff and lightweight. According to Kelly, this driver, which is assembled at the Scan-Speak factory in Denmark, was developed over two and a half years and required 28 prototypes to get right. It's "the star of the show," he told me, because it was so difficult to get it to integrate seamlessly with the ribbon tweeter. It's clearly the driver of which he's most proud.
Every speaker design is a compromise, and in the 20T the biggest compromise is the midrange driver, which needed to be larger than would otherwise be considered optimal in order to keep the sensitivity high enough to ease integration with the tweeter. The integral phase plug is there to help control beaming at the midrange's highish upper-frequency limit of 3.5kHz. The midrange features an unusual one-piece, CNC-machined yoke system of high-quality steel produced for Aerial by Danish Sound, the parent company of Vifa and Scan-Speak. Kelly told me that while this assembly is expensive to make, it results in a rigid system that offers an extremely strong and uniform magnetic field. As in the woofers, the copper-clad aluminum voice-coil is wound on a Japanese-made titanium bobbin. In short, said Kelly, the midrange is a totally tweaked-out drive-unit capable of rapid acceleration and tight control, its performance based not on a single "silver bullet" but on the sum of all of its design elements.
The tweeter is a new, genuine ribbon design, not a "vibrating circuit board." That is, it's not one of those etched pieces of Mylar often incorrectly called a "ribbon," but a true ribbon—a single, light aluminum conductor with no backing material of any kind. Made by Raven, in California, the ribbon is redamped and built to Aerial's specs, which include a custom-tooled waveguide made for Aerial by a California company specializing in small-airplane parts, and a magnet structure consisting of three "domino-sized" blocks of neodymium supplied by a third California firm.
"We listened to everything, every ribbon I could get my hands on," Kelly told me, "before choosing this one to be the basis of our design." The tweeter, assembled by Raven from the supplied parts, weighs 5 lbs and costs "eight times" what a "very good" dome tweeter would, according to Kelly. He claims it features quick response time and smooth on-axis response flat to 30kHz, and down only 7dB at 40kHz. He also says the tweeter is down only a few dB at 20kHz when listened to 45 degrees off-axis. And it can handle plenty of power.
"You can't burn it out," says Kelly. "So far, we've shipped more than 100 pairs, and there have been no tweeter failures." He did caution that a vacuum cleaner could easily suck the ribbon out of the gap. "It's replaceable in the field," he assured me.
Kelly's design partner on the 20T was Dave Marshall, who, Kelly said, came up with most of the critical crossover network, which uses French-made polypropylene-film capacitors, large air-core (tweeter) and high-nickel-steel (woofer) coils, and multiple parallel resistors in a 24dB/octave design said to offer "seamless" phase and amplitude integration. All internal wiring is insulated with Teflon and soldered with silver.
Heavy lifting, light tweaking
Michael Kelly and Dave Marshall arrived one Saturday late last fall to install the 20Ts—definitely a two-man job. The Aerials ended up very close to where the WATT/Puppy 7s and every other speaker I've reviewed have ended up in my room: a few feet from the front wall, somewhat farther from the side walls, about 10' apart, and angled toward the listening position.
The bass unit has two sets of WBT five-way binding posts, normally linked with flat, gold-plated jumpers. Internal wiring links the upper binding posts to a third set mounted atop the bass unit and hidden when the head unit is put in place. Aerial supplies a short length of high-quality wire to link the upper posts to a fourth set on the head unit. I opted to hook up directly to the head unit's binding posts, for the best connection to the midrange and tweeter, with the bass unit's signal traveling down through the link.
A three-position woofer control on the bass unit offers two levels of bass boost, centered between 40 and 50Hz, while one on the head unit can boost the tweeter output, depending on listening distance and taste. My preferences for both controls changed over the review period, mostly depending on associated equipment; to begin with, I left both at their flat, "11 o'clock" settings.
My review of Theta Digital's Enterprise power amplifier was in progress when the Aerials arrived, so I used them for my first listen to the Theta. With more than 400Wpc available from the Theta into 4 ohms, and the Aerials rated at 90dB efficiency at 4 ohms (and dipping down to 2.8 ohms), there was clearly enough power to drive the 20Ts, but that first listening session resulted in disappointing, somewhat anemic sound, as reported in the Enterprise review in March. Despite the promised low-frequency extension, the Aerials sounded cut off at the knees on Sundazed's superb 180gm mono reissue of Bob Dylan's John Wesley Harding, which was the last album I played through the WATT/Puppy 7s before I made the switch. And the WATT/Puppys themselves had been sounding somewhat lackluster in the bass compared to their performance driven by the Musical Fidelity kW monoblocks, which I eventually used to drive the Aerials.
I've reviewed several Aerial speakers over the years, and if they shared any negative characteristic, it would be a low-end "thickness" and too much bass, not too little. What's more, the WATT/Puppy 7 doesn't actually go that low, substituting a little 50-60Hz bump to give the pleasing illusion of deep bass. Yet at first, the Wilson sounded fuller in the bass than the 20T, which can go genuinely low.
I switched to the 100Wpc Music Reference RM-200 power amp, which had just been fitted with new KT88 output tubes. From a cold start, the system jelled nicely, the Aerials sounding sweet, open, extended, incredibly airy, and especially detailed on the Dylan LP and a few others we tried. But it would take more power and more current to coax the 20T completely out of its shell; after Kelly and Marshall headed back to Massachusetts that evening, I brought in the forklift and reinstalled the huge, powerful Musical Fidelity kWs.
Driven by a moderately powerful tube amp or a transistorized monster, the 20T's ribbon tweeter let know it was producing the high frequencies—and that's not a criticism. A ribbon sounds different from a moving-coil dome tweeter; without a doubt, it resolves much more information. There wasn't a familiar LP or CD in my collection that didn't surprise me with heretofore hidden details that were now suddenly, obviously there through the 20T.
These were genuine, musically significant details, not peak-induced accentuations, because the 20T's tweeter was also one of the smoothest, sweetest, airiest high-frequency reproducers I've ever heard. I'm sure ribbon aficionados reading this are saying, "So what else is new?"
When I stood up, however, I didn't hear much of the ribbon at all. Severely restricted vertical dispersion is the downside of such a design, but when I sat within the prescribed vertical window, I could luxuriate in the sweetness of the sound. Not every ribbon yields such sweetness. At this year's CES I heard a ribbon design that was so infested with resonances and nonlinearities that a familiar Pentangle track (bell-toned guitars, wispy female vocal) was almost unrecognizable. Aerial's ribbon was sweet, controlled, and subjectively linear; I'm sure that JA's measurements will confirm the smoothness and extension of its response, both on and off its horizontal axis.