Snell Acoustics XA Reference Tower loudspeaker
Peter Snell might not recognize the new XA Reference Tower's distinctively thin columns as his Type A at all. Long gone is Snell's "upright brick of polished wood and stretched cloth" (The Audio Critic, Vol.1 No.4, 1977, pp.38-39). In its place is David Smith's thin, 73"-tall, 210-lb floorstander, with which he's attempted to meet three design goals: uniformly flat amplitude response between the speaker and the room boundaries, powerful reproduction of deep bass, and uniform vertical dispersion.
I first heard the original Snell Type A 22 years ago, at the Listening Room in Scarsdale, New York, when I was introduced to its tall, thin, bearded designer, Peter Snell. In the next two decades Snell Acoustics changed hands four times, from Snell to William Osgood to Peter Lyngdorf to, finally, Boston Acoustics.
Along the way, I reviewed many Type As. First came the Type A/III system (July 1984, Vol.7 No.6), followed in 1989 by Kevin Voecks' Type A/III Improved (March 1990, Vol.13 No.3). Next was the 605-lb, $18,999 Type A Reference, designed by Voecks (March 1996, Vol.19 No.3) and shipped in 12 cartons. Setup involved positioning and connecting four loudspeaker cabinets, two external passive crossovers, one active electronic crossover, two stereo amplifiers, six interconnect cables, and 32 speaker cables. But the Type A Reference rewarded all that setup with a majestic sound, excellent soundstaging, deep and powerful bass extending to well below 20Hz (its subwoofer met early THX deep-bass specifications for a 9000-cubic-foot room), and an ultra-transparent midrange. It served as my reference for a large loudspeaker system for many years.
Creating a wider sweet spot
With the David Smith-designed XA Reference Tower, the Snell Type A returns to its original four-way, single-cabinet design with a complement of seven forward-facing drivers: a 1" titanium tweeter, two 4.5" midranges, and four 8" aluminum woofers. Its main innovation is Smith's eXpanding Array of drivers, to deliver very broad and even vertical dispersion, an array first developed for the Snell XA90ps (July 1999, Vol.22 No.7) to produce a wider sweet spot.
The term eXpanding Array refers to the increasing distance between drivers the lower their assigned frequency range. The XA Reference's drive-unit lineup begins with what Smith calls the "MTM array," for the central midrange-tweeter-midrange elements. Computer modeling generated the exact crossover topology and the necessary spacing between the to optimize the speaker's vertical polar pattern. The XA Reference's owner's manual states that its "constant vertical directivity means that the system has a very wide sweet spot with virtually no change in frequency response or perceived balance."
Smith said that for speaker design he uses "a very complete system and crossover simulator. It takes driver magnitude and phase curves, driver impedances, crossover topologies, crossover values, and physical system layout to give an accurate system response simulation on- and off-axis." In the case of the XA90ps, the computer modeling smoothed the vertical dispersion to within 1dB variation through ±15 degrees. Outside that zone, the array's vertical polar pattern drops off by 4-5dB at 40 degrees, which reduces the energy that bounces off the floor or ceiling. Instead of the XA90ps's two 2.5" upper-midrange units bracketed by two 6.5" midbass drivers, the larger XA Reference has four 8" woofers—set at 4.3x and 6.8x the distance between the MTM units—to extend the Reference's smooth vertical dispersion pattern down to 60Hz.
"I tried a number of alternative schemes," said Smith, "but none of these alternative systems looked promising, so a more conventional symmetric array was settled on...The simulations showed that constant directivity came from having a constant element spacing relative to the crossover frequency. That is, if the highest crossover frequency is six times the next crossover frequency, the mid-to-tweeter spacing should be one sixth the woofer-to-tweeter spacing. Both of these spacings should be one third (at most, one half) the crossover wavelength. In practice, this means the mids had to be placed very close physically to the tweeter, and the top crossover point should be as low as practical.
"The one breakthrough was finding a high-output, low-resonance tweeter from Audax that has a compact-diameter magnet structure. By rear-mounting this driver on a computer-cut aluminum MTM plate, the Vifa midrange units' driver baskets can overlap the tweeter's metal frame to keep the center-to-center spacings minimized."
The XA Reference's crossover points are 300Hz and 2.4kHz. Smith noted that the crossover filter characteristics "are mostly electrical second-order except for the tweeter high-pass, which is quasi-third, more for the phase shift than the rolloff slope. Like the XA90ps, they are all Linkwitz-Riley and in phase in nature. But my latest simulations show the vertical directivity is more even if the high-pass sections (roll-in) are higher slope or "squarer" than the low-pass sections (rollouts). This means that the tweeter is squarer, higher-order, at 2400Hz while the mids roll off with a more gradual slope."
The XA Reference is a vented system with a low crossover point for the woofers, and Smith found that the upper impedance bump of the vented woofers made crossing over difficult. A conjugate network was added to the crossover that fixed the impedance to flat. Some very-large-value parts, such as a 70mH inductor, were used.