Wilson-Benesch Torus Infrasonic Generator (Subwoofer)

Equipment report
Wilson-Benesch Torus Infrasonic Generator
Wilson-Benesch Torus Infrasonic Generator (Subwoofer)

Before I heard the Torus—the revolutionary “Infrasonic Generator” from Wilson-Benesch, the British hi-fi firm that put carbonfiber enclosures, drivers, tonearms, and turntables on the audio map—my rule of thumb about adding a subwoofer to a fast, bass-shy speaker was the same as my rule of thumb about switching from an analog to a digital system: If you want to live happily with a subwoofer, then don’t ever again listen to your main speaker without one. (And if you want to go all-digital, then don’t ever listen to a record player again.)

Granted, good subs add “floor” and air to the soundstage, lifelike weight and color to the bass (and midrange) response of lower-pitched instruments, and floor-shaking extension and dynamics to grand pianos, bass drums, organs, acoustic and electric basses, synths, big bands, and full orchestras. The trouble is subs don’t do these things for free. There is always a sonic tradeoff involved that, to a greater or lesser extent, degrades the sound of the main speaker.

If subs stopped playing exactly at their crossover point—producing great bass below, say, 40Hz and no sound above that—everything would be dandy. But, of course, subs don’t do that. All you have to do is turn off your main speakers while listening to a recording of a baritone vocalist to hear the sub playing on, ever so faintly, up into the midrange. Moreover, it isn’t just the sub’s driver that keeps sounding where you don’t want it to sound. The big, generally-rectangular box the driver is housed in keeps speaking from all its planes and angles, too, as do the floor and walls around that box. The result is that you now hear two substantially different sound sources—each with its own timbral signature, phase and transient characteristics, radiating pattern, distortion, resonance, coloration, room interaction, and (generally) amplification—layered on top of each other, like a coarse wool sock pulled over a sheer nylon one. It is no wonder that this combination often sounds like two different things or like one satyr-like thing that degrades the coherence, transient speed, timbre, resolution, transparency, imaging, and soundstaging of the quick, coherent, high-resolution, low-noise, bass-shy loudspeakers you started with.

After better than a decade of experimenting with subs of every size and type, I’ve concluded that if you want deep bass from a hi-fi system then buy a full-range speaker designed to reproduce deep bass. Don’t buy a mini-monitor, add a sub, and hope for the best.

This, in fact, was the advice I gave potential purchasers of the MAGICO Minis—the 2006 TAS Product of the Year Award-winning two-ways that I reviewed in Issue 163. Great—in fact, virtually unbeatable—on chamber music, folk, acoustic rock, small combo jazz, and much symphonic music, the Minis are not the speakers for powermusic lovers. Though they play with unusual clarity and nuance in the low end, like any two-way they won’t do the 20–40Hz octave with lifelike authority and extension. While your first thought might be “subwoofer,” asking the Minis to play alongside a lumbering sub is, as I once said about another speaker, like asking a sprinter to pass the baton to a weightlifter; it will just screw up most of what the MAGICOs do so well.

Or so I thought.

How I came to try the Minis with a pair of Wilson-Benesch Torus Infrasonic Generators (W-B doesn’t use the word “subwoofer,” for good reason as it turns out) is a story you can read on-line, at our AVguide.com Web site. (See the threads “War Declared over Magico Mini” and “MAGICO Minis and the Wilson- Benesch Torus” in the Speaker forum.) In a nutshell, I was goaded into it by a couple of readers who’d bought Minis and were curious to know if the MAGICO’s virtues could somehow be extended into the bottom octave. As noted, I didn’t think so, but the Toruses (then unknown and unreviewed in the U.S.) were novel enough to prick my interest.

The trouble with subwoofers is that they are being asked to do two incompatible things: to start and stop with the same speed, low distortion, and high resolution as the main speakers, and to play with chestcrushing power, volume, and extension down into the region where sound isn’t so much heard as felt. To do the first thing you need an extremely nimble, relatively short-throw, small-diameter driver; to do the latter, you need a stiffly-suspended, long-throw, large-diameter driver. Alas, a large-diameter driver cannot start and stop with the speed of a smaller-diameter woofer; and a smaller-diameter driver can’t move air like a big woofer. In both cases, you still have to house your drivers in an enclosure that is, itself, likely to interfere with stopping and starting on schedule because of its own resonances.

There have been any number of schemes intended to solve this conundrum—push-push woofers, push-pull woofers, servo-woofers, bipole/ dipole woofers, woofers loaded via transmission lines, woofers loaded via ports, woofers in sealed boxes, woofers without boxes. Wilson-Benesch’s is, I believe, unique.

The problem was to develop a driver that had a large surface area (to move lots of air at very low frequencies) but that also had very low mass (to move that air with high speed and low distortion). Wilson-Benesch’s brilliant chief engineer, Craig Milnes, reasoned that a large carbon-fiber diaphragm would have the necessary stiffness and low mass (the Torus’ 18" carbon-fiber diaphragm tips the scale at less than an ounce and can withstand mass loads 100,000 times its own weight), but how could Milnes make it function, make it move air, without attaching a stiff surround, a spider, a basket, a large magnet, and an enclosure—in other words, without adding substantial and highly resonant mass?

There was an obvious solution, of course, but it involved a completely different kind of drive system. Electrostatic and planar speakers, which comprise extremely lightweight electro-conductive diaphragms that are essentially pushed and pulled between fixed poles in a electromagnetic field, can generate low frequencies without the attachment of stiff surrounds and massive spiders, baskets, magnets, and enclosures. But an electrostatic or planarmagnetic bass panel has to be very large and, of course, it will function as a dipole, with subsequent room-cancellation/ augmentation effects that make it a generally poor choice for low-frequency reproduction.

Strokes of genius are rare in any field, but I think Milnes’ Torus qualifies. Putting two of one and two of another together, as it were, Milnes reasoned: “Why not make a push-pull cone driver?” By attaching a voice coil to the top of an upward-firing, lightweight 18" toroidal diaphragm (essentially a ridged circle of carbon-fiber with a hole in its center) and another to its bottom, and connecting these voice coils to fixed magnets that aren’t otherwise attached to the driver but, rather, to a precisely machined thirty-four-pound steel pole (called “the core” by W-B) that runs through the hole in the center of the toroidal diaphragm and is itself grounded not to the enclosure but to a massive steel plate at the foot of the enclosure, a lightweight cone could be made to function like an electrostat or planar-magnetic diaphragm. Voilà, a cone Magneplanar!

All sorts of side benefits accrue from this push-pull design. First, because of the equal opposing force of the two massive rare-earth magnets between which it is suspended, the lightweight carbon-fiber diaphragm will return instantaneously and automatically to the zero point between the twin poles as soon as the signal it is being fed ends—in other words, it will stop on a dime. Second, because a stiff surround is no longer necessary to act as a “spring” to restore the driver to the zero point (the highly flexible surround of the Torus functions solely to seal off the acoustic-suspension carbon-fiber cylinder in which the diaphragm and core are housed), the one-ounce diaphragm does not have to overcome the resistance of a suspension (or its own mass) to begin moving—in other words, it will start in a flash. Third, because of its extremely low mass, the Torus’ diaphragm can be made large enough to move lots of air without any sacrifice of transient speed—velocity and control and massive air-moving power achieved through a single engineering masterstroke. Fourth, because the core and the diaphragm are more or less “freestanding” (not physically connected to the cylindrical enclosure around them but to the steel base plate and, through tiptoes on the plate’s bottom, to the floor beneath it), all structural-borne energy is essentially grounded to earth, rather than to the enclosure (and hence to the room). Fifth, because the Torus is little more than an extremely lightweight, freestanding piece of carbon-fiber, the enclosure itself does not have to support a lot of mass and can thus itself be extremely lightweight—a small, sleek, beautiful black cylinder, like a small bass drum made of carbon-fiber, with no parallel sides and very low material resonance.

To drive the Torus Infrasonic Generator, Milnes made another series of thoughtful choices. Most modern-day subwoofers have built-in crossovers and amplifiers, a setup which is compact and convenient but which also adds considerably more vibrating mass to the enclosure. Milnes chose to make an outboard crossover/amplifier—removing another source of structure-borne resonance. Like REL and Vandersteen, he equipped his amp with high-level inputs that are fed, via a supplied Neutrik connector, from the positive and negative posts of the main amp. The W-B amp—a 200W discrete solid-state unit—then boosts the current it samples from the outputs of the main amp, giving the entire system the sonic “flavor” of the main amplifier. (There are low-level inputs, as well, for systems sourced from hometheater controllers with their own crossovers—but see below for qualifications on home-theater use.)

Though its crossover slope is fixed at 24dB/octave, the Torus’ amp/crossover control unit does allows you to choose a low-pass crossover point from 30Hz up to 150Hz (in 3Hz increments), to set the Infrasonic Generator’s output level via a rotary control knob at virtually any level within its 360º radius, and to adjust the phase anywhere from 0º to 180º. It also has a set of amplifier binding posts on its back panel, which are connected via speaker cable (not supplied) to one of two sets of binding posts on the Infrasonic Generator. A short pair of jumpers (supplied) is then hooked from the first (connected) set of binding posts to the second (unconnected) set of binding posts (from the hot terminal to hot terminal and ground to ground), so that both of the push-pull magnetic engines of the Infrasonic Generator’s core are being fed the same signal. On the bottom of the control unit are a series of DIP switches that can be adjusted to provide boosts at select frequencies. (For more on optimizing these parameters see the sidebar on setting up and optimizing the Torus.)

Since I used the Torus—a stereo pair, actually—with the MAGICO Mini loudspeakers, there is really no way to talk about how well the Infrasonic Generator works without talking about the combination. In many ways, the Minis are a worst-case scenario for a subwoofer. Where they play (from the mid-40s up) they are so fast and clean and of a piece (and so free of box colorations) that I was convinced no “subwoofer” could keep up with them. That conviction was only reinforced by experiments I made with other subwoofers (prior to the arrival of the Toruses), all of which seemed to be playing in a different time zone than the Minis. Oh, the subs boosted the bass, of course, but at the same time they dragged the Minis down like a boat anchor caught in the weeds, slowing transients, obscuring details, and thickening textures to a point where the MAGICOs were no longer recognizably the world-class two-ways that won our 2006 Product of the Year award but merely good loudspeakers with nothing extraordinary to commend them.

The Toruses, however, were an entirely different breed of infrasonic generator. The moment I put them in I was impressed—not so much by what I heard from the subs but by what I heard from the Minis. The thing was they still sounded like Minis. Oh, beyond the improvement in the low bass, there were, I suppose, very slight losses in speed, transparency, and detail through the Mini’s lower midband (and actually in the mid and upper bass). But you had to strain to hear them and with further refinements in setup even these small losses were reduced to a more-thanacceptable minimum—to a point where I preferred the Mini/Torus to the Mini Solus, to a point where I thought I had hit on a “super-system” that virtually any wellheeled music lover would want to hear.

As I did when I reviewed the Minis, let me show you one reason I’m so high on this combo. Though it is not typically in character for me to trot out frequencyresponse charts like a string of show ponies, this is a case (as with the Minis) where the test results speak volumes, simply because they jibe so closely with the way the Mini/Torus system sounds. Printed below you will see the quasianechoic frequency response (made with Bill Waslo’s Praxis suite of measurement tools) of the optimized MAGICO Mini/ Wilson-Benesch Torus super-system.

Folks, from about 26Hz up this is astonishingly flat response for a two-way mated to a sub (or for any speaker system), with far fewer of the peaks and dips than you might expect to see (and certainly to hear) in the crossover region—and no major peaks at all. This combo truly speaks with one voice—and at one speed.

I’m not going to kid you—it took considerable time and effort to achieve this blend and balance, much experimentation with placement, crossover point (I’m still vibrating between 33Hz and 36Hz, though this chart was made at 36), gain (in my 17' x 16' x 11' room a setting on the rotary dial of about 9:15–9:30 worked best), phase (in my setup, 90º produced the best Mini/Torus blend), speaker cables, and room treatment. But the sonic results were well worth the effort. All told, with acoustic instruments and analog sources this is the most lifelike stereo system I’ve yet heard.

However, it has its limitations. Somewhat ironically, the Mini/Torus sounds like a “super-Mini,” which is to say that its strengths are the Mini’s strengths: unusually neutral and natural timbres, extraordinary transparency to sources (hardware and soft), sensationally lifelike transient speed, superior imaging and soundstaging, a disappearing act second only to the MBL 101 Es, the most (and most realistic) inner detail I’ve heard from a dynamic speaker, and the most consistently realistic reproduction of voices and acoustic instruments I’ve heard from any speaker. What the Mini/ Torus is not—and here the MBL 101 E comes in again—is thrillingly visceral, sonically spectacular, limitlessly dynamic, and seemingly unfettered in the subterranean range, circa 25Hz and below.

Oh, the Mini/Torus will reproduce the big bass drum strikes at the start of George Crumb’s A Haunted Landscape [New World Records] with the kind of through-the-floor impact that’ll make you jump (and it will sustain those big bass waves and keep them rolling towards you from the back of the stage for what seems like forever). It will reproduce the dark, clanking, bottom-octave sforzandos of the Steinway on Decca Head’s great recording of Roberto Gerhardt’s Astrological Series with something very close to the power, size, and color of an actual Steinway played sforzando. It will reproduce the explosive, deep-reaching Bösendorfer and gunshot percussion on Nova’s superb LP of Reiner Bredemeyer’s Schlagstück 5—an amusing post-modernist piece that, like so much avant-garde music of the 70s and 80s, sounds a little like acoustic instruments imitating the pitches and durations of electronic ones—with such presence and immediacy that you would swear you were listening to a vintage Sheffield direct-to-disc recording. And, speaking of Sheffield, it will capture the 16- and 32-foot pipes of the E.&G. G Hook organ on Sheffield’s famous LP (not direct-to-disc, BTW) of the Mendelssohn First Sonata with perfect pitch and room-shuddering authority, and it will do this without any lumps in the sonic gravy, from the organ’s deepest notes right up to its fluting highest (as you would expect from a speaker system that is this flat and clean and high in fidelity). Keep in mind that the Mini/Torus system does all this in the 26-100Hz range while retaining in full the Mini’s magical ability to make voices and higher-pitched instruments sound “there in the room with you.” Indeed, it hasn’t been since the days of the Magnepan 1-U that I’ve heard a system that can more consistently give you the kind of goosebumps you get when you hear something or someone on record sound “real.”

Now for what the Mini/Torus system won’t do. Because the Torus is an acoustic-suspension speaker back-loaded by a relatively small volume of air, its resonance point is higher than what you would get in, say, a massively boxed feedforward subwoofer like the Krell MRS. Were its enclosure (and the volume of air it is working into and against) larger, the Torus would go somewhat deeper. As it now stands, Wilson-Benesch (realistically) rates it as down 6dB at around 18–20Hz, and that is the way it sounds and measures in my room (where it is down about 6–8dB referenced to 1kHz at 20Hz, not counting room lift). In other words, the Infrasonic Generator is more of a truly great woofer (or a great cross between a woofer and a subwoofer) than a subterranean noise generator. This works out just swell with a two-way like the Mini, which, frankly, could use a great woofer. But it does not work out so well with certain kinds of music.

For instance, the lava-flow synths on Paula Cole’s “Tiger Lily” go through-the-floor deep with the Toruses, but not quite as powerfully deep as they do with the MBL 101 Es’ bandpass subwoofers (but only, let me quickly note, when the 101 Es are driven by the $76k MBL 9011 amps at very loud volumes). Ditto for the synths and percussion on Trent Reznor’s “The Perfect Drug” from the Lost Highway soundtrack CD. And neither the MBLs nor the Mini/Torus approach the sledgehammer impact and room-commanding authority of the mighty Krell MRS on any bottomoctave noise or note. All of which is a roundabout way of saying that if really low synthesizer is your thing, you can probably do better in a subwoofer. And if you’re planning to use the Torus in a home theater—or a dual-function system—you can probably do better in a subwoofer (and for much less dough).

There is this, as well. While the Torus is relatively unlimited in acoustical power output, the two-way Mini isn’t. At very high volumes (steady +100dB SPLs), the Mini rather runs out of dynamic steam ahead of the Torus, ironically holding it back dynamically. The Torus still wants to go (although it won’t go far beyond +100dB SPLs!); the Mini does not.

However, neither the Mini nor the Torus were designed for head-bangers or for home theaters. Both were designed to reproduce acoustic music with the highest fidelity, and separately and in combination both do just that. I have never heard a subwoofed system that is more of a piece or (with the right sources) more consistently “foolyou” realistic—or a subwoofer that does less sonic damage to the sound of a superlative main speaker. For classical, folk, jazz, and much rock, this is, indeed, a “super system,” well worth the $45k that two Toruses ($19k with amps/crossovers) and two Minis ($26k with MAGICO stands) currently command. You can do better in a sub below 25Hz, and you can do better in a speaker system in overall dynamic range and sheer visceral thrills (or what’s an MBL 101 E for?), but, in my opinion and in the here and now, you will have a hard time getting closer to the absolute sound, particularly in a small-tomedium- sized room, than you get with this MAGICO/Wilson-Benesch combo. If ever I’ve heard a five-star speaker, the Torus/Mini super-system is it.

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