Sunfire True Subwoofer
The True Subwoofer from Sunfire is the latest example of the unconventional audio equipment designs (and nomenclature) of Bob Carver.
As its name implies, the latest Sunfire product is designed to extend the deep-bass coverage of a music or home-theater system all the way down to the limits of human hearing, and even beyond - that is, when a recording contains such information, rare in music but not in today's movie soundtracks. Audiophiles familiar with Bob Carver's previous component designs, most of them developed during his many years at the Carver Corporation (which he left in 1994), will not be surprised to find that the True Subwoofer incorporates some highly original concepts and technology.
The basic design principles of the True Subwoofer are discussed in a white paper available from the manufacturer (and well worth reading). Although I cannot go into all of its details here - the paper is much longer than this entire report - I will attempt a brief explanation of the operating principles of this unique product.
A subwoofer is a loudspeaker designed to reproduce deep bass, the frequencies that fall below the range of most ordinary woofers. Ideally, it should operate as low as 20 Hz, usually considered to be the lower limit of human hearing, with an upper limit from 50 to 100 Hz or so. Reproducing these frequencies at realistic levels requires that the speaker move a lot of air, which is commonly achieved by using one or more large-cone drivers, or several smaller ones, in a big, heavy enclosure designed to complement the driver characteristics. Lower-priced "subwoofers" usually operate in vented enclosures and typically have a low-frequency limit of 30 to 40 Hz.
In sharp contrast to conventional subs, Sunfire's True Subwoofer is a sealed cube measuring only 11 inches on a side, although it weighs close to 50 pounds. The enclosure contains two unusual flat-diaphragm drivers with huge magnets, which account for most of the weight, and each of the drivers, mounted on opposite sides of the cubical box, has an effective cone diameter of 8 inches. They operate as a bipolar system, with both diaphragms moving in and out simultaneously. There are no grilles covering the drivers; the flat, rigid surface of each diaphragm and the rubber edge supporting it are fully exposed.
The subwoofer's rear panel is a metal plate that contains all the connection terminals and controls and serves as a heat sink for the resident power amp. Instructions in the owner's manual note that the heat-sink plate can reach 65°C (149°F), which is uncomfortable to the touch but not unsafe. (The plate never became more than moderately warm during our testing and listening sessions.)
The control panel of the Sunfire True Subwoofer has smoothly operating knobs for adjusting the low-pass crossover frequency (40 to 120 Hz), volume (from zero to room-shaking levels), and phase (from 0 to 180 degrees).
The subwoofer offers both speaker-level and line-level inputs for connection to a receiver or amplifier. Two pairs of insulated binding posts can be connected to the speaker-level output terminals of a system's main amplifier (this was the way we used them in our listening tests). Alternatively, two gold-plated phono jacks can be connected to the line-level outputs of a receiver or an amplifier. There is also a pair of gold-plated phono jacks, identified as Hi-Pass Out, that carry line-level frequencies above 70 Hz to the amplifier powering the main speakers.
A miniature toggle switch marked Flat/Video Contour modifies the system's low-frequency response, reducing the output below 30 Hz when it's set to Video Contour, though the instruction manual recommends using the Flat setting for most audio and video applications. In any case, when setting up the Sunfire subwoofer in a music system it is a good idea to experiment with the controls to achieve a satisfactory result, since there will inevitably be some interaction between the volume and crossover-frequency controls, and to a much lesser degree the phase control. Adjusting the phase control during our evaluation produced no audibly significant result.
Another minor idiosyncrasy of the True Subwoofer (and possibly of other powered subwoofers) was its sensitivity to hum from power-line ground loops. The owner's manual details a simple procedure (reversing the AC plug from the power amp or the subwoofer in its wall socket) to minimize this effect. During our tests, we occasionally heard a faint hum close to the drivers in the absence of an audio signal, but it was never audible while the subwoofer was playing music.
The Sunfire subwoofer turns on automatically when a signal is detected, and it switches itself off after about 3 minutes of silence. The red LED power indicator on the control panel lights up when the unit is active and dims in the standby (off) mode. In our setup, however, the subwoofer did not automatically shut down, apparently because of the aforementioned ground loop, which I was unable to eliminate totally. At any rate, this anomaly never had any audible or measurable effect on system performance.
So, how are the True Subwoofer's small drivers able to move enough air to generate low-bass frequencies - more air then the large cones (or multiple smaller cones) used in most subwoofers? Unlike those cones, which generally have a very limited maximum (peak-to-peak) excursion, the Sunfire subwoofer's drivers have a maximum excursion of 2-1/2 inches, far more than the drivers in any conventional speaker. The catch, however, is that it takes a lot of power to drive them that distance, particularly because the efficiency of a woofer is proportional to the volume of its enclosure. The Sunfire sub's small enclosure makes it extremely inefficient, so it requires a huge power input to achieve output levels comparable to those of larger conventional subs.
It turned out that some 2,700 watts (that's not a misprint!) were needed to produce realistic acoustic levels into the True Subwoofer's 4-ohm load. Bob Carver's approach to the seemingly impossible requirement of building that much power capacity into a tiny space was to use an innovative circuit similar to the one in his Sunfire power amplifier. The incredibly small amp, which occupies a circuit board about the size of a large chocolate bar, delivers enough power to drive the 0.4-cubic-foot True Subwoofer to the same levels as would be generated by several larger woofers driven by a 200-watt amplifier in a 10-cubic-foot box.
Suffice it to say that the Sunfire True Subwoofer lived up to its claimed performance in full measure. We measured the frequency response with two microphones, each positioned close to one of the speaker diaphragms. The combined response was a remarkable +/-0.5 dB from 85 Hz down to 20 Hz (and a very respectable +/-3 dB from 110 to 18 Hz, the sub's rated response range). At a typical (fairly high!) listening level, which produced a mild skin-tingling sensation, the distortion was only 3 to 5 percent over much of the range from 25 to 80 Hz.
In the white paper mentioned earlier, Bob Carver describes a comparison between the True Subwoofer and his previous favorite subwoofer (unnamed), a large, heavy, and expensive model whose enclosure is more than ten times the volume of the True Subwoofer. Carver says that the larger sub required considerably more amplifier power to reach its maximum output, while the Sunfire sub delivered twice as much acoustic output with substantially less input power (from the AC line). In other words, the Sunfire sub was considerably more efficient.
For our own listening tests, I used the Sunfire subwoofer in conjunction with a compact speaker system that was on hand for testing. The two-way speakers were placed atop a pair of large (and very good) three-way columnar speakers containing formidable built-in powered subwoofers, and the True Subwoofer was placed on the floor near the left column.
In A/B comparisons, I found surprisingly little difference between the large tower speakers with their own subs and the subwoofer/satellite system I assembled using the True Subwoofer with the compact two-ways. In fact, most of the time it was virtually impossible to tell which was playing without looking at the switch settings, since the two systems had generally similar colorations. But when it came to reproducing truly deep bass, below 30 Hz, the Sunfire subwoofer was the clear winner. It had the flattest and deepest bass response I have ever heard or measured, which is doubly impressive considering its size!
The point of this comparison was not to establish which speaker system was better - that would be like arguing about the relative flavors of apples and oranges. What it did, to my satisfaction, was confirm that Sunfire's True Subwoofer is a clear contender for top honors among powered subwoofers of any size or price, in addition to being a real value for the money. And its compact size is a bonus that's especially desirable in dimensionally challenged installations.
