Main Panels: During its years of development, the Dvorak underwent several iterations before maturing to its current five-piece configuration of two main panels, two subwoofers, and an active crossover/equalizer. While the overall panel is 56" high, the speaker proper consists of a folded MDF baffle measuring 26.5" H by 12" W by 4" D. This is supported about 20" above the base by a structural spine and two side legs running from top to bottom. These legs serve double duty as the folded portion of the baffle. On the rear of the main panel, three narrow horizontal boards complete the folded portion of the baffle. The rear spine, folded sides, and horizontal structural supports are all made from rigid Baltic Birch. A full-length black Crimpoline cloth covers the entire structure, and an Avonite top-piece puts the finishing touch on an appearance that's reminiscent either of a small Acoustat or a large Room Tune.
The folded sides of the baffle serve to minimize the frontal width of the panel, reducing the edge diffraction while achieving the desired separation between the positive-polarity, forward-firing wave and the negative-polarity, rear-firing wave. Both Marshall and Siegfried emphasized that the precise shape and construction of the Dvorak's baffle were critical for maintaining uniform directivity and consistent dispersion characteristics across the frequency region where the transition between dipole and quasi-dipole/monopole radiation occurs.
Though free from typical cabinet resonances, the baffle does vibrate a little. However, as for the most part these vibrations also radiate in dipole fashion, Siegfried feels that effective cancellation of the front and back waves minimizes any audibility. Though the main panel is fairly rigid, I did notice a very subtle increase in focus by loading the rear of each base with the optional, $150/pair "Top Hat" -- a black rectangular box filled with a 25-lb bag of lead shot.
Two Vifa 8" cone drivers are clamped in place on the baffle by the rear spine (no screws are used). These form a symmetrical array by being mounted above and below a 1" aluminum-dome tweeter, also from Vifa, which stands 41.5" from the floor. This tweeter is found in a number of well-respected high-end speakers, and was chosen for the Dvorak because of its excellent room response and power-handling capability around the rather low crossover point of 2kHz. Use of a dome tweeter means that the moderately directional radiation pattern of the dipole midrange-units and woofers is maintained through the treble -- sans the rear wave -- but with a uniform forward dispersion characteristic in lieu of the narrow, "hot-spot" beaming common with traditional dipoles. As a result, the speaker should avoid the placement sensitivity and off-axis colorations typical of some panel designs.
The dipole drivers cover a range of 40Hz to 2kHz when operated full-range without the optional subwoofers. With the subwoofers, the active crossover rolls-off the 8" drivers at a more comfortable 100Hz. In addition, a soft absorbent material placed in the shallow cavity between these drivers and the grillecloth covering the back of the speaker partially absorbs and attenuates the upper-midrange portion of the rear wave. This helps achieve a smooth transition to the tweeter's forward-firing radiation pattern. (Linkwitz found dipole operation in the treble not beneficial, as it degrades overall tonal quality, produces high-frequency "splatter," and makes precise speaker placement more tricky.)
Driver selection is a critical feature in any speaker's performance, but particularly so for a dynamic dipole design. Without the assistance of an enclosure's loading properties, dipole radiation demands large excursion capabilities from a driver over portions of its frequency band as well as good power handling, inherently low distortion, and smooth cone break-up characteristics near the edge of its performance envelope. Excessive excursion can produce higher levels of inductive modulation distortion, degrading sonic clarity. By employing two 8" dipole drivers per channel instead of just one, more-than-adequate volume levels are possible in a normal-sized room without inordinate excursion. (Interestingly, even though they excelled in some performance areas, some of the more exotic high-ticket drivers evaluated by Audio Artistry didn't pass the grade for dipole use due to unacceptable high distortion when pressed near their operating limits.)
At the bottom of each main panel an 11" by 6" MDF board is braced vertically between the outside leg, further stiffening the structure against torsional vibration. This board also carries the passive crossover components, including Solen polypropylene capacitors, air-core inductors, and Caddock power resistors. The crossover is a 4th-order Linkwitz-Riley, with electrical compensation provided to minimize the time delay resulting from the physical offset of the drivers. Although the crossover point is a low 2kHz, the steep 24dB/octave filter slope makes life much easier on the tweeter. It also minimizes the consequences of any cone break-up artifacts produced when the 8" drivers are driven hard. A rigid aluminum plate on the lower portion of the center spine directly adjacent to the crossover contains two sets of gold-plated, multi-way binding posts to facilitate bi-wiring.
Dipole Subwoofers: The Dvorak's main panels sound fine when used alone in a small room, offering good dynamics and the essential open perspective of the full system. However, I strongly encourage adding the stereo subwoofers even if your room is not large.
A black grillecloth encircles the subwoofer enclosure, but the side-panels are made of solid MDF. The cabinet is coupled to the floor with adjustable spikes threaded into a 2"-thick plinth. A black Avonite top matches that of the main panel. Each subwoofer contains two long-throw 12" woofers, each with a free-air resonance of 18Hz. By using equalization to flatten the natural dipole roll-off, you can get LF extension with adequate power through the full audible range.
The top woofer faces forward, the bottom woofer rearward. Both radiate sound in a dipole pattern. In order to reduce the overall width of the subwoofer cabinet, the drivers are mounted on angled baffles, one atop the other and crisscrossing at 45o, and separated by a horizontal board that runs through the center of the enclosure. The dimensions of the enclosure, as well as of the space between the driver cones and the cabinet openings, are calculated to enhance the drive-unit sensitivity. The potential cavity resonance formed by the internal space is much higher than the 100Hz maximum frequency the subwoofer is intended to reproduce, so it should not create a problem.
A pair of gold-plated binding posts attached to the lower rear of the cabinet connects the amplifier to the big woofers. This doesn't waste amplifier power in a passive crossover. In addition, the amplifier directly damps the woofers, providing superb control. A 50Wpc amplifier is more than sufficient to use with the Dvorak's subwoofers, as it only takes about 25W to drive the big woofers to their peak excursion.