Balanced Audio Technology VK-5 preamplifier & VK-60/75 power amplifiers
Designers who take a middle-of-the-road approach to this issue use single-ended circuitry internally, but include balanced inputs and/or outputs. This is effective from the marketing point of view, and is not expensive to implement—but it's arguably a poor compromise in that it introduces additional components in the circuit path without achieving the benefits of a fully balanced design. In a preamp, for instance, a balanced input signal is first converted to single-ended, then back to balanced via a phase splitter at the output.
There are those who feel that balanced designs, properly executed, are inherently superior to single-ended designs. There's no prize for guessing that Balanced Audio Technology (hereafter referred to as BAT) falls into this group. In an interesting pair of white papers on the VK-5 preamp and the VK-60 power amp, BAT designer Victor Khomenko candidly admits that the theoretical benefits of balanced design (cancellation of common-mode noise, resistance to RF) don't really account for what he feels is the superiority of balanced topologies (footnote 1). Thus, he points out, there's little correlation between sound quality and Common Mode Rejection Ratio (CMRR): some excellent-sounding balanced designs have only average CMRRs. "Then, how do we know that balanced is superior? We listen."
Which was exactly what I planned to do.
Tube electronics from a new, small company—the stuff probably looks as if it's been assembled in someone's basement by people who've just learned to solder, right? Not in this case. Victor Khomenko and his partner Steve Bednarski have worked at Hewlett-Packard for a number of years, and they've used their high-tech industry experience to ensure that there's nothing amateurish or homemade-looking about their first products. Examine the insides of the VK-5 and the VK-60 and you'll see neatly-laid-out circuit boards (0.093" thick, with 3oz unplated pure copper traces), premium metal-film and foil resistors, custom oil-filled capacitors, gold-plated Neutrik XLR connectors, and high-quality parts throughout. Tubes are mounted in ceramic sockets with silver-plated contacts. The VK-60's toroidal transformers are made by Plitron, supplier to Krell, Mark Levinson, Spectral, and Bryston.
The VK-5 has a simple, uncluttered front panel, with large knobs for volume and source selection, a pair of smaller gain-trim knobs for each channel, and toggle switches that allow On/Off/Sleep selection (the Sleep mode reduces tube-filament voltage) and Play/Mute. All five inputs and the three outputs (two main and one tape) are balanced XLRs. (XLR-to-RCA adaptors are available from the factory.) The VK-60 has three sets of output taps, matching speakers with high (nominally 8 ohm), medium (4-6 ohm), and low (3-4 ohm) impedances. BAT suggests experimentation to find out which of these is optimal for a given speaker.
The Dunlavy SC-IV's impedance stays within the 4-6 ohm range, dipping to 3 ohms only in the very low bass. I tried the medium- as well as the low-impedance taps; there was very little difference. The low bass sounded perhaps a little better-controlled through the low-impedance tap, so that's the connection I used for subsequent listening. The VK-5 and the VK-60 have rubber feet; any spikes or cones with standard ¼" threads may be substituted.
The most striking aspect of the VK-60 is its array of output tubes: fat, three-nippled 6C33s (two per channel). This Russian power triode, considered one of the hottest tubes around (literally and figuratively), is famed for its use in the guidance system of the MiG-25 fighter aircraft (footnote 2). (I can't think of a more fitting swords-into-plowshares conversion.) BAT first used 6SN7s (also Russian-made) for the eight input tubes, later replacing them with military-spec 6H8Cs. There are three potted toroidal transformers, one for power and two for output. A handle runs the length of the chassis in the rear and permits use of the VK-60 for weightlifter training. The VK-60 can be configured as a 60W stereo or a 120W mono amplifier; converting a stereo amplifier to mono is a simple matter of placing a jumper on a circuit board and using jumpers to connect the left and right speaker terminals.
Footnote 1: The VK-5 white paper makes reference to the "limitations of single-ended structure with its half-signal processing" (italics in the original). This seems misleading. Single-ended circuits process all of the original audio signal—which is, in fact, single-ended. Balanced circuits also process a polarity-inverted version of the signal. While this may have advantages, it might be more accurately referred to as double-signal processing
Footnote 2: The 6C33 has no equivalents, and is made only in one factory in Russia, so availability is a potential concern. However, demand for this tube is increasing (especially in Japan), and there's no reason to believe that production will cease in the foreseeable future. In any case, BAT has a good stock of 6C33s for production and replacement use.