Tube Preamp Reviews

Rudyard Kipling said that "never the twain shall meet." He was speaking of East and West, but in the world of audio, his adage has most often been applied to what has been the traditional chasm between the sounds of tubes and solid-state. Tube advocates thump the tub for the timbral and spatial glow, the absence of harsh, odd-order harmonic distortions, the harmonic completeness and holistic spatiality that only fire bottles can provide. Solid-state advocates point out the superiority of their preferred gear in terms of bass depth, power and control, low noise, and ultimate detail resolution. That chasm between the characteristic sounds of tube and transistor has narrowed appreciably in the latest generations of gear, as each type of circuit has become capable of embodying some of the other's trademark characteristics. But between the camps, friendly competition continues.

Balanced Audio Technology's VK-51SE tubed line preamplifier made a profound impression on me at the 2003 Consumer Electronics Show. Paired with BAT's VK-D5 CD player and VK-150 monoblocks driving Wilson Audio WATT/Puppy 7s, the '51 was the heart of a system of stunning drive and power, dazzling resolution, and earthshaking dynamic range. Could the VK-51SE be the line stage that would complete the Hegelian synthesis of the best of the tube and transistor sounds into a new Third Way—and in an all-tube package? This kind of fun I had to experience for myself. BAT was more than ready to oblige.

Old School, New School
The VK-51SE represents a significant upgrade from the well-respected VK-50SE; BAT's Steve Bednarski describes it as "evolutionary" in terms of actual circuit design but "revolutionary" in sonic effect. The '51 is an intriguing blend of traditional and cutting-edge all-tube design techniques. Like its predecessor, the '51 uses four Russian-sourced, twin-triode 6H30 Super-Tubes in each channel for gain. BAT's Victor Khomenko pioneered the use of this high-current, low-impedance, military-spec tube in audio applications, and now has years of experience in designing circuits to complement its unique strengths.

Despite the VK-51's all-tube configuration, an octet of 6H30s permits a universally friendly output impedance of a mere 200 ohms, as well as a complete lack of cathode followers or buffer circuitry. Also carried over from the '50 is the BAT Six Pak capacitor bank in the output stage. No beer or ale here—Khomenko's Six Pak consists of half a dozen brass-jacketed, paper-in-oil capacitors per channel. That's about as old-school as you can get, and man, do they look cool. The '51 is a single-stage component—one gain stage, no global feedback—and is a wholly differential, completely balanced design from input to output. The "Balanced" in Balanced Audio Technology means just that: tight-fitting, high-quality XLR jacks are the only connections provided for the five inputs and two outputs.

What really sets the '51 apart from the '50, even in the latter's SE configuration, is Khomenko's thorough rethinking of the power supply. According to him, the power supply, with its 325 joules of energy storage, would be sufficient for a moderately sized power amplifier. The benefits of a preamplifier with such mammoth reserves are obvious: a dramatic increase in available headroom, particularly when that power is modulated by the 6H30, a tube capable of hair-raising dynamic performance even in more conventional applications.

The cherry atop the sundae of power-supply redesign is the addition of another Six Pak as bypass caps. Quoth Mr. Bednarski: "the quality of filtering and noise immunity is improved by using the oil-caps in the power supply." (footnote 1) The power supply also features dual-mono transformers.

More evidence of the new school of tube component design is in the '51's controls and interface. A mere 10 buttons and one knob grace the BAT's front panel, and the necessary information is provided via a large LED display that's easy to read even from across the room. The 10 buttons control power On/Off/Standby, selection among five sources, absolute phase, Mute, Mono, and a Function selection for the display. Similarly to the Mark Levinson No.32 Reference and the VTL TL-7.5 Reference line stages, the BAT lets the user vary the relative volumes of each source and set (or not) a maximum volume setting, along with absolute phase, mono/stereo, fixed gain (for audio/video applications), the units in which the volume control reads out, and to name each of the five inputs. A neat touch is that the LEDs can be set to display messages in the Latin, Greek, or Cyrillic alphabets. All of the information programmed is individually remembered for each input. The display can be set to three brightness levels or turned off entirely. All of these options are accessible from the remote control (footnote 2).

All of this good stuff is wrapped up in a brawny, broad-shouldered package that made me think Stealth fighter or Darth Vader every time I looked at its stylish, all-black, vaguely menacing countenance. There's a hint of macho in the '51's looks. Nor is the BAT's visual beefiness just for show—this is a heavy and very solidly made piece of equipment.

Iron Fist, Velvet Glove
If the VK-51SE has one sonic characteristic that stands out above the rest, it is a dynamic envelope that can flatten walls, most especially in the bass range. The teeming maelstrom of two bass guitars and two drum kits on King Crimson's "Thela Hun Ginjeet" and "Frame by Frame," from Vrooom (CD, Discipline Global Mobile DGM0105), had not just tremendous power, but wondrous dynamic subtlety as well. Everything from the proverbial whisper to a scream fell effortlessly within the '51's compass.

Distinguished dynamic performance in the bass is fine, but the big BAT maintained that standard of excellence throughout the spectrum. Listening to Al Stewart's "The Dark and the Rolling Sea," from Modern Times (UK LP, CBS 80477), I was particularly struck by Gerry Conway's variations of touch on the drum kit, especially the gently tapped cymbals, and the cyclical reedy wheeze of the accordion that intensifies the song's sea-shanty feel. The sorrowful irony in Stewart's voice as he sings "So don't call on me / when your ship goes down / on the dark and the rolling sea" was chilling through the BAT. The pizzicato strings in the first movement of Colin Davis and the Boston Symphony's performance of Sibelius' Symphony 2 (Italian LP, Philips 6998 026) were caught with uncommon subtlety and individuality in the varying force of the plucks.

Transient response was the polar opposite of the "traditional" sound of tubes. Bass had a stick-your-finger-in-the-socket immediacy. Charley Charles' drums on Ian Dury's "Reasons to be Cheerful, Part 3" (UK 12" 45rpm, Stiff 12-BUY-50-A), and the kick drum and bass guitar samples on Alex Gopher's "Ralph and Kathy," from youmybabyandi (CD, V2 27059-2), had definition on the leading edges that equaled or surpassed the best solid-state components I have heard.

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Footnote 1: VK-50 owners aren't left behind. Any VK-50 or VK-50SE can be fully upgraded to VK-51SE status. Contact BAT for details.

Footnote 2: Oddly, pressing the Function key on the remote didn't switch the '51 out of adjustment mode, but cycled it repeatedly through the available options. I had to press the Function key on the front panel to return to normal status.