Digital Processor Reviews

I find it more than a little ironic that in 1990 the only two digital-to-analog converters to employ a new state-of-the-art DAC also use vacuum tubes. Many in the audio community consider tubes an anachronism, and find it surprising and humorous that they are still used in newly designed audio products. The fact remains, however, that these two tubed digital processors achieve the best digital playback currently available—and by a wide margin. Moreover, their respective designers' technical savvy and passion for building leading-edge products is reflected in their choice of these superlative and very expensive new DACs. Is it mere coincidence that both designers also chose vacuum tubes to realize their vision of no-compromise digital playback?

The two converters to which I refer are the extraordinary $12,000 Stax DAC-X1t Vacuum Tube Output Reference D/A Processor reviewed in Vol.13 No.8, and the new $7000 Vacuum Tube Logic Digital to Analogue (sic) Converter reviewed here. The two processors are distinguished not only by their use of vacuum tubes, but also by their inclusion of the UltraAnalog DAC modules, devices that redefine what can be expected (both musically and technically) from digital to analog converters (footnote 1).

Despite the outward similarities between the Stax DAC-X1t and the VTL DAC, they are substantively different in design. First, the UltraAnalog DACs in the VTL are custom-made to VTL's specifications, with more signal processing done outside the module with tubes. Second, the tubes in the VTL assume more functions (de-emphasis, analog low-pass filter, gain) than the Stax's cathode-follower output buffer. However, the VTL is much less tweaky and more modest in build quality. You may remember the Stax used three AC power cords; Ohno Continuous Crystal (OCC) copper throughout, from transformer windings to pcb traces; leadless resistors soldered to the pcb top; and massive power supplies; all in a gorgeous and luxurious chassis.

In my review of the Stax, I concluded that it was significantly superior in musical (and technical) performance than any existing digital converter. Indeed, many digital processor designers hold the DAC-X1t as the benchmark against which their efforts are measured. Does the VTL D/A, priced $5000 less than the Stax, represent a serious challenge to the Stax's reign as the best digital playback currently attainable?

Technical description
The VTL D/A converter is a slim, 19"-wide unit with rack-mount holes in the front panel. Although well made, it embodies a no-frills design philosophy that places sonic performance above user convenience and styling elegance. The front panel and chassis are machined from aluminum and screwed together, rather than the less expensive and commonly used bent sheet metal. The side panels and a border around the front panel are anodized red, while the top, rear, and bottom are black. The hand-machined metalwork is excellent, with a nice bevel to the front panels' edges.

The front panel holds three toggle switches and four red LEDs. A power on/off switch is located to the far right-hand side, with the power-on indicator appearing at the panel's center. At the far left, a row of three LEDs indicates the incoming digital signal's sampling rate: 32kHz, 44.1kHz, or 48kHz. Next to these LEDs, a polarity inversion switch marked "0°" (up) and "180°" (down) allows the user to invert absolute polarity (in the digital domain). The third toggle switch selects one of the converter's digital inputs.

Moving to the rear panel, three digital inputs are provided on gold-plated RCA jacks, and three parallel UHF jacks are located just above the RCAs. UHF jacks are found primarily on older Tektronix oscilloscopes: this is the first time I've seen them an a piece of audio equipment. VTL supplies a UHF to BNC adapter with the processor for those applications where the digital source appears on a BNC jack. (The Wadia transports, for example, use BNC outputs.) Note that no optical inputs are provided. In the rear panel's center, a pair of XLR connectors are mounted above a pair of RCA jacks. Balanced operation (via a transformer) is available in the professional version of the converter (in conjunction with other professional features like AES/EBU input), which adds $2000 to the consumer price of $7000. In converters equipped with the balanced option, a small toggle switch selects between the balanced or unbalanced output pairs. The consumer version does come with the XLR jacks wired (pin 3 hot), but in an unbalanced configuration. An IEC line-cord jack and fuse holder finish off the rear panel.

Removing the top cover revealed the VTL's construction and layout. The right-hand third of the unit is dedicated to the power supply, with a thick aluminum shield isolating it from neighboring circuitry. A pcb holding four 6201 tubes lies horizontally next to the power-supply shield. Vents in the chassis top and bottom panels provide tube ventilation. The remaining half of the chassis is filled by a large pcb that contains the digital circuitry, some power-supply regulation, and the very large UltraAnalog DAC modules. Unlike most IC DACs that are fairly small, the UltraAnalog units are 2" by 3", consuming a sizeable portion of the board's real estate. The ubiquitous Yamaha YM3623B decodes the incoming S/PDIF signal to 16-bit audio data—higher bit depths are not supoorted—and an NPC SM5813APT performs the 8x-oversampling digital filtering.

Looking at the VTL's topology in more detail, the power supply is comprised of four transformers and four full-wave bridge rectifiers driving a total of eight independent power supplies. Five supplies, each with their own three-pin voltage regulator, are located on the DAC board and supply the DACs and logic circuits. The other three supplies serve the tube circuits, which share common heater and high-voltage supplies. The high-voltage supply filtering includes eight 68µF, 450V electrolytic capacitors.

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Footnote 1: The UltraAnalog DACs (and ADCs) represent the state of the art in D/A conversion. The modules, a combination of monolithic and discrete devices, undergo rigorous calibration during manufacture. They are also very expensive, about 20 times the price of a Philips TDA1541, Burr-Brown PCM64, or Analog Devices AD1860. For a complete technical description of the UltraAnalog DACs, as well as a discussion of R-2R ladder conversion, see my review of the Stax DAC-X1t in Vol.13 No.8.