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Oversampling, or, more correctly, the resampling of the original 16-bit x1 digital audio code, is conventionally performed in the frequency domain. It is claimed by Wadia that time-domain curve-matching provides superior accuracy with asymmetric music waveforms, resulting in improved transient definition, better intertransient silence, and a more solid low-frequency range. The "Frenchcurve" code is resampled at a 16x rate and processed at 36MHz on a 36-bit–wide number path, the computations running at 36MIPS (million instructions per second) per channel. CD replay does demand quite…
Sidebar 1: Specifications
Description: Standalone D/A converter unit.
Price: $4950 (1989); no longer available (2009). Approximate number of dealers: 40.
Manufacturer: Wadia Digital Corp., Hudson, WI 54016 (1989); 1556 Woodland Drive, Saline, WI 48176 (2009). Tel: (734) 786-9611. Fax: (734) 786-0163. Web: www.wadia.com.
Sidebar 2: Review System
For the bulk of these tests, the Wadia 1000 was installed as part of a system comprising the Apogee Duetta Signature speakers, a Krell KSP-7B preamp, and a bi-amped pair of Krell KSA80s. Custom silver cable was used throughout. References included my customized Cambridge Audio CD1, the Sony DAS-R1, and the Marantz CD12, their performances locked into my continuing primary reference, a well-aligned Accuphase DP80/DC81. For secondary listening, a Pink Triangle PIP II preamp and ARC Classic 30 power amplifier were employed, with Celestion SL700 and Spendor SP2-2…
Sidebar 3: Measurements
A possible cause of the mildly variable performance of the WD1000 with different systems was its high level of spurious output beyond 20kHz. For example, with a 100pF/1M-ohm loading, the output contained 20mV p–p of 20MHz content, not a happy input for some pre- or power amplifiers. Moreover, despite a premature response rolloff at the edge of the audio band, –5.11dB by 20kHz (fig.1), the level of spuriae just above 20kHz was considerable. A good illustration is given by the high-frequency intermodulation spectrum for 19/20kHz 1:1 full modulation (fig.2). On this…
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…
The VTL converter uses two 6201 dual triodes per channel, which is a military version of the 12AT7WA. The four triode elements per channel provide gain after the DAC, perform active de-emphasis, and form an ultra-low–impedance totem-pole cathode-follower output driver. One of the 6201s is also used in the fourth-order Chebyshev anti-imaging filter, of which two poles are passive, two active.
Gain is required after the DAC because of the custom nature of the UltraAnalog parts used in the VTL converter. A stock UltraAnalog DAC D20400 (as used in the Stax DAC-X1t) puts out 5V at full level,…
The treble was exquisitely smooth and free of edge. Cymbals had more of the delicate quality of brass being struck, with their harmonic structure intact, rather than sounding superimposed on a layer of whitish grundge. The treble grain that often obscures musical detail in CD playback was noticeably absent, allowing the listener to hear the wealth of nuance at the lowest level of presentation. Similarly, sibilance was not spitty and annoying, as it often tends to be from CD. Instruments' high-frequency harmonic characters seemed natural, utterly free from stridency or edge. The brittle…
Sidebar 1: Specifications
Description: Two-channel, tubed D/A converter. Tube complement: four 6201. Inputs: three S/PDIF on RCA or UHF jacks. Outputs: unbalanced on RCA and XLR jacks. Sample rates handled: 32kHz, 44.1kHz, or 48kHz. Resolution: 20 bits. Dynamic range: 112dB. Filtering: 8x oversampling.
Dimensions: 19" W by 2 5/8" H by 10 3/8" D.
Price: $7000 (1990); no longer available (2009). Approximate number of dealers: 25.
Manufacturer: Vacuum Tube Logic of America, 4774 Murrietta, Suites 9 & 10, Chino, CA 91710. Tel: (909) 627-5944. Fax: (909) 627-6988. Web: www.vtl.com…
Sidebar 2: Review System
The VTL D/A converter was auditioned in my usual reference system: Hales System Two Signature speakers driven by VTL 225W Deluxe monoblocks via 3' runs of bi-wired AudioQuest Clear cable. Level control and matching between processors under audition were through the Electronic Visionary Systems Stepped Attenuator, while interconnects were Expressive Technologies IC-1 (Stepped Attenuator to power amplifiers) and Music Metre (processors to Stepped Attenuator). The VTL was driven by the Esoteric P-2 CD transport (reviewed in this issue) and with a 48kHz signal from a…
Sidebar 3: Measurements
The VTL's output level when decoding a 0dB, 1kHz signal was 3.26V (left channel) and 3.22V (right channel). This is 4.2dB higher than the standard 2V output for a full-scale signal. Frequency response was flat (fig.1), down just 0.2dB at 20kHz. Note also that the right channel is about 0.1dB lower in level than the left. Fig.2 shows the VTL's de-emphasis error. This plot is actually the decoder's frequency response when playing an emphasized test signal, but since the VTL's frequency response was flat, this graph represents only errors in the VTL's de-emphasis…