Sidebar 3: Measurements
In the main, the Apogee PSX-100's D/A section was tested using the digital AES/EBU digital output of an Audio Precision System One Dual Domain. I also made use of a two-card PrismSound DScope v2.02 and a Miller Audio Research Jitter Analyzer, which runs on a National Instruments DSP PC card.
Looking first at the Apogee's D/A section, with its output set to –10dBV (which is how MF used the converter), the front-panel multiturn pots can still be adjusted to give a maximum output level of 7.3V. With the pots set to give a standard CD output level of 2V RMS,…
A reader who asked to remain anonymous wrote to tell us the results of some tests he saw conducted on one of our top-rated loudspeaker systems. Frequency-response checks showed that the system had virtually no deep bass, a midbass peak, a midrange slump, and a high-end rise. Further checks had shown gross distortion at input levels of over about 6W, and a definitely limited (although adequate for Row-M listening) maximum output-level capability. Said reader then went on to ask how we could possibly consider such a speaker to be one of the best available.
Well, we aren't going to argue…
Sidebar: Letters You'll Never Believe
From a well-known amplifier manufacturer
Editor: Thank you for your letter expressing your dissatisfaction with our SPA-140 amplifier. The "hardness" that you hear in the sound is not due to your associated equipment, but stems from a drastic increase in the amplifier's distortion at low power levels. This, we hasten to add, is really quite a common thing in solid-state amplifiers that were designed in a hurry in order to meet the New York hi-fi show deadline.
There is nothing you can do about the problem, except to buy our next-year'…
Rather a mouthful, the name of this digital decoder is derived from that of the designer, Robert Wadia Moses. The "computer" part of the title relates to the custom digital filter function generated by a set of 32-bit microprocessors: for simplicity's sake, I shall abbreviate the name to "WD1000." A more expensive version, called the '2000, sells for $6995, and carries some additional features and details. The resampling rate is increased to 64x in the '2000, with the additional optical and digital input switching and the main power supplies each contained in separate additional enclosures.…
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,…
