CD Player/Transport Reviews

In 1995, as the compact disc enters its second decade of commercial reality, it's fair to say that the associated hardware has come of age, exorcising at last the digital gremlins of time-base jitter and quantization noise. Digital-processor maturation is particularly evident in the design of the all-critical D/A processor. The simplistic digital circuitry of yesterday has given way to considerable design sophistication that deals directly with jitter and low-level nonlinearities.

It should be realized that no DAC is perfect. The conversion process from the digital to the analog domain introduces a conversion error that manifests itself as noise and distortion products. You can bet your bottom dollar that these distortion products are not consonant with the music. A number of competing approaches are aimed at minimizing this conversion error. Broadly speaking, these can be categorized as based on either single-bit or multibit DAC technology. The 1-bit Delta-Sigma DAC is currently quite popular, the Crystal CS4328 being a widely used example. To judge from two recent examples I'm familiar with (the California Audio Labs Alpha and the Bel Canto Design Aida), the CS4328 chip succeeds in achieving excellent sonics in a cost-effective package.

MMB
Rather than go the low-bit route, Accuphase has taken aim at improving the weak point of multibit-type DACs—namely, inferior linearity at low-level signals. Their assault on the state of the art can best be described as brute force in nature. A Multiple MultiBit (or MMB, for short) type of DAC is used in the DC-91 processor and the DP-65 CD player (see TJN's review in Vol.17 No.10). This type of DAC uses a number of multibit DACs in a parallel drive configuration (fig.1).

Theoretically, the noise floor, hence the conversion error in an MMB system, decreases by 3dB every time the number of paralleled identical DACs doubles. If 16 DACs are used per channel, then the error would decrease by a factor of 12dB. There is, of course, a point of diminishing returns, as the degree of improvement increases slowly with the increasing number of DACs used.

Believe it or not, that's exactly what Accuphase did! Imagine sixteen 20-bit, hand-selected, Burr-Brown, multibit DACs (each a PCM63P-K) in parallel for each channel—that's a total of 32 DACs! The payoff is a substantially lower noise floor and amazing linearity down to the theoretical capability of an ideal near–20-bit DAC. The CD-91's signal/noise ratio of 120dB is truly state of the art.

A transport of delight
The DP-90 transport is designed to complement the DC-91 processor both cosmetically and in performance. Hiding inside the 1/3"-thick aluminum chassis are two separate power supplies, each of which has its own beefy transformer. One supply is dedicated to the digital signal processing, microprocessor, and display circuitry. The other serves the spindle drive motor, sled, and the focusing and tracking assemblies. The disc-drive motor features sapphire bearings and a heavy-duty spindle, which should offer many years of stable drive.

No stone is left unturned in Accuphase's relentless pursuit of perfection. The laser pickup is provided with an ultracompact RF amplifier that's integral to the pickup assembly. Ordinarily, the RF amplifier is external to the pickup, which renders its feeble output signal vulnerable to externally induced noise. Another design aspect that's been well attended to is the mechanical coupling between the disc tray and the rotating assembly. Remember all those disc dampers that were touted as cures for digititis? I bet everyone with an early-'80s CD player has purchased one of them in the hopes of finally arriving at Sony's promised land of "perfect sound forever." Well, partner, you can check your CD damper at the door: the DP-90's tray is firmly locked in place during playback, to eliminate the possibility of tray resonances.

The DP-90 offers every conceivable type of digital output, but the star attraction is an ST-type optical that uses a Hewlett-Packard transmitter operating at a data transfer rate of 150 million bits per second. A quartz fiberoptic cable is supplied for this connection. I experimented with every available connection, and can tell you that the ST-type rates a clear first sonically. In fact, the only way to experience the full potential of the DP-90/DC-91 system is through this connection. The balanced XLR output was a close second, giving up a touch in the areas of detail resolution and timing precision. The coax output finished in third place; not surprisingly, the bandwidth-limited TosLink brought up the rear.

Digital control
In reality, the DC-91 is more than just a digital processor. Think of it as a digital control center. The unit has 13 digital inputs. In addition to the ST-type and XLR inputs, three TosLink and three coax inputs are provided. Separate terminals are provided for connecting up to three digital-recording devices, such as DAT, DCC, and MD. All digital inputs are designed to handle 24-bit audio data, to allow for future expansion. Two 24-bit digital outputs are also provided to maintain compatibility with the next generation of digital components. A smart Input Selector allows easy switching between the various inputs while retaining in memory all settings (eg, level and phase) associated with each input.

Absolute polarity selection and level control are accomplished in the digital domain. The signal level is adjusted by a DSP chip with 24-bit output precision. The control range is from 0dB to –40dB in 1dB steps. I found the digital level control function not only quite sonically transparent, but also quite convenient. It allows volume adjustment from the listening seat using the remote control, and makes it possible to drive the processor's analog output (either balanced or unbalanced) directly into a power amp. The DC-91 offers the user the choice of using dither (triangular probability density) at the 20-bit level. This will optimize sound quality for those lucky enough to own true 20-bit sources.

The C.E.C. experience
The DP-90 transport arrived at my house first; the DC-91 processor appeared several months later. My only previous long-term experience with a "super" transport had been the C.E.C. TL 1—and that was a shocker, to be sure. It wasn't so much that it sounded so different from my reference at that time (the original Theta Data); rather, the unique manner in which it affected the sound startled me. It sounded as though I had plugged a tubed preamplifier in to the signal chain. The tonal balance became laid-back; the highs were softer and a bit rolled-off, lending a sweeter disposition to the upper mids. Even the bass quality was impacted, the lower registers sounding fuller and less precise than before. How tube sound managed to filter through the C.E.C.'s digital bitstream remains a mystery to me. The effect, of course, was musical, but euphonic. The C.E.C.'s tonal colorations were permanently painted over the original tonal colors.

Accuphase sound
The Accuphase transport, on the other hand, didn't meddle with the overall tonal balance. Its top-to-bottom voicing was neutral, allowing the recording's inherent balance to shine through. Recordings with lush midranges were given full expressive scope; threadbare recordings were revealed for what they are, without any editorializing.

The DP-90's resolution of low-level detail was exemplary (I'll have more to say about this later), but its most endearing attributes by far were its timing precision and resolution of rhythmic nuances. Because of its split-second duration, the attack portion of musical transients is most susceptible to corruption by digital circuitry. The coherence of analog sound derives from the fact that the attack and decay portions of each transient are inexorably bound together in time. However, in the digital domain, the music's time base must be reconstructed. And, as we all know by now, jitter can play havoc with timing precision, so a reconstituted musical transient can sound as if it were pasted together. Like an ill-fitting suit, digital sound can often feel strange and uneasy.