A common question we get from Stereophile readers is why we don’t review
products from
Digital. The five-year absence from these pages of
reviews of products by a major manufacturer invites all kinds of speculation).
The explanation is simple:
decided it was not in their best interests
to have reviews in the US press, and declined our requests for review samples.As we were contemplating buying products on the open market for
review, the company had a change of heart, not coincidentally connected
with a change of management.
They provided review samples of their $8450
27 Digital Decoding Computer and $12,500 Wadia 7 transport. 27 DIGITAL DECODING COMPUTERThe 27 Digital Decoding Computer represents ’s latest thinking in
digital processor design. Although the 27 costs less than the flagship
$13,500 9, the newer 27 has some circuit refinements not found in
the top-of-the- line processor.The 27 is housed in a beautifully made chassis with no bolts visible from
the top or sides. The chassis is built of separate panels of thick aluminum
joined at the corners by foot rounded structures; these accept short spikes
on the bottom for resonance control. If you don’t like the idea of spikes
poking into your rack, the 27 comes with metal “coasters” that fit between
the spike points and the rack shelf surface.The 27’s front panel has no knobs, switches, buttons, or controls. Instead,
an alphanumeric display shows the unit’s operating status, which is controlled
exclusively by the remote control. The blue display indicates the output
level (volume) in numbers between 0 (mute) and 100 (frill-scale). Input
selection, phase inversion, and left/right balance adjustment are all made
by the remote control and displayed on the 27’s front panel.
The supplied remote control also runs ’s transports. At the time
of this writing was replacing the plastic remote with a cast aluminum
remote more befitting the construction quality and appearance of their
processors and transports. It’s a good thing, too: the plastic remote is
an ergonomic nightmare, with lots of tiny, identically shaped, and poorly
laid-out buttons. If your 27 was purchased with the plastic remote,
will replace it with the aluminum one at no charge. I didn’t have the new
controller in time for this review, but saw a prototype at ’s factory.
It looks like a big improvement.
The 27’s rear panel provides six digital inputs: two ST-type glass-fiber
optical, two S/PDIF coaxial on BNC jacks, one AES/EBU, and one TosLink.
was a pioneer in applying glass-fiber transmission technology to
consumer digital audio, and recommends using the ST type optical connection
with their products. All the digital inputs are mounted on a separate panel
attached to the machined chassis for easy upgrade if a new transmission
technology comes along.Balanced analog output appears on XLR jacks, single-ended on RCAs. The
27’s output stage was designed to drive both outputs simultaneously in
systems (like mine) that use a separate woofer amplifier.Using the 27 without a preamplifier: The 27’s adjustable output level
lets you drive a power amplifier directly from the 27 without the need
for a preamplifier. is a big proponent of this technique, which removes
from your signal path a pair of interconnects, input and output jacks,
a volume control, wires, switches, circuit-board traces, resistors, and
the active gain stages of a preamplifier. No matter how good your preamplifier
is, it can’t be better than no preamplifier.Removing your preamplifier from the signal path has undeniable sonic ad
vantages, but some drawbacks as well. First, the digital-domain volume
control in the 27 reduces resolution at the rate of 1 bit for every 6dB
of attenuation. The greater the attenuation (the lower the number on the
27’s front-panel display), the greater the degradation.Second, removing the preamplifier poses the problem of source switching:
how do you play your tuner or turn table? ’s solution is to digitize
your other sources with their 17 A/D converter, whose digital output
drives another input on your 27 processor. Or you could reconfigure your
system each time you switched sources. Finally, some listeners may use
only a CD source and not need input switching.To minimize the amount of lost resolution resulting from digital-domain
attenuation, the 27 has an analog- domain output level adjustment. A pair
of tiny switches inside the chassis set the 27’s maximum output level from
0.41V to 9.02V, with 14 positions in between. If you have a power amplifier
with a high input sensitivity (the input voltage required to drive the
amplifier to clip ping) and high-sensitivity loudspeakers, you’d set the
27’s switches for a low out put level. This way, for the best sound, you’d
run the 27 with very little digital- domain attenuation. Remember, the
higher the number on the 27’s front- panel display during normal listening,
the better.My system worked best with the 27 set for a maximum output level of 2.63V.
The next lower setting, 2.03V, didn’t have quite enough output to pro duce
satisfying levels with CDs re corded at low levels, or those with a high
peak-to-average ratio. I calculated the resolution loss between these set
tings at about a third of a bit—probably not audible. Most of my listening
was with the display showing between 80 and 100. With some discs I ran
the 27’s volume control wide open, with no attenuation and thus no degradation.Some users may not take hill advantage of the 27’s variable analog output
level feature. doesn’t provide instructions on setting the switches;
you must rely on your dealer for this information. If you find yourself
listening most of the time with the display showing below 70, you’re needlessly
throwing away resolution. It’s well worth the trouble to open the chassis
and set the switches to get the best possible performance from the 27.
recommends having your dealer set the output level, but you need
to adjust it while it is in your system to find the right level. If your
dealer won’t come to your home to correctly set the analog gain switches,
get the information on the switch set tings and do it yourself. Taking
the time to play with this adjustment to get it just right is well worth
the effort.While we’re on the subject of digital-domain volume controls, I should
mention the exemplary technique used in the Enlightened Audio Designs DSP-9000.
That unit automatically switches-in analog-domain attenuation so that the
digital volume control never attenuates the signal by more than 6dB. For
example, as you reduce the volume from full-scale output the DSP-9000 attenuates
the signal in the digital domain until 5.9dB of attenuation is reached,
then switches-in 6dB of analog attenuation and puts the digital volume
control back to no attenuation. Consequently, if you had 10dB of attenuation,
6dB of it would be in the analog do main, 4dB in the digital.Inside the 27: During a visit to ’s factory I spent some time
with the 27’s principal designer, Jim Kinne. The unit uses some innovative
design techniques, the first involving the power supply. Two, large toroidal
transformers are housed in a sub-enclosure within the 27’s chassis. The
transformers are sandwiched .between layers of Neoprene damping material
on the sub-enclosure bottom and the machined aluminum plate that covers
the transformer sub-enclosure. This technique isolates the transformers
mechanically, acoustically, and electrically from the rest of the chassis.
The transformers’ outputs are rectified, filtered, and regulated on a
separate pcb. Unusually, the regulation stages for the I/V converter and
analog output stages are current regulators, not the typical voltage
regulators. These constant-current supplies are then regulates with voltage
regulators on the audio board next to the audio circuit.In a current regulator the output current is held constant. When followed
by a shunt voltage regulator any current not use by the supplied circuit
is dumped to ground. believes that this power-supply topology produces
superior sound quality, primarily because a current source is less affected
by inductance and capacitance of the power-supply traces.In all, the 27 uses a whopping 36 regulators, most of which form cascaded
regulation stages (the output of a pre-regulator feeds the input of the
final regulator) for a cleaner and more stable power supply. The eight
current pre regulators are in TO-220 packages with 2”-tall heatsinks. Many
of the cascaded voltage regulators next to the audio circuits are surface-mount
devices.Aiding these 36 regulators are loads of filter capacitors, some of them
distributed near the audio circuits. The 27 has more than 30,000uF of filter
capacitance — as much as some small power amplifiers. This huge power supply
makes the 27 run very hot.
The pre-regulated power-supply rails aren’t connected to the second regulators
on the audio board through conventional circuit-board traces. Instead,
the power supply is routed to the audio board by a separate dedicated board
underneath the audio board; the supply voltages are “popped up” to the
audio board where they’re needed. This separate power-sup ply board can
have huge traces and direct routing, in contrast with the smaller traces
and indirect routing of conventional power-supply distribution on a single
board shared with the audio circuits. The 27’s power-supply design and
execution are impressive, to say the least.The input section is based on a Crystal CS8412 receiver chip implemented
on the DSP board. Signals present at unselected inputs (if you have ST-type
optical and AES/EBU connected simultaneously, for example) are disconnected
from the circuit to prevent contamination. Although the C58412’s jitter
rejection is only mediocre by today’s standards, the clock that drives
the DACs isn’t generated directly by the 8412. Instead, the 8412 is used
to demodulate the incoming S/PDIF signal, and the 8412’s recovered clock
synchronously drives a Xilinx programmable gate array. The gate array generates
a clock at its output that is de-jittered by a Voltage Controlled Crystal
Oscillator (VCXO), which is de-jittered again by another VCXO located near
the DACs. In all, three stages of VCXO-based re-clocking reduce clock jitter.
This scheme is a more sophisticated implementation of ’s “Rock-Lok”
de-jittering circuit, first used in the 2000 Digital De coding Computer.
By the way, a single Xilinx gate array—a square chip about half the size
of the bar code on the front, of a magazine — replaces literally dozens
of separate ICs used in earlier products.All the clock lines are differential (two traces carry the clock and its
inverted version) to minimize the effect of noise on the crucial timing
signals. Moreover, the clock lines are not just traces on the PCB, but
are miniature transmission lines, with a precise output impedance from
the driver and termination resistors on the receiver to create a matched
input impedance. A carefully executed transmission line prevents reflections
in the conductor that would add noise to the clock signal, making the edges
less precisely timed and introducing jitter. was at the forefront of using a custom digital filtering algorithm,
a tradition continued in the 27. Two Motorola 56004 DSP chips, under the
control of ’s software contained in Read-Only Memory (ROM) chips,
form the digital filter. The filter runs at 16x.oversampling, meaning it
interpolates 15 new samples for every incoming sample read from the CD.
The DSPs execute ’s patented “Digimaster” filtering algorithm, an
update of the algorithm used in ’s original 2000 Decoding Computer.
The two Motorola DSPs replace the four AT&T DSPs used in the 2000 while
nearly doubling the amount of computing horsepower.The Digimaster “spline” filter algorithm is based on mathematics first
developed in the 18th century for shipbuilding. Shipbuilders needed to
fit curved horizontal planks of wood to the fixed points represented by
the vertical hull members. Similarly, digital-interpolation filters attempt
to “fit the curve” by creating new points along an arc (a musical waveform)
between existing points (the original sample values) (see Note 2 below).The Digimaster filter is optimized for time-domain performance.
Consequently, the 27’s filter will repro duce an impulse with no pre- or
post- echo, and its square-wave response has none of the ripple of conventional
digital filters that follow a sin(x)/x function. However, there is a penalty
in the frequency domain for this improved time- domain performance: a rolloff
of nearly 3dB at 20kHz. (See Note 3 below).Volume control and muting are also performed by the DSP chips. To attenuate
a digital signal, each sample is multiplied by a number less than 1. For
example, if you set the digital volume control for 6dB of attenuation (half
the voltage), the DSP multiplies each sample by 0.5. Similarly, pressing
the remote control’s mute button simply ramps down the volume to zero by
software control.The 16x-oversampled data are put through a Xilinx gate array that retimes
the over-sampled data for output to the DACs. Specifically, the gate array
creates four data-streams and delays three of them by slightly different
amounts. To create a balanced signal, the gate array also inverts the second
and fourth datastreams. Each datastream is converted to analog with its
own DAC: The result is an increase in the effective oversampling from
16x to 64x (16x in the digital filter, multiplied by four by the time-staggered
DAC technique). To look at it another way, 16x-oversampling is performed
by the DSPs under control by the Digimaster software, and 4x-oversampling
is performed by hardware (the DACs).The DACs are Burr-Brown PCM 1702s in a fully balanced configuration, with
the positive and negative phases of the balanced signal handled by two
DACs each. The 1702’s intrinsically good linearity and lack of an MSB trimmer
must have made life easier for the 27’s designers; setting the trimmers
on previous DACs, so they all perform identically, must have been a challenge.The DAC outputs of each phase are summed with their own current-to-volt
age (I/V) converters, the super-fast (400MHz) Burr-Brown OPA-642 op amp
chip. This is followed by another Bun-Brown device, the INA-103 instrumentation
amplifier. The variable analog output level described earlier is implemented
by varying this stage’s gain. The IMA- 103 processes each phase of the
balanced signal separately, and also acts as a differential amplifier to
convert the balanced signal to single-ended to drive the RCA jacks.The output buffer is a Burr-Brown BUF-634 op-amp, which can reportedly
swing more than 9V RMS and source a quarter of an ampere of current. Each
phase of the balanced signal, along with the single-ended signal, is buffered
by its own BUF-634. The output stage, with its high output level and specified
out put impedance of less than one ohm, is designed to drive a power amplifier
directly through any length of cable. Although calls this output
stage the “Sledgehammer,” it is completely unlike the Sledgehammer stage
used in earlier products.Note that the signal remains balanced from before the DACs to the XLR
jacks (parallel signal paths for each phase of the balanced signal). In
addition, the single-ended signal is created by combining the balanced
signal with a differential amplifier, then buffering it independently.
This technique provides single-ended users with the benefit of the 27’s
balanced topology. The alternative is to simply connect the positive phase
of the balanced output to the RCA jack.This inferior method effectively throws away half the DACs and output
stage when using the single-ended outputs.The four-layer analog output board that contains the DACs, I/V converter,
and analog output stages sits above the power-supply board. The four-layer
digital board with the Xilinx gate array and DSP chips sits behind the
audio board. The left and right channels on the audio board have been designed
as mirror images of each other so that their electromagnetic fields cancel.Most of the 27’s components are surface-mounted devices. The DSP chips,
Xilinx gate array, DACs, I/V converter, and output buffer are all surface-mount
devices. Surface Mount Technology (SMT) eliminates the long leads of “through-hole”
construction, instead mounting the electronic component on the printed
circuit board’s surface. touts the many advantages of SMT, including
the removal of parasitic resistance and inductance of component leads,
along with the ability to put components closer together on the circuit
board. SMT resistors and capacitors are so tiny you almost don’t see them
on a circuit board.Previous products have, in my view, put the emphasis on the digital
side of the design at the expense of the analog stages. For example, the
original 2000 broke new ground with its sophisticated digital filtering
and time- staggered DAC technique, but subjected the analog audio signal
to decidedly audiophile push-on connectors. (The 2000 was designed by telecommunications
engineers, not tweaky audiophiles.) Fortunately, ’s thinking seems
to have shifted in the last eight years: the 27’s analog section appears
to be as care fully realized as the digital sections.Some might be perturbed by the 27’s use of an op-amp output stage rather
than a discrete circuit. The conventional wisdom is that any contender
for crown of State of the Art in digital playback — to which the 27 appears
to aspire — must use a discrete output stage. Al though few would argue
that an op- amp can outperform the best discrete stages, it should be noted
that virtually all digital processors use an op-amp in the current-to-voltage
converter. Every UltraAnalog DAC module, for example, has an integral op-amp
current-to-voltage converter (except for the Spectral SDR-2000 Pro, which
performs current-to-voltage conversion with a discrete circuit outside
the DAC). Consequently, many superb-sounding processors —including the
Mark Levin son No.30.5—have an op-amp in the signal path (the No.30.5 has
a discrete output stage, however).But what ultimately matters is how the product sounds. Any preconceptions
engendered by the product’s de sign vanish in the listening room.I found the 27 gorgeous in cosmetics and build quality—it has a substantial
solidity and beautiful metalwork. My only complaint about its operation
is the narrow window over which the remote control works. I had to point
the remote almost straight-on to get it to accept commands — a real nuisance
when listening. Moreover, if you push and hold the volume control while
trying to find that window, the 27 doesn’t accept the command until you
release the button, then activate it again while it is pointed directly
at the front panel. This drawback won’t be a factor if you have a direct
line of sight between the listening position and the 27, but I found it
frustrating in my listening setup. 7 CD TRANSPORTAt $12,500, the 7 is the most expensive CD transport I’m aware of
its look and feel, and its beautiful construction and finish quality certainly
reflect its astronomical price.The 7 comes in two chassis: the transport itself, and a power supply.
The chassis construction is the same as that of other products, with
machined aluminum panels connected at the corners with rounded structures
into which spikes can be threaded. The drawer is a thin slit in the center
of the main unit’s front panel, just above the display.The front-panel display shows all the usual information (track number,
playing time, etc.), and has a total of two controls: Play and drawer Open/Close.
Track skip, stop, pause, search, and other functions are provided only
on the remote control. The remote itself is a massive metal unit that requires
two hands to operate. Rows of burtons of identical color, size, and shape
nearly cover the remote’s front, though many of these are for controlling
other products and don’t apply to the 7. I didn’t have this remote
control for the review; advised me to use the 27’s remote, which
also controls the 7. Nonetheless, the 7’s remote looked intimidating.The rear panel holds one each of the four main output categories: AES/EBU,
ST-type glass-fiber optical, TosLink, and coaxial on a BNC connector. Three
small screwdriver adjustments adjust the front- panel display’s brightness
and drawer open and close speeds, respectively.The 7’s high cost is due largely to the expensive Esoteric P-2S transport
mechanism. The P-2S is the top-of-the-line mechanism made by Esoteric (a
division of TEAC). (The P-2 transport I reviewed in Vol.13 No.12 used the
transport one model down from the P-2S.) This transport is probably the
best engineered and most solid yet devised. It is also the most costly—by
a wide margin. It was reported to me that the raw P-25 mechanism alone
costs $2500 —and that’s the wholesale OEM quantity price.The device uses the Vibration-Free Rigid Disc Clamping System (VRDS) first
seen in the P-2 transport sold in North America under the Esoteric name.
The VRDS clamping system, just larger than the CD itself, holds the entire
disc in a viselike grip. Compare this method with the usual technique of
holding the CD in the center by a small spindle. Al though all the Esoteric
VRDS mechanisms use this clamping technique, the various models are built
very differently. The P-2S used in the 7 features a machined aluminum and
bronze turn table and clamp, the clamp mounted on a massive structure spanning
the mechanism. Lower models in the Esoteric VRDS line are less massive,
use stamped rather than cast parts, and plastic elements where the P-2S
uses metal ones. sent me a raw mechanism so I could get a better
look at its construction. This is one serious piece of mechanical engineering—it
makes the ubiquitous Philips transport mechanisms look like toys. Wadia 7 CD transport and remote modifies the standard P-2S mechanism by replacing the oscillator,
changing the digital output board, and adapting the unit for their external
custom power supply. They also mount the VRDS assembly on an integral suspension
system to isolate it from vibration. has established a tighter jitter
tolerance for their mechanisms than the standard P-2S transports.Note that damper discs, rings, and other mechanical CD tweaks interfere
with the VRDS mechanism. The VRDS clamping system would seem to obviate
the need for such tweaks anyway.SYSTEMI listened to the 27 and 7 in a variety of configurations. Before
the 7 arrived, the 27 saw lots of action driven by the Mark Levinson No.31
transport. Conversely, the 7 fed the Mark Levinson No.30.5 processor and
a Classe DAC-1 so it could be compared with the Levinson No.31 transport.
Of course, the 27 and 7 were also used as a pair, connected through the
-recommended glass- fiber optical (ST-type) cable. Other digital links
included the Illuminati coaxial and AES/EBU cables — my references in digital
interconnects.Loudspeakers were Genesis 11.5 dynamic/ribbon hybrids. Most of my time
listening to the 27 was with the processor driving the Audio Research VT15O
tubed mono-block power amplifiers directly from the 27’s balanced outputs.
The Genesis woofer amplifier was fed from the 27’s single-ended outputs.
To judge whether the sound quality I was getting was due to the 27’s intrinsic
quality or to the removal of the Sonic Frontiers SFL-2 preamplifier, I
experimented with the SFL-2 in and out of the signal path. For the matched-level
comparisons between the 27 and Mark Levinson No.30.5 processor, both units
fed the SFL-2.
The long balanced interconnect between the 27 and VT15Os was Wire-World
Gold Eclipse, with Magnan Type V feeding the Genesis woofer amplifier’s
unbalanced inputs. Other interconnects called into play when using the
SFL-2 included shorter runs of Gold Eclipse or AudioQuest Diamond X3. The
loudspeaker cables were also Gold Eclipse.The products under review sat on a Billy Bags 5500 series rack or a Merrill
Stable Table. My Well Tempered Turn table was temporarily ousted from its
place atop the Stable Table because I’d eliminated the SFL-2 from the system
and because I needed the rack space to house all these digital products.
I removed the Genesis Digital Lens from the system so I could evaluate
the products on their own merits. Power to the system was conditioned
by the entire MIT Z-System, including the Z-Stabilizer II, Z-Iso-Duo, Z-Center,
and Z-Cord II power cords.
