| Introduction The Pass D1 D/A Converter is a high
performance D/A Converter for use with digital sources. It uses a new innovative design
and some very expensive parts to deliver the highest quality performance obtainable from
today’s state of the art.
The Pass D1 D/A Converter contains several functional elements:
The input system offers AT&T, AES/EBU, SPDIF, and Toslink inputs. These are
switched by low capacitance relays through a Scientific Conversions digital transformer to
a Crystal CS8412 digital receiver.
Please note that initial shipments of the D1 show 96 KHz operation on the front panel
frequency indicator, however this feature is not as yet enabled as the receiver chips are
not yet available. This upgrade will be made available as a "drop in" as soon as
the chips become available. Contact your dealer, distributor, or Pass Labs for
availability and pricing. (If the 96 KHz indicator lights up on your D1, then the newer
receiver chip is installed.)
The Crystal’s clock is used to feed a special phase lock loop system that removes
the jitter and clocks the digital filter and D/A Converter circuits. Jitter is reduced to
a typical 35 picoseconds for high swing signal and as low as 20 picoseconds at digital
"0". This is achieved through the new Fujitsu PLL with the Lithium Tantalate
crystals designed for DVD and 96 KHz data rates, and incorporating our proprietary loop
filter.
The digital receiver and the phase lock loop both drive an NPC digital filter with 24
bit input and output. The digital filter provides 8 time oversampling of the digital
signal with 32 bit internal precision.
The digital filter sends digital signal through impedance matched buffers to four Burr
Brown PCM-63 (K) D/A Converters in a fully balanced configuration. The D/A Converters are
selected for matched characteristic to minimize distortion and noise.
The output of the D/A Converters is fed to the most dramatically innovative
current/voltage (IV) D/A Converter in the industry. In conventional D/A Converter systems,
the I/V D/A Converter consists of either an operational amplifier operated as a phase
inverting summing junction or the same variety of circuit rendered with discrete
transistors. The conventional approach uses negative feedback to achieve low distortion
and filtering. Unfortunately negative feedback around a high gain circuit creates
stability problems when exposed to the very high frequency transients generated by digital
circuits.
While the energy spectrum of the transient noise is above the audible spectrum and not
heard directly, the feedback loop must process both it and the desired signal, creating
audible artifacts.
The I/V conversion in the D1 is accomplished with single Mosfet device operated in
common-gate single-ended Class A mode. It presents the D/A circuit with the desired
virtual ground, and turns the D/A output current into voltage across an output resistor.
It operates entirely without feedback, and does not invert the phase. This circuit is
totally impervious to high frequency transients, so that no artifacts are reflected down
to the audio region.
The D1 offers fully balanced outputs lines with each of the four lines having dedicated
digital and analog circuitry. The balanced and unbalanced output connections of the
preamplifier have an output impedance of 150 ohms, and will drive a 1000 pF cable load
flat to 1 Megahertz. The output level is adjustable through the use of a 24 position
Swiss-made switch and precision metal film resistors, for precise tracking and complete
neutrality.
The power supply of the Pass D1 D/A Converter uses two shielded oversize toroidal
transformers, one each for the digital circuitry and analog circuits. Both sets of power
supplies are at least triply regulated, with separate individual regulators for each
circuit section.
Given the quality of the design approach and the parts, you expect good measured
performance:
The distortion of the digital and analog stages is extremely low, with less than .02%
THD and noise at full scale and from 20-20KHz. 1 bit below this figure, the distortion
drops to .005%.
The noise floor is extremely low, approximately 135 dB below maximum output. The jitter
floor of the clock is well below 10 picoseconds, with signal related components peaking at
30 picoseconds.
These are great specs, but are only a secondary source of pride with this product. All
design decisions were listening tested extensively against the most highly regarded D/A
products available, and we did not stop until the circuit sounded better than anything
else on the market.
You will experience this for yourself.
The PD1 is the work of Wayne Colburn, with some analog assistance by Nelson Pass. Feel
free to contact either of us with questions or comments.
Thank you for purchasing this piece. It is our sincere hope that you will enjoy its
sound as much as we do.
Setup
The D/A Converter has four input connections, and two sets of output connections and an
AC power connection.
The D1’s AC voltage and current rating are indicated on the rear. It will be
either 240 volts, 120 volts, or 100 volts. A .5 amp 3AG slow blow fuse is provided with
100 and 120 volt units, and a .25 amp slow blow fuse is provided with 240 volt units. The
frequency rating of the power supply is 50 to 60 Hz. The D/A Converter typically draws 25
watts during operation.
The power supply of the D1 regulates down to about 100 volts on a 120 volt setting, 200
volts on the 240 volt setting, and 90 volts on the 100 volt setting.
We have provided a standard AC power cord that fits into the line receptacle at the
rear. The D/A Converter is equipped for operation with an earth ground provided by the AC
outlet. Do not defeat this ground. The chassis and circuit ground of the D/A Converter is
connected to earth through a power thermistor, which gives a ground connection but helps
avoid ground loops.
The D1 is designed to be operated continuously, and is not provided with an on/off
switch. Our experience is that it takes an hour of initial warm up time for full
performance, and we suggest two hours for most critical listening.
The four connections on the rear for input sources conform to industry standards for
impedance of SPDIF and AES/EBU and the optical connectors are the industry standard
AT&T and Toslink. The two sets of output connections on the rear are balanced XLR
males with Pin1 ground, Pin 2 plus, and Pin 3 minus polarity. Also provided are RCA
connectors for single-line operation which are the same as Pin 2 of the XLR outputs.
CAUTION The AT&T connectors are regarded as the highest quality obtainable
in a digital connection, however, the plastic components on the connection surfaces are
quite delicate and were obviously designed for professionals to handle. Use care when
connecting and disconnecting to the AT&T connectors to avoid damaging this part. It is
a very expensive part, and we will charge for its replacement.
The output signal of the balanced connector is twice the level of the unbalanced
circuitry, by the virtue of having the same amplitude signal plus its inverse. Because the
D1 is a truly balanced design we suggest using it with the balanced outputs where
possible.
Full scale digital will deliver 3.6 volts rms balanced output with the volume control
at maximum. The output impedance of the balanced output connection is 150 + 150 ohms. The
output impedance of the single ended output connection is 150 ohms.
The front panel has switches for input selection and polarity reversal. The input
switch cycles through the four inputs as labeled top to bottom. The polarity switch
inverts the signal polarity digitally and the LED indicator lights up for inverted
polarity.
The front panel knob on the right is used to set the output level to match your preamp.
This control functions as a differential shunt between the two polarities of the analog
signal on the output of the current to voltage D/A Converter. This control can be used as
a volume control when directly driving a power amp. You may feel free to set the level
control anywhere you like, as there is no performance degradation (or improvement, for
that matter) at lower levels.
The level control is a high quality Swiss made switch with 24 discrete precision
resistors. You may notice some slight switching noise with this control when the D1 is
still warming up, but this will slowly go away.
Initial shipments of the D1 show 96 KHz operation on the front panel frequency
indicator, however this feature is not as yet enabled as the receiver chips are not yet
available. This upgrade will be made available as a "drop in" when the chips
become available. Contact your dealer, distributor, or Pass Labs for availability and
pricing.
The Pass D1 DA D/A Converter is warranted by Pass Laboratories to meet performance
specifications for 3 years from date of manufacture. During that time, Pass Laboratories
will provide free labor and parts at the manufacturing site. The warranty does not include
damage due to misuse or abuse or modified products and also does not include consequential
damage. |
