PassLabs D-1 Owners manual
Pass Laboratories
D1 D/A Converter
Owner's Manual
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 commongate 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.
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