DAC Project D1 Circuit available?

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Retired diyAudio Moderator
Joined 2002
I doubt it, but here are some product links:

Product Information page

Owner's Manual (pdf)

The owners manual goes over what components are inside of the D1:

"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

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."

An externally hosted image should be here but it was not working when we last tested it.

Retired diyAudio Moderator
Joined 2002
The David Broadhurst DAC looks like a great project. I could not imagine designing a circuit board of that complexity, with just a circuit.

$99+shipping for the boards, socket and manual does not seem bad at all, considering the cost to get those boards manufactured by yourself.

Definately a good project to consider when there is time. I could not imagine designing boards for the D1.

The Broadhurst DAC seems kind of weird.
Why would you run the clock directly into
the DAC? The clock should run to the digital
filter and the filter should clock the DAC. If
the filter introduces a delay -- and it does --
the master clock will be meaningless to the
datastream from the filter to the DAC.

The other weird thing with this DAC is the
signal lines aren't loaded except by their
PCB trace. That could leave CMOS outputs

Running the master clock straight into the DAC chips results in much lower jitter than clocking the DAC chips in the conventional method ie. using the BCLK output of the DF1704. This is because the DF1704 will introduce signifcant jitter of its own.
As long as the timing is maintained into the DAC chips there is no problem - it works very well.
Note that this simple method works because the BCLK output of the DF1704 is continuous. Had it not been a more sophisticated re-clocking system would have been needed.

What signal lines are you refering to in your second comment???
The one and only
Joined 2001
Paid Member
We have had internal discussions about posting the
D1 schematic, and a decision has just now been made.

The hangup? The circuit is difficult to render and we
don't want to devote a lot of energy to supporting
the people who can't build it without help. It's tough
enough keeping up with the support demands for the
simpler circuits.

We have decided to go ahead and post the service manual
to the first version at www.passlabs.com with the notice
that this is not a kit, and we are not encouraging people
to build it.

Sorry that we can't do better in this regard, but we have
limited resources.
I'm also shouting my thank's to you Nelson and people at PassLabs.

Now a little question. Where can one find the service manual on PassLabs site or is it not up just jet.....

I think it will at least be something to think about and I'm sure the analog output stage would be interesting.

Big Thankyou to Nelson Pass!

Hey thats amazing! I think that people who are able to complete this without help are the only ones that would undertake it, personally I'm not expecting to build a straight copy, but you never know!

Thanks again Mr Pass, you are truly unique in this bithchy world of audio.

You've done it now!

You madman! You have opened pandoras box. Is this product still
in production. Let me be the first to make you question your sanity and ask the part number and vendor for the VCO for U3. I will sit back and wait for my deserved abuse now.......


P.S. Way cool I to V stage!

Thanks Nelson for taking the plunge... May the force be with you...


Just thought that I'd point out the RED SENTENCE at the top of the service manual... I particularly noted the "DO NOT CALL US" part... I would think that email or a board post is pretty much synonymous to a phone call, wouldn't you agree?

Also the Pass site states
D1: Currently out of production.
... Just in case you missed it...

Ahem..... The value for U3 is listed on the schematic...

Did you ride the "special" bus as a child? :)



This opportunity was simply too good to pass up! LOL!
Pass DAC

Oh...... like I'm the only one that is going to ask that! Just wait for the questions to start flying. This DAC will be discussed so much that we will long for heat sink and device matching questions to break up the monotany. I know what U3 is (a VCO), I want to know who makes it. I could find that out without help if I wanted to spend the time tracking it down. Mr. Pass is free to ignore me without the annoyance of a phone call.

I rode the long bus to school. My favorite driver was Grace Tazap. She once had a kid's pants freeze to the top of the bus when he climbed on the bus roof to unstick the windshield wiper. Can you remember someone's name from 35 years ago......


"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."

There's want I wanted to know in another post...........

The one and only
Joined 2001
Paid Member
Just a note of design attribution: Wayne Colburn
designed the circuitry. He based the IV converter on
an earlier Pass circuit, and the trick here is that we simply
cascode the output of the DAC chips with a Mosfet, and
voila! an IV conversion with very high linearity and
no feedback. The bandwidth is close to infinite, and the
RF of the converters doesn't bother it at all.

The Lithium Tantalates come from the same vendor we
get our Dilithium crystals.
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