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6moons audio reviews - Nagra HD DAC
06/10/14 18.02
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Reviewer: Srajan Ebaen
Financial Interests: click here
Source: 27" iMac with 3.4GHz quad-core Intel Core i7, 16GB 1.333MHz RAM, 2TB hard disc, 256GB SSD drive, ADM
Radeon HD 6970M with 2GB of GDDR5 memory, PureMusic 1.89g in hybrid memory play with pre-allocated RAM and
AIFF files up to 24/192; Audirvana 1.5.10 in Integer mode 1, Metrum Hex, AURALiC Vega, SOtM dX-USB HD with Superclock upgrade & mBPS-d2, Aqua Hifi La Voce 2
Preamp/Integrated: Nagra Jazz, Esoteric C-03, Bent Audio Tap-X, Crayon CFA-1.2, Bakoon AMP-12R, Gato Audio DIA250, Job Pre2 [on review]
Amplifier: First Watt SIT1, FirstWatt F6, Goldmund/Job 225, AURALiC Merak [on loan]
Speakers: soundkaos Wave 40 + Zu Submission, Albedo Audio Aptica, Boenicke Audio W5se, German Physiks HRS120
Cables: Complete loom of Zu Audio Event, KingRex uCraft and Light Harmonic LightSpeed and Zu split USB cables, Van
den Hul AES/EBU cable, Tombo Trøn S/PDIF cable, AudioQuest Diamond Toslink
Stands: Artesania Audio Exotyeric for front end, Rajasthani hardwood rack for amps
Powerline conditioning: GigaWatt power strip on amps, Vibex Granada on all components
Sundry accessories: Extensive use of Acoustic System Resonators, noise filters and phase inverters
Room size: Irregularly shaped 9.5 x 10m open floor plan combines the living/listening room, kitchen and office. Added to
this space the speakers see the air volume of the entry hall and a long corridor plus the 2nd-storey 6 x 9.5m loft. Woodpanel ceiling slopes up to the loft. Parquet flooring. Lots of non-parallel surfaces ('vertical gable' windows, twin-angle
ceiling, spiral staircase enclosure, fireplace enclosure). For a pictorial tour, see here.
Review component retail in Europe [incl. VAT]: €19'900 with 2 x ACPS II power supplies; €22'900 with MPS and 2 x
1.25m DC cables, HD VFS €1'750 [for DAC], VFS €1'650 [for MPS]j
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6moons audio reviews - Nagra HD DAC
06/10/14 18.02
1.25m DC cables, HD VFS €1'750 [for DAC], VFS €1'650 [for MPS]j
The ultimate DAC would arguably need a dual-core engine: true 1-bit for DSD, discrete R2R for PCM. Nagra's HD
DAC—something even more ambitious was in R&D to explore viability of a dual-core circuit—neared completion by the
close of 2013. The team in Romanel-sur-Lausanne was working hard to wrap the project. This spanned from conceptual
to mechanical, from multi-layer PCB layout to advanced power supplies, from analog and digital circuits to control logic,
display coding, firmware authoring and more.
This spin-off from Nagra's secret extreme converter research project was previewed as prototype at Munich HighEnd
2013 but then still in a temporary box. Final production would adopt classic compact Nagra livery. Even though that builds
out real estate by going considerably deeper than my Jazz preamp, it still sprouts two outboard power supplies for digital
and analog respectively. That's because the trademark ¾ width low-rider casing is otherwise too crammed with signalpath circuitry, interstage and balanced output transformers plus two very large custom coupling capacitors and a valve to
leave room for anything else.
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6moons audio reviews - Nagra HD DAC
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Nagra audition room with Verity Audio Lohengrin II speakers.
Fully DSD64/128 and 24/384kHz PCM compliant, the HD DAC is based on a small core module from Andreas Koch of
Playback Designs as probably hifi's ultimate authority on DSD. From him Nagra commissioned a custom ceramic board.
The heart of its circuit is a Xilinx FPGA coded to operate as a true 1-bit DAC since commercial true 1-bit chips no longer
exist. "The sampling frequency reaches 5.6MHz and the internal calculations use 72-bit precision. This method produces
a signal with such quality that the steep-slope input filters on the analog section can be eliminated. The result is that the
harmonics and transients are perfectly maintained, allowing the music to conserve its natural depth and essence."
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6moons audio reviews - Nagra HD DAC
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Bread-boarded HD DAC prototype circuit during my April 2013 visit to Nagra.
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6moons audio reviews - Nagra HD DAC
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The board in the foreground shows an early version of the Koch module.
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6moons audio reviews - Nagra HD DAC
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'Jumper central' was installed for easy listening comparisons of various circuit and parts options during prototyping.
Everything pre/post the custom Koch board is Nagra's own They tap directly into Koch's analog reconstruction filter to
follow up with their own ultra-precision chip-based impedance converters and drivers for each half of the symmetrical
output. This direct-coupled stage applies neither voltage gain nor feedback but operates at a truly colossal slew rate.
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6moons audio reviews - Nagra HD DAC
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Next come ultra-complex Teflon-layered mumetal enclosed interstage transformers with
very thin primary wire and even thinner
secondaries. They load the Koch module at a
very specific impedance which the listening
team arrived at by careful auditions.
These ITs sport extended bandwidth and linear
phase response. After 12 prototypes which
took Nagra's in-house expert half a day each to
wind, engineering finally signed off on this
challenging magnetic part with the bright blue
metallic glass core.
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6moons audio reviews - Nagra HD DAC
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These mono ITs generate passive voltage gain—+7/+11dB for 1V/2.5V outputs—further impedance conversion and signal
summing since the Koch module output is symmetrical but Nagra's circuit deliberately single-ended. The ITs then see an
auto-bias single mil-spec ECC82-related inverted triode buffer.
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This US JAN valve at the time was built with special treatment on its larger plate and offered lower microphonics, higher
spec consistency and longer life expectancy over its common ECC82 equivalents. This cathode-coupled tube stage has
negative voltage gain, zero feedback but off-the-charts bandwidth. Its noise performance is superior to even the ~130dB
SNR of Koch's module. During my two visits the final coupling caps were giant silver/gold Mundorf issue because Nagra
were still waiting on their very own custom caps.
4-in-1 MPS power supply (three AC supplies, one DC feed, green LEDs still pre-production).
For Nagra whose chief PCB layout architect has worked in sixteen layers for a previous military contract employer, these
DAC boards are very basic 4-layer affairs (two for signal, two for power supplies). Digital and analog grounds operate at
deliberately different potentials. There are 25 dedicated power supplies with stout Elna Silmic 2 capacitive storage and 32
decoupling capacitors. Two of the power supplies are for high-speed USB alone to bypass buss power.
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The MPS in bits'n'bobs. The open case here still lacks the transformer, battery and battery logic board. The final production fascia shows the
smaller now yellow LEDs marketing manager Matthieu Latour insisted on.
There are separate miniature isolation transformers for each of the two AES/EBU and two coaxial digital inputs plus three
more for the 1GHz Ethernet-formatted I²S input. There's also a discrete headphone circuit, optional transformer-coupled
fixed true balanced outputs, remote-controlled analog volume for the RCA outputs and a menu-driven display for custom
options like digital filter and fixed volume settings. Owners of multiple Nagra components can use the new MPS shown
above. This eliminates running multiples of the compact stock power supplies shown below and centralizes Nagra power.
Out of its four feeds the MPS runs one on pure battery power. The MPS can thus drive the HD DAC, Jazz or Melody
preamp and a Nagra phono stage whilst only taking up a single wall or conditioner power outlet.
The type of stock Nagra PSU of which the DAC has two (one for digital, one for analog).
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6moons audio reviews - Nagra HD DAC
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To better appreciate the gestation of any Nagra product, we'll now take a look at the team responsible for the HD DAC.
Otherwise empty prototype case at Munich HighEnd 2013 - in production one of the coaxial S/PDIF inputs became BNC.
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