S/PDIF, USB and HDMI DAC with balanced preamp output
Made by: NAD Electronics International, Canada
Supplied by: Armour Home Electronics
Telephone: 01279 501111
Web: http://nadelectronics.com; www.armourhome.co.uk
NAD Masters M51 (£1500)
Not only the first stereo DAC to feature an HDMI input but another of NAD’s ‘Masters’
to employ the Zetex DDFA processor. It’s mathematics made into music
Review: John Bamford Lab: Paul Miller
t’s not every day that NAD introduces
a new product into its Masters Series
line-up. This intriguing M51 DAC was
previewed to the hi-fi trade in January
at CES and it boasts a wealth of features
that tick pretty much all the boxes required
by a 21st century audiophile.
Yes, it has an asynchronous USB input
that accommodates data up to 24-bit/
192kHz to take advantage of 99.99% of
all available hi-res downloads. It also has
an AES/EBU (XLR) and two S/PDIF inputs
(one RCA coaxial, one Toslink optical); it
comes with a remote control handset and
features built-in volume control allowing
it to be used as a preamp in an all-digital
system; and it even sports balanced
analogue outputs. NAD’s M51 really looks
the business.
With its classy brushed aluminium front
panel and high quality VFD display it might
have come from the Scandinavian school
of design. The face the NAD presents to
the world is the epitome of minimalist
understatement. But as our photograph
of the NAD’s rear panel on p33 reveals,
the M51 goes a step beyond any specialist
audiophile DAC so far produced by adding
a couple of HDMI inputs to its feature set.
You might want a high quality DAC to
improve the sound quality of an ageing
CD player, of course. Most likely you’ll be
interested in getting the best sound quality
possible from a ‘digital library’ these days
as well (this accounts for the resurgence in
popularity of standalone DACs).
But here’s an audiophile DAC that can
also be hooked up to the HDMI output of a
DVD and/or Blu-ray disc player to improve
its sound quality through a hi-fi enthusiast’s
two channel system. You might well ask,
‘Why on earth hasn’t a hi-fi manufacturer
made a DAC like this before?’ And therein
lies a story…
While chipsets and ‘kits of parts’ for
HDMI inputs/outputs are just as readily
available as those for S/PDIF and USB
interfaces, there’s a fundamental difference
with HDMI in that it doesn’t transmit data
in the clear. Developed to replace the
(analogue) SCART connector, it allows data
transfer of copy-protected material. As a
consequence, a manufacturer must join the
‘HDMI club’ and become a licensee – an
‘adopter’ – privy to confidential decryption
keys. HDMI adopters pay an annual fee of
$10k, plus (admittedly small) royalties to
the HDMI licensing organisation of four
cents per product sold, for so long as the
manufacturer implements the ever-sonecessary High-bandwidth Digital Content
Protection [HDCP] and uses the HDMI logo
on its product and promotional materials.
Adopters must license HDCP separately
from Digital Content Protection LLC, a
subsidiary of Intel. This costs a further
$15,000 per annum, on top of which
manufacturers then purchase quantities
of ‘Device Set Keys’: $1000 for 10,000
sets; $2500 for 100,000; and $5000 for 1
million sets. It’s all geared towards largescale consumer electronics manufacturers
whose highly automated production lines
RIGHT: Plenty of processor power under the
hood – Analog Devices DSP for HDMI, XMOS
for USB and the core Zetex DDFA silicon that
converts all LPCM to PWM [see boxout]
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will deliver more products in a morning
before a factory’s tea break than a
boutique company – the type of specialist
firm that makes niche products for audio
enthusiasts, such as standalone DACs –
might build in an entire year.
Full marks, then, to NAD for deciding
to include HDMI on a DAC aimed at
audiophiles. The company can afford to
implement it, after all. Just consider all its
DVD/BD players and AV receivers produced
over the years. But don’t get too excited
(as I did on first hearing about the M51),
as unfortunately its HDMI inputs cannot be
used to transform the sound quality of that
legacy DVD-Audio or SACD player that you
might be using in your system, via line-level
analogue connections.
The Content Protection for Pre-recorded
Media [CPPM] encryption employed on
most DVD-A recordings ensures that a
player’s digital outputs are downsampled
to 48kHz and with SACDs the digital output
is completely muted. This is a minefield,
however – it varies not only from player to
player but also from disc to disc – so if your
local dealer is a NAD stockist you might
want to first try the M51 with your hi-res
disc player if you consider this an important
factor in your purchasing decision.
Nor should you be overly wooed by the
numbers bandied about in NAD’s marketing
literature for the M51. A ‘35-bit/844kHz
DAC’ sounds terribly exciting on paper.
However, the numbers relate to the core
Zetex PWM processor
employed in the DAC, the
same device that NAD first
used in its Master Series
M2 digital amplifier [see
the Ed’s boxout]. Several
years ago NAD partnered
with the UK design team
of Zetex plc, based in
Oldham, Lancs, who had developed a novel
switching amplifier technology employing
feedback around its output stage.
By the time NAD’s M2 digital amplifier
came to market, Zetex was in fact acquired
by the Texas-based semiconductor
company Diodes Incorporated, whose
website [www.diodes.com] refers to
it still having an engineering centre in
‘Manchester, England’.
NAD has furthermore trickled down this
Diodes Zetex ‘engine’ to a more affordable
2x150W integrated amplifier dubbed
C 390DD (priced at €2500) that should be
available here by the time you read this.
Via the partnering remote control
handset, a set-up menu configures the DAC
for fixed or variable output (including its
output level in ‘fixed’
mode), while buttons
also control five levels of
display brightness and
absolute phase inversion.
Don’t lose the handset if
you’re using the DAC as
a preamp as there’s no
volume control on the
front panel, only an ‘input’ button to scroll
through its six digital inputs.
‘The rhythm
section was
served up with a
vigorous power’
Resisting temptation to dive straight in
to the ‘novelty’ feature of the M51 – its
HDMI inputs – and with the sound of
various D-to-A converters fresh in my
head following our June ’12 Group Test,
I first installed NAD’s USB Audio Class 2.0
The big Zetex processor at the heart of the M51 DAC is the same PWM (Pulse
Width Modulation) device we saw in NAD’s M2 ‘digital’ amplifier [see HFN Jun
’10]. Zetex calls this DDFA or Direct Digital Feedback, but instead of driving a
high power Class D output stage, here it’s servicing a thoroughly audiophile
analogue preamp. Volume is also handled digitally, but as Zetex employs 35-bit
processing it offers a full 66dB of attenuation (that’s the top 11-bits) before
squeezing the dynamic range of even the highest resolution 24-bit digital inputs.
You’ll see some other numbers in NAD’s blurb, doubtless repeated verbatim
in the red tops, that at first sight make less sense. Converting incoming LPCM
to a PWM bitstream at a final 844kHz sample rate seems plain wrong – after
all, why pick a number that’s not an integer multiple of the common 44.1kHz
or 48kHz base rates? The answer, according to my maths, lies in the choice of
commercially available crystal clocks and the 108MHz chip chosen by NAD and
Zetex (it’s commonly used in flat panel TVs).
Because all incoming digital data, whether natively 16-bit or 24-bit, is
truncated down to a 7-bit PWM bitstream, this yields a total of 128 discrete
pulse ‘widths’ to describe the audio signal per sample (27 equals 128). Each of
these 128 pulse widths is an integer multiple of the minimum ‘width’ which, to
ensure accurate resolution or ‘timing’ of its cycle, demands a minimum sample
rate of 108MHz divided by 128, or 844kHz. Mystery solved! PM
ABOVE: Large vacuum fluorescent display
shows input, volume setting and sampling
frequency of incoming data. The only buttons on
the spare fascia are standby and input select
driver, in order to listen to the DAC with
all manner of music tracks from my Dell
PC’s digital library. The driver must be
downloaded from NAD’s website, but of
course this is not required for Macintosh
users running MAC OSX v10.6 (Snow
Leopard) or later, that natively supports
audio up to 24-bit/192kHz.
The sound of the M51 is not without
character – I’m not entirely convinced
that it’s wholly neutral and transparent
to the source recordings one feeds into
it – however it’s a charming sound that I
warmed to within minutes of having it up
and running in my system. (You can see the
system and listening room by visiting www.
The M51 sounds meaty and powerful in
the bass, warm and almost ‘tubey’ through
the midband, and sweetly balanced at
the top end: making for a relaxed sonic
presentation that’s particularly forgiving of
harsh recordings and rarely grates. If you’re
looking for a DAC that gives digital sources
a degree of analogue warmth, this is it!
The demeanour of the M51 was readily
apparent when listening to modern, ‘hot’
tracks such as ‘Letters From The Sky’ from
Civil Twilight’s eponymously-titled debut
album [Wind-up 60150131522]. The piano
sounded bold and immediate, the lead
vocal was richly textured with plenty of
‘air’ around it, and the sound remained
consistently controlled and intelligible.
Where more explicit components tend
to highlight the ingress of hard-edged
sibilance in the recording’s production, the
M51 remained couth and civilised.
The M51 threw up countless delectable
surprises. Vintage recordings such as
Hendrix’s Band Of Gypsys recorded live at
New York’s Fillmore East on New Year’s Eve
of 1969 sounded wholesome, energetic
and deliciously fresh. Remastered umpteen
times over the decades, mine was a rip of
a 1997 CD reissue [MCA, MCD 11607-2]
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NAD MASTERS M51 (£1500)
ABOVE: A choice of single-ended (RCA) and balanced (XLR) preamp
outputs are joined by AES/EBU (XLR), S/PDIF (coax and optical), USB and
two HDMI inputs. There’s also an HDMI pass thru’, USB upgrade port and
RS232 for system controllers. The M50 remote is essential for navigation
and it sounded fabulous through the
M51, the rhythm section of Buddy
Miles and Billy Cox served up with
vigorous weight and power.
The DAC’s opulent sound was
further demonstrated on Caravan’s
epic ‘Nine Feet Underground’
from In The Land Of Grey And Pink
[Decca 8829832], showcasing the
rounded, throaty texture of Richard
Sinclair’s Fender Jazz bass guitar
and the primal growl of David
Sinclair’s Hammond organ redolent
of yesteryear.
I then tried the free 44.1, 96 and
192kHz downloads of an excerpt
from Stravinsky’s Firebird Suite
performed by the Budapest Festival
Orchestra, from Channel Classics.
Again, the sound was luxurious when
playing the 44.1 file and even more
splendid at 96kHz… but imagine my
chagrin when the audio stuttered to
a halt and my PC (running Windows
7) completely froze once I selected
the 192kHz file.
NAD’s perfunctory M51 user
manual gives no clue as to what
might have been amiss, but on
revisiting the company’s website
I clocked the notice that advises:
‘Any drivers previously loaded must
be uninstalled prior to installing
this driver’. With a heavy heart I
set about purging my PC of myriad
drivers installed for auditioning USB
DACs in recent years. Our editor
experienced no such problem when
testing the M51 using Windows XP,
by the way. Nevertheless, you have
been warned!
As already mentioned, not all
DVD-As are downsampled by legacy
hi-res disc players, and the M51’s
smooth and refined sound remained
evident via HDMI when playing
one of Mark Waldrep’s excellent
24-bit/96kHz recordings on his
AIX Records label. I was listening
to Dorian Michael – Acoustic
Blues [AIX 80016] featuring guest
performances by guitarists Albert
Lee and Laurence Juber, an exemplar
of modern media’s ability to describe
beautifully the subtle variations in
tonality and timbre of instruments.
The sound lacked a little sparkle and
pizzaz, but it wasn’t so creamy that
it robbed the recording of vibrant
detail and dynamics.
And while I could criticise the
M51 for softening images and
sometimes proving too syrupy, it
certainly transformed the lacklustre,
anaemic sound of the analogue
outputs of an inexpensive three-yearold Pioneer BDP-320 Blu-ray player.
With the Pioneer set to output LPCM
via HDMI the 24/96 Dolby TrueHD
audio track of Dave Matthews and
Tim Reynolds concert performance
Live at Radio City Music Hall was
enthralling via the M51 DAC, as was
the sound from Jeff Beck: Performing
This Week... Live at Ronnie Scott’s,
albeit encoded only in 16-bit/48kHz
for the Blu-ray release. While Beck’s
guitar variously sang and cried
throughout, the combined power
of Vinnie Colaiuta’s drumming
alongside the bass playing of the
terrifyingly talented Tal Wilkenfeld
made a compelling argument for
employing the M51 as a digital hub
in a high-end two-channel system.
Aside from the advantage of managing volume in the digital
domain, this ‘preamp’ version of the Zetex DDFA [see boxout,
p31] also brings with it a unique technical ‘fingerprint’.
Premature digital clipping is evident at the nominal 0dB volume
position and so all testing was performed at –1dB, at which
point the M51 delivers a full 4.13V (phase-inverted) balanced
output from a low 90ohm source impedance. In a parallel with
most Class D amplifiers, output impedance increases at HF
as does its response, reaching +0.08dB/20kHz (44.1/48kFs),
+0.37dB/45kHz (96kFs) and +2.8dB/85kHz (192kFs). Distortion
also increases very markedly indeed at high frequencies, from
a vanishingly low 0.00025% through the midrange to 0.06% at
20kHz and 0.6% at 40kHz (all re. 0dBFs). Once again, this is the
behaviour of a typical Class D amp than a typical outboard DAC.
Low-level linearity is spot on, true to within ±0.5dB over a
full 120dB dynamic range, but the pattern of distortion versus
digital level is less uniform than usual [see Graph 1, below, and
compare with DACs in previous issues of HFN]. Importantly, the
M51 locks flawlessly onto all 44.1-192kHz sample rates via
S/PDIF, USB and HDMI interfaces and its performance, including
the 115dB S/N, is remarkably unvarying (further suggesting
that all this ‘character’ is determined by the central onboard
processor). Jitter is low but also low rate and decorrelated, from
200psec (S/PDIF) to 300psec (USB) [see Graph 2], this noise-like
pattern often linked to a not unwelcome softening of ‘edgy’
material but also a softening of sharp stereo imaging. Readers
can download full QC Suite test reports for the NAD M51 DAC’s
S/PDIF, USB and HDMI performance by navigating to www.
hifinews.co.uk and clicking on the red ‘download’ button. PM
ABOVE: Distortion vs. 24-bit digital signal level over a
120dB range at 1kHz (black) and 20kHz (blue)
Keenly priced given its feature
set and refined sound quality,
NAD’s M51 is a fabulous DAC that
can also be used as a preamp in
an all-digital system. It sounds
sophisticated and sumptuous,
never edgy or mechanical. I’ve
heard DACs that are sharper
focused and more transparent
with hi-res sources; nevertheless
it is sure to find favour among
many audiophiles, thanks to its
sound being notably free of grain.
Sound Quality: 82%
- 100
ABOVE: Zoomed, high res. jitter spectrum (±500Hz)
comparing low-rate jitter with 24-bit/48kHz data
over S/PDIF (black), HDMI (green) and USB (red)
Maximum output level (Balanced)
4.13Vrms at 91ohm
A-wtd S/N ratio (S/PDIF / USB)
115.3dB / 114.9dB
Distortion (1kHz, 0dBFs/–30dBFs)
0.00025% / 0.024%
Dist. & Noise (20kHz, 0dBFs/–30dBFs)
0.06% / 0.11%
Freq. resp. (20Hz-20kHz/45kHz/90kHz)
–0.0dB to +0.1dB/+0.4dB/–3.6dB
Digital jitter (48kHz/96kHz/HDMI/USB)
222ps / 200ps / 265ps / 300psec
Resolution @ –100dB
Power consumption
12W (1W standby)
Dimensions (WHD)
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