Nad Master Series M51 Direct Digital Dac Review

The huge rise in demand for instant digital music has led to a rise in demand for separate DACS (Digital to Analogue Converters). Audiophiles who wish to keep with the trends in digital technology, without suffering the inherent loss of sound quality that are a by-product of the commonly available digital formats, are turning to high-quality DACs to provide an interface between their digital library and their hi-fi system.

The NAD Master Series M51 ‘Direct Digital’ DAC, a 35-bit 844KHZ digital to analogue converter, is part of NAD’s master series suite of components. Designed to be paired with the M50 music server/streamer and the M52 storage volt, the M51 aims to squeeze the best possible sound quality from all o fyour digital sources.

Key to the M51’s sound is nads proprietary Direct Digital™ technology, first demonstrated in the M2 direct-digital amplifier. Using a 108MHZ processor, the M51 converts the standard PCM signals output by your digital devices, and converts them to PWM signals – with a sampling rate of 844KHZ. This completely eliminates digital jitter. A DSP-Based volume control allows to the M51 to act as a digital preamp – with no reduction in resolution, even when attenuating the highest quality 24-bit signals.

A pure class A low-impedance output stage, and a choice of balanced XLR or single-ended RCA outputs mean the M51 will interface with any hi-fi system or power amplifier. And while we’re on the subject of connectivity, the M51 has another unique trick up its sleeve – in the form of 3 HDMI jacks. 2 Of these are inputs – allowing you to connect devices such as games consoles, or blu-ray players, and bypass their inferior internal DACs for better sound quality. The 3rd acts as a video pass-through – to be connected to your AV receiver or TV, allowing the video from these sources to pass through to that device. And – of course, a USB port –supporting USB audio Class 2.0 – is provided for connection of a computer. Window’s users will need to download NAD’s free USB audio drivers – if you’re a mac user, you can get listening right away.

It's clear that a lot of thought has gone into the M51’s packaging – lifting the large foam insert with its neatly situated accessories box from the top reveals the M51 nestled within a large, perfectly formed block. It’s wrapped in plastic, with a further cloth bag helping to protect the finish.

The accessories box contains the remote control, batteries, 2 power cables and a quick-start guide – no interconnect cables (hardly surprising), though a USB A-B cable for your computer would’ve been nice.

The unit feels well put together, despite the rather tinny, wrap-around top cover – the side panels are held on with screws on either side of the unit, as opposed to the ‘wrap-around, screw on the bottom’ designs that manufacturers are opting for in modern stereo components. There’s a gap between the bottom edge of the side panels and the bottom, which results in some flexing of the sides – and the case does flex slightly when moving it on a shelf. It’s nowhere near as solid as, for example, the Cambridge Audio 851C. The feet have little padding – there’s fassioned from a plastic material with a tiny amount of soft rubber, and certainly don’t do much, if anything, to prevent vibrations being transmitted to and from the unit. Only the front panel has some redeeming qualities – it’s a solid block of aluminium that undoubtedly ways more than the rest of the casing it’s affixed to.

Look at the back panel, and the RCA outputs instantly grab your attention. Not your standard RCA jacks – no, these are truly massive, securely held in place with large nuts. Unfortunately – the coaxial connector is a standard RCA connector – and the other connectors, while decent, don’t scream quality.

As previously mentioned, both single-ended RCA and balanced XLR outputs are available. There’s an AES-EBU XLR input, an RCA coax input, and an optical input with a protective dust flap – a nice touch. The aforementioned 3 HDMI jacks (2 in, 1 out) and joined by 2 USB jacks – one for connection of a computer, and 1 used to perform firmware updates. RS-232 and 12V trigger in/out are provided for integration into custom instillations or for use with other AV gear, and an IEC power socket with hard on/off switch means you’re free to upgrade the cable to something better than that provided in the box.

Look to the front – and there’s a lot less going on. A power button, standby indicator, display, IR sensor and input selection button are all that adorn the units facade. There’s no volume control, and no ability to control the units onboard menu system from the front panel – for those functions, you’ll need the provided M50 remote. This is a disappointment; NAD could’ve at least provided a means to alter the volume from the unit itself. For me, this isn’t a deal breaker – I use analogue sources, so would never use the M51 as a digital preamp or have a need to enter the menu system – but it’s disappointing none the less.

That remote is a thick, traditionally-shaped affair – in fact, it’s much thicker than your standard remote. It’s almost cylindrical – save for the flat faceplate, and the indents in the sides designed to improve your grip. There’s a definite edge to the aluminium faceplate – no-where near the fit and finish of other remotes. It feels nice to hold, however, and the central navigation controls have a satisfying click to them when pressed.

The first thing I noticed when setting the M51 up in my system was the quiet output – The M51’s output is much less powerful than that of many other line-level devices. Whether this is a good thing will depend on your system – some setups work better with devices with higher outputs, while some, such as mine, aren’t fussy. Still; it’s something to be aware of if you switch inputs frequently or are comparing the M51 to your other components.

Like many NAD products, including the budget-oriented D3020, the M51’s sound is smooth – very smooth, in fact, leaning towards neutrality rather than warmth. The sound is bursting with detail, with clear separation between instruments. Unfortunately, however, the sound stage isn’t as broad and expansive as rivals, and all that smoothness comes at a price – excitement.

The M51 draws so much detail from your tracks, and presents them in such a well-timed, well-balanced way, that excitement is virtually non-existent. This is far from a good thing – it leaves music sound bland and dry. Music (well, good music anyway) starts with emotion – strip that away, and you’re left with a recording with a very un-subtle lack of personality that is difficult and tiresome to listen too.

Play Nirvana’s Lithium, and the muted guitars, simple-but-heavy drum fill, and pounding rhythm is delivered with precision. However; the M51 lacks a sense of enthusiasm as portrayed by other players, such as the Cambridge 851D or even the Stream Magic 6. Sure, it’s listenable – but you’re left feeling disappointed by an uninspiring performance.

Switch to something harder, such as Story Of The Year’s ‘Pay Your Enemy’, and the M51 redeems itself somewhat – the fast punchy drums delivered as accurately as you could hope for, and the guitars being more wide-spread than first impressions of the M51 would suggest. However; again, the performance is somewhat uninspiring. There’s no sense of fun to put a smile on your face. Play this track on an 851C, and you’ll quickly find yourself joining in with the catchy chorus – not here, though.

SO would I buy an M51? The answer to that is simple – and it’s not one to please NAD. The M51 has the potential to be a great DAC – however, a lack of useful inputs (I really don’t need HDMI for hi-fi use), minimalist controls (where that front-mounted volume control?), the slightly flimsy casework, and – of course – the sound mean that the M51 has a long way to go before before it can compete with the best at this price level.

If you’ve got money to burn and prefer specs over sound, then this may be the DAC for you. If you want a DAC with most of the functions of the M51, plus an awesome headphone amp, front-panel control, a better remote  – and better sound, get a Cambridge Audio 851D.

Categorised as DACs Tagged ,

By Ashley

I founded Audio Appraisal a few years ago and continue to regularly update it with fresh content. An avid vinyl collector and coffee addict, I can often be found at a workbench tinkering with a faulty electronic device, tweaking a turntable to extract the last bit of detail from those tiny grooves in the plastic stuff, or relaxing in front of the hi-fi with a good album. A musician, occasional producer and sound engineer, other hobbies include software programming, web development, long walks and occasional DIY. Follow @ashleycox2


  1. Interesting review. I’ve read about 15 reviews of the m51 and you’re the only one saying it does not match up to the price. Maybe it just didn’t go well with the 851E?

    1. I just don’t care for the typical audiophile sound, I.E I’d rather have a little fun and excitement even if it comes at the expense of some detail. Though I don’t own one any more (this review is getting on a bit now), I don’t think I ever found a component that didn’t match well with the 851E.

      1. Hi,
        Did you had the chance to compare the nad m12/m22 to your 851e/w.

        I own a 851e and 2 851w and i have problem integrating a good subwoofer. The lfe signal of the 851e is too weak and the sub does’nt do anything unless i put the preamp to -40 db and more…

        Thx for your advice,

          1. Yes it has one volume, X-over and polarity control.

            To get something, I have to put the volume at max level. The phase is good as it does’nt cancel my speakers bass level and the X-overt is not too high, I tried it from 80Hx to 40 Hz.

            I tried with a Rel-R328 and a Focal Electra Sw1000S. Same issue twice.

            1. Strange. I’m afraid I’m not much help as I never use a sub. Assuming there isn’t a setting in the 851E settings menu (I no-longer have mine to check), the only thing I can suggest is to try the sub with the second preamp outputs of the 851E as opposed to the mono sub out. That is of course assuming you’re using the 851E with balanced connections to your 851Ws. I’m not sure whether that would make a difference but it’s worth a try.

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