The NAD C 658 BluOS Streaming DAC, Part Two

By Bill Heck

In my previous article, I introduced a new way of thinking about audio components, a view that moves us into a new digital audio realm. We are leaving an audio world in which, while music may have been recorded and stored digitally, the playback system remained essentially and straightforwardly analog. We are entering a world in which the norm will be, or at least should be, seamless combinations of sources and functions, all driven by digital processing power.

But we also saw that finding and using all the sources and functions that we would like without duplicating functionality – and without amassing a raft of components – is difficult.

Enter the NAD C 658. The 658 certainly is not the only audio component that leans into our new digital world. But a remarkable slice of that world lurks inside the conventional gray metal case of what looks at first glance like a rather ordinary preamp. It’s the do-it-all (almost) workhorse of the two-channel audio world, and perhaps the only unit out there with such a full range of capabilities: streaming, DAC, preamp (with phono inputs), headphone amp, subwoofer crossover, and room correction, with serious digital processing power and advanced software. Find yourself a CD transport, a turntable if you still play LP’s, a hard disk drive if you have stored music files, and a power amp – you’re all set.

There are three major advantages to packing all this functionality into one unit.

- You avoid expensive duplication of functions across multiple components.

- You minimize unneeded complication and connections. Consider what it would take to duplicate the functions of the 658: a streaming device cabled to a DAC cabled to a preamp cabled to a room correction device cabled to a crossover (for the subwoofer). Or consider the 658: one box, with one integrated setup and one integrated control system. Which setup is less expensive to implement? Which is simpler to set up? Which takes up less space on your equipment rack? (For that matter, which equipment racks have enough shelves for all those boxes plus a CD player and a power amp?) Is any of this sonically relevant? Maybe or maybe not, but surely simpler can’t hurt. Meanwhile, it is relevant to your sanity.

- You control everything through one interface. All functionality in the same unit means one control method, especially for streaming and locally stored music files. In the case of the NAD, that’s BluOS (see below). The only other control you’ll need is a remote for the CD transport – and if you have a BluOS-enabled CD player, you won’t need that remote, either.

There are two potential drawbacks, though:

- If you want to upgrade some specific function, say the DAC, you’re stuck. Yes, you could buy a separate DAC. But not only are you back to duplication, but the streaming function in the NAD is now behind your shiny new DAC. If you are worried about the last nuance among high-quality DACs or streaming devices, whether real or supposed, and you know that you will pine after the next year’s claimed breakthrough, the 658 may not be the device for you.

- Perhaps some completely new function will come along, making the unit obsolete. In this case, however, NAD has you covered. The 658 employs “Modular Design Construction” (MDC), which allows for the addition of circuit boards to add or change functions. The only additional board currently offered is for HDMI inputs and outputs; if you need that functionality, just add the circuit board. Naturally, it’s up to NAD as to what MDC boards they offer in the future, so there are no guarantees that everything we might ever want will be available. Still, it’s nice to know that NAD is thinking ahead.

Inputs and Controls
This section will be short indeed: you can read the product description on the NAD website as well as I, so no point in rehashing the basics here. I would like to clarify a few points, though:

- The online “product story” is inconsistent with the downloadable “white paper” on a few small points. (The product story is on the page at ; look under “Downloads” on that same page for the white paper.) NAD support assures me that the white paper is to be considered authoritative and that the product story will be corrected.

- That white paper is, however, wrong on one point: the subwoofer output is indeed high-filtered, meaning only low frequencies are output to the subwoofer. A NAD support rep was kind enough to test and confirm this point and assures me that the white paper will be corrected.

- As befits a modern DAC, the 658 can handle high resolution inputs up to 24/192 PCM; it also handles unfolding of MQA.

- A good set of inconsistencies may arise because the BluOS software is updated frequently, and the updates can modify or add to functionality. See the discussion below for details.

- One concern that people have raised in online forums, as did the salesperson with whom I chatted before ordering, is that one cannot play back music from a computer attached to the USB port. At first this sounded worrisome, but after using the unit for several months, I found that this concern mostly is a vestige of the old paradigm in which you needed a separate component (a computer), to play music files. Not with this unit: no matter where your music files are, there are simple ways to play them directly through the 658. And if you have been relying on the typically mediocre sound card in your computer for digital to analog conversion, you will have better audio quality to boot.

- The included remote control is a particularly nice one – but realistically, it’s mostly irrelevant, as you probably will use the BluOS app on your smartphone instead. Indeed, I suspect that most users would be perfectly happy if the unit did not even ship with a remote.

In general, operation of the unit is as expected and completely straightforward, with no noticeable oddities or “gotchas”.

In my previous article, I stressed the need for software to control our new digital functionality, in particular for playing stored music files and using streaming services.

NAD’s parent company, Lenbrook, produces the BluOS software platform, which naturally is used for the 658. (BluOS is also licensed by a few other brands.) The BluOS ecosystem has all sorts of options, such as multi-zone control (64 zones?!). For our purposes, it’s enough to know that you can use BluOS to select and play music as well as to control the functions of the 658. There’s no way to cover BluOS in detail here, but three points are worthy of note.

- First, if you do have stored music files, you can attach the storage device, e.g., a hard drive, to your router to make it part of your in-home network. BluOS will index the files and allow you to select the files and control playback. Meanwhile, you can use your computer to download music files, which you then can send directly to the storage device to add them to your music library. No specialized music server, with its own software, controls, and special connections, is needed. More duplication and expense avoided!

- Second, if you use a streaming music service, BluOS tells the 658 what to stream, but the streaming is done directly by and through the 658. Unlike some competitors, BluOS does not route the streaming through the app, e.g., through your phone or tablet. To my still-remaining IT sensibilities, the BluOS method is truly righteous.

- Third, BluOS natively interfaces with a lot of music streaming services and internet radio services, 25 at last count. These include the big players, such as Idagio (classical music), Tidal, Qobuz, and Amazon, as well as a host of lesser-known names (Deezer, Radio Paradise, etc.). The biggest miss for classical music fans is Primephonic, but we cannot blame Lenbrook for this: Primephonic’s service does not integrate with software from any manufacturer other than Sonos, which is not exactly an audiophile favorite. (I contracted Primephonic customer service about this issue. They assured me that they are working on integrations but provided no timetable.)

Overall, I found the BluOS app to be well-organized, easy to use, and stable. For example, when I first set up the 658, it wanted to update the software. Wonder of wonders, the update process worked automatically and correctly the very first time. Since then, I have applied four additional updates; the process is trivially easy and has worked flawlessly. Those who have struggled with software update failures, fiascos, and new bugs introduced with every release will share my amazement. I should note that internet discussion forums had plenty of complaints about earlier versions of the software: I can’t say how justified those complaints were at the time, but it appears that Lenbrook has addressed them successfully.

Earlier I mentioned that BluOS updates might add or modify functions of the 658. I already have seen several improvements, most subtle but some more obvious. Here’s one example: the subwoofer crossover frequency originally was fixed at 80 Hz, as described in the white paper. A software update made that user selectable from 40 – 200 Hz. Another, more recent update added several more streaming services to the list of those supported when I purchased the unit. Imagine that: an audio component that gets better with age.

Do not let the brevity of the discussion here cause you to underestimate the importance of well-executed software like BluOS. In terms of the user experience, think of the parallel to your smartphone’s OS. Imagine the frustration that you might feel if you found iOS or Android so opaque and confusing that you could not use your phone’s functionality properly. (Right about now, some of you are thinking of exactly such experiences.) Of course, we buy audio components to improve our experience of listening to music, but part of that experience involves getting those %$#& components working, playing the music we want when and how we want it. In my experience, BluOS does the job well.

Performance, Part One
Finally, the heart of the matter: how does this thing perform? Before answering that, a few qualifiers and confessions.

First, I have neither the skills nor the equipment to measure the performance of a unit like this, but have no reason to doubt the specs provided by NAD. Second, I have not done painstaking comparisons to similar products, mostly because there are not a lot of (any?) similar components combining such a range of audio functions. For comparison, I would need to assemble a combination of DAC / preamp / room correction system / subwoofer crossover, with far too many variables for meaningful comparison.

With those qualifications out of the way, I can tell you that my system sounded good when I originally installed the 658. The sound was clear and well-balanced, with no obvious shortcoming. The audiophile boxes were checked: imaging was good, the sound was detailed without being edgy. Transients were clean. Dynamics seemed fine. One particularly noticeable aspect was that backgrounds were absolutely silent, with music emerging from and fading back to stillness. So far, so good.

A word about subwoofing – because why should surround sound receiver users have all the low frequency fun? The 658 has line level outputs for two subwoofers. (If you have only one, bass from both channels is steered to a single output.) By the way, if you have not considered a subwoofer (or better, two subwoofers) for your two-channel system, the 658 might encourage you to take another look. I’ve had mine for about a year and would not dream of letting it go; the included subwoofer output from the 658 make it easier than ever to use.

But…but… It felt as though I was missing something. I was by no means dissatisfied: the 658 already had improved my system compared to the components that it replaced, and it was great having everything in one easy to use package. Still, it somehow felt that there should be more. I had no desire to spend hours swapping components or fiddling with tweaks to try to gain some small increment; surely there were better ways to make serious improvements. Perhaps I could try, ummm, let’s see – how about room correction?

DIRAC Room Correction
For several years, some form of room correction has been mostly available in AV receivers, with different vendors implementing proprietary algorithms with varying degrees of success. Recently, DIRAC and a few other similar software companies have arrived on the scene. DIRAC – named after the scientist, Paul Dirac, of Dirac equation fame – has developed acoustic correction software that it licenses to equipment manufacturers, such as NAD. DIRAC uses a set of measurements (see below) to create a “correction curve” that is implemented by the 658’s processor when playing music. The version included with the 658 is DIRAC LE (aka “Lite”), which does correction from 50 – 500 Hz, the range where the worst room issues normally arise. An upgraded version that corrects across the full audio spectrum costs $99.

The measurements require the user to position a microphone, included with the 658; a laptop or smartphone is needed to process the results. The procedure is slightly tedious: ideally you move the microphone through a series of 9 to 17 measurements. Then again, this effort certainly is less taxing than endless fiddling with speaker placement! I spent a couple of hours on my first attempt, including reading the directions, unpacking and hooking up the microphone, digging out an old tripod, setting levels, etc. A more careful second try when I knew what I was doing took less than an hour.

After the measurements are complete, the generated correction curve is saved and sent to the 658; up to five curves (up from the original three via a software update) can be stored there. Why more than one? Curves can be for a single seating position, i.e., a narrow area for one person, or for a wider area, perhaps a couch for two people. In any case, corrections include both equalization in the usual sense, i.e., increasing or decreasing the signal at various frequencies, and correction in the time domain. I would love to tell you how it all works, but the algorithms obviously are proprietary, as they are the heart of DIRAC’s business. Meanwhile, you can adjust the frequency equalization curve to fit your taste. For example, on my first attempt, I made a very slight tweak to reduce a mid-bass region that seemed just a bit prominent.

While playing music, you can select among the stored correction curves or turn Dirac off completely at any time. Thankfully, the most recently used choice stays in place until you change it.

The result? My first attempt resulted in sound that was different, but I was not quite sure that it was an improvement. However, I knew that there were two things going on. First, we all adjust to the vagaries of our rooms and accept what we hear, warts and all, as “normal”. Maybe I just needed time to adjust. Second, my measurement technique had been a little sloppy. I tried the entire measurement process again, using a better microphone stand for more careful positioning (a boom attachment for a camera tripod works well). The resulting correction curve removed all doubt. This was it.

Performance, Part Two
After applying room correction, the usual audiophile boxes are checked with very large, dark checkmarks indeed.

Perhaps the most noticeable improvement is clearer placement of musicians and their instruments in all dimensions, including depth. Indeed, the solidity and precise placement on a good recording can be startling, especially for solo piano and small ensembles. Moreover, without DIRAC correction, moving my head slightly often resulted in an obvious shift in instrumental placement and change in tonal balance; with DIRAC engaged, the audible effect of movements on my part is as if I am moving in front of a live performance, as the instruments stay where they are while my perspective on them changes. This latter effect may not sound that important, but if you’re like me, occasionally swaying to the music or “air conducting” the orchestra, listening just feels more natural. (For the record, I try not air conduct when attending live concerts.)

The sound is nicely detailed but not etched or edgy, by turns full and powerful or floating and delicate as the music dictates. Transients pop nicely yet naturally. Frequency response seems smoother: in particular, the lines played by cellos and especially double basses are easier to follow, as major peaks and valleys in the nether regions are sufficiently smoothed to let me hear all the notes. (The vagaries of this sort of subjective analysis are on display in online forums, where one camp of users complains that the sound of the 658 is horribly aggressive, bright, and hard, while other reviewers remark on the warm, relaxed, even laid-back sound of the unit. It can’t possibly be both!). With the Legacy Audio PowerBloc2 driving the speakers, dynamics are excellent.

In general, music just seems to be “cleaned up” and quite natural, and this is particularly noticeable in longer-term listening. In summary, the more I use DIRAC, the more important it feels: turning it off results in an immediate desire to turn it right back on.

While the above effects are quite noticeable, DIRAC will not make your $500 bookshelf speakers sound like $10,000 floor standing behemoths. All the same, the improvements wrought by DIRAC would have taken considerable effort and no little expense, if indeed they could be produced in any other way – and they were accomplished with the version of DIRAC included with the 658 at no extra cost.

Given that the worst of room effects are encountered in the range covered by the LE version of DIRAC, many users will be satisfied to stop here. Personally, I intend to spend the extra $99 to upgrade to the full spectrum version of DIRAC. My assumption is that even a relatively small improvement will be more than I could get by spending the same amount on any other accessory. I’ll report back later.

Does all this mean that I think that the 658 is the epitome of digital sound? Frankly, I have no idea. It seems to me that we have reached a level of sophistication in digital equipment that makes it difficult and often outrageously expensive, given the law of diminishing returns, to make real improvements by swapping components. Most front-end components with any high-fidelity pretensions have vanishingly low levels of noise, distortion, and, for digital components. even jitter. What I can tell you is that my system, now with the 658 in place and room correction on, seduces me into listening more than I have in ages.

The NAD C 658 retails for $1649. For most mortals, this is not exactly cheap for a single component – but the 658 is not your typical single component. When even the low end of audiophile preamps would be around $1000, a streaming “node” might be $500 or more, a dedicated phono preamp is a few hundred bucks, a dedicated subwoofer crossover is at least that much, and we still don’t have room correction, the price of the 658 starts to look very reasonable indeed. Add in the fact that the entire collection is run by already mature but still improving software (BluOS) and the 658 looks a downright bargain.

The Bottom Line
By now, it should be obvious that I am impressed with and pleased by the NAD C 658. As I write this, I am enjoying music entirely new to me, the Saint-Saens Piano Quintet No. 1, Op 30. The piano is close but not in my face, its sound blooming through its full range; each of the stringed instruments is in its appointed place; and the effect is that the musicians are grouped starting perhaps 15 feet in front of me. The sound is relaxed and natural, as it would be in real life. How could I be anything but delighted?

The 658 not only sounds fine, but it also makes the music so easy. Consider how I ended up listening to that quintet: I opened the app, noticed this album in the list of new releases on my streaming service, tapped a button, and started listening – in high-resolution, no less. One app (BluOS), no fiddling with disks (although I still do listen to CDs, particularly old favorites), no adjusting multiple components – not too many years ago, I would have regarded all this as magic.

Which brings us back to the bigger picture from my previous article: the 658, and components like it, are redefining how audio systems are put together. The combination of functions and the rich feature set of the 658 seem to presage what more components will be like in the future: digital “centers” that bring together formerly disparate functions in useful, even graceful ways, and that sensibly use the power of digital processing to enhance our systems. In a few years, we may look askance at any preamp that lacks a high-quality DAC, and even be annoyed by other components that include redundant DACs. We may feel cheated by any preamp that does not offer room correction and subwoofer outputs. We may be frustrated by any preamp that fails to index and sort our collection of downloaded music – if we even bother to download music given the ubiquity of high-quality streaming options. We may expect that every preamp not only has streaming built in, but also natively interfaces with every streaming service out there. A few years from now, we may not even recall that we once had to buy more than one component to build the front end of an audio system.

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Meet the Staff

Meet the Staff
John J. Puccio, Editor, Publisher, Reviewer

Understand, I'm just an everyday guy reacting to something I love. And I've been doing it for a very long time, my appreciation for classical music starting with the musical excerpts on the Big Jon and Sparkie radio show in the early Fifties and the purchase of my first recording, The 101 Strings Play the Classics, around 1956. In the late Sixties I began teaching high school English and Film Studies as well as becoming interested in hi-fi, my audio ambitions graduating me from a pair of AR-3 speakers to the Fulton J's recommended by The Stereophile's J. Gordon Holt. In the early Seventies, I began writing for a number of audio magazines, including Audio Excellence, Audio Forum, The Boston Audio Society Speaker, The American Record Guide, and from 1976 until 2008, The $ensible Sound, for which I served as Classical Music Editor.

Today, I'm retired from teaching and use a pair of bi-amped VMPS RM40s loudspeakers for my listening. In addition to writing the Classical Candor blog, I served as the Movie Review Editor for the Web site Movie Metropolis (formerly DVDTown) from 1997-2013. Music and movies. Life couldn't be better.
Karl W. Nehring, Contributing Reviewer

For more than 20 years I was the editor of The $ensible Sound magazine and a regular contributor to its classical review pages. I would not presume to present myself as some sort of expert on music, but I have a deep love for and appreciation of many types of music, "classical" especially, and have listened to thousands of recordings over the years, many of which still line the walls of my listening room (and occasionally spill onto the furniture and floor, much to the chagrin of my long-suffering wife). I have always taken the approach as a reviewer that what I am trying to do is simply to point out to readers that I have come across a recording that I have found of interest, a recording that I think they might appreciate my having pointed out to them. I suppose that sounds a bit simpleminded, but I know I appreciate reading reviews by others that do the same for me -- point out recordings that I think I might enjoy.

For readers who might be wondering about what kind of system I am using to do my listening, I should probably point out that I do a LOT of music listening and employ a variety of means to do so in a variety of environments, as I would imagine many music lovers also do. Starting at the more grandiose end of the scale, the system in which I do my most serious listening comprises an Arcam CDS50 CSD/SACD CD player, Goldpoint SA4 Passive Preamp, Legacy Audio PowerBloc2 amplifier, and a pair of Legacy Audio Focus SE loudspeakers. I also do a lot of listening while driving in my 2016 Acura RDX with its nice-sounding ELS Studio sound system through which I play CDs (the ones I especially like I rip to the Acura's hard drive so that I can listen to them whenever I want) or stream music through the system using my cell phone. For more casual listening at home when I am not in my listening room, I often stream music through the phone into a Vizio soundbar system that has remarkably nice sound for such a diminutive physical presence. And finally, at the least grandiose end of the scale, I have an Ultimate Ears Wonderboom Bluetooth speaker for those occasions where I am somewhere by myself without a sound system but in desperate need of a musical fix. I just can't imagine life without music and I am humbly grateful for the technology that enables us to enjoy it in so many wonderful ways.
Bryan Geyer, Technical Analyst

I initially embraced classical music in 1954 when I mistuned my car radio and heard the Heifetz recording of Mendelssohn's Violin Concerto. That inspired me to board the new "hi-fi" DIY bandwagon. In 1957 I joined one of the pioneer semiconductor makers and spent the next 32 years marketing transistors and microcircuits to military contractors. Home audio DIY projects remained a personal passion until 1989 when we created our own new photography equipment company. I later (2012) revived my interest in two channel audio when we "downsized" our life and determined that mini-monitors + paired subwoofers were a great way to mate fine music with the space constraints of condo living.

Visitors that view my technical papers on this site may wonder why they appear here, rather than on a site that features audio equipment reviews. My reason is that I tried the latter, and prefer to publish for people who actually want to listen to music; not to equipment. My focus is in describing what's technically beneficial to assure that the sound of the system will accurately replicate the source input signal (i. e. exhibit high accuracy) without inordinate cost and complexity. Conversely, most of the audiophiles of today strive to achieve sound that's euphonic, i.e. be personally satisfying. In essence, audiophiles seek sound that's consistent with their desire; the music is simply a test signal.

William (Bill) Heck, Contributing Reviewer

Among my early childhood memories are those of listening to my mother playing records (some even 78 rpm ones!) of both classical music and jazz tunes. I suppose that her love of music was transmitted genetically, and my interest was sustained by years of playing in rock bands – until I realized that this was no way to make a living. The interest in classical music was rekindled in grad school when the university FM station serving as background music for studying happened to play the Brahms First Symphony. As the work came to an end, it struck me forcibly that this was the most beautiful thing I had ever heard, and from that point on, I never looked back. This revelation was to the detriment of my studies, as I subsequently spent way too much time simply listening, but music has remained a significant part of my life. These days, although I still can tell a trumpet from a bassoon and a quarter note from a treble clef, I have to admit that I remain a nonexpert. But I do love music in general and classical music in particular, and I enjoy sharing both information and opinions about it.

The audiophile bug bit about the same time that I returned to that classical music. I’ve gone through plenty of equipment, brands from Audio Research to Yamaha, and the best of it has opened new audio insights. Along the way, I reviewed components, and occasionally recordings, for The $ensible Sound magazine. Recently I’ve rebuilt--I prefer to say reinvigorated--my audio system, with a Sangean FM HD tuner and (for the moment) an ancient Toshiba multi-format disk player serving as a transport, both feeding a NAD C 658 streaming preamp/DAC, which in turn connects to a Legacy Powerbloc2 amplifier driving my trusty Waveform Mach Solo speakers, supplemented by a Hsu Research ULS 15 Mk II subwoofer.

Mission Statement

It is the goal of Classical Candor to promote the enjoyment of classical music. Other forms of music come and go--minuets, waltzes, ragtime, blues, jazz, bebop, country-western, rock-'n'-roll, heavy metal, rap, and the rest--but classical music has been around for hundreds of years and will continue to be around for hundreds more. It's no accident that every major city in the world has one or more symphony orchestras.

When I was young, I heard it said that only intellectuals could appreciate classical music, that it required dedicated concentration to appreciate. Nonsense. I'm no intellectual, and I've always loved classical music. Anyone who's ever seen and enjoyed Disney's Fantasia or a Looney Tunes cartoon playing Rossini's William Tell Overture or Liszt's Hungarian Rhapsody No. 2 can attest to the power and joy of classical music, and that's just about everybody.

So, if Classical Candor can expand one's awareness of classical music and bring more joy to one's life, more power to it. It's done its job. --John J. Puccio

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"Their Master's Voice" by Michael Sowa

"Their Master's Voice" by Michael Sowa