Metrum Acoustics Hex NOS Differential DAC
Input: 2x optical Toslink, 2x Coaxial (1x BNC and 1x RCA), AES/EBU module optional, USB module optional
Output: 1 pair RCA, 1 pair XLR
Dimensions: 320 x 320 x 85 mm
Weight: 5500 grams
Availability: online and through Authorized Dealers
Price: from $3,288.99 w/USB, as reviewed $3,510.00 includes AES/EBU and USB inputs
US Distributor: www.stereodesk.com
The Hex DAC is, according to Metrum Acoustics, a non-oversampling (NOS) filterless DAC in the strictest sense. This means that inside, including inside its mysterious* DAC chips, the PCM signal remains a PCM signal and it passes through the D/A process un-oversampled and un-filtered, as it were. Many of today's DACs, even some calling themselves NOS DACs, employ delta-sigma DAC chips that, among other things, oversample, interpolate, convert the PCM signal to a PDM signal (pulse-density modulation), and filter our precious musical data. Filterless NOS DACs skip these steps and some feel that the resulting analog signal sounds more analog-like as a result.
Dutch-based All Engineering is the company behind Metrum and the Hex is the third and most costly DAC in the Metrum DAC lineup that includes the Quad and the Octave both sporting the same NOS topology. If I told you the Quad had 4 DAC chips/2 per side, and the Octave 8, would you be able to guess which DAC chips are in the Hex?
*Metrum Acoustics goes to the trouble of hiding, literally scratching out, the identifying information stamped on the "industrial high speed dac-chipsets" in use in their Hex DAC. It is, therefore, a trade secret and we'll have to take their word for what they do and more importantly what they don't do. But we can see that there are 8 DACs per side in this fully differential design (if they sell 100 DACs that = a lot of scratching). We can also see three toroidal transformers that provide the DAC boards, control logic, and USB input each with their own, separate power supply.
The optional USB input is sourced from M2Tech and is a HiFace asynchronous USB-S/PDIF converter which Metrum modifies to skip its standard USB bus power. There are a total of 6 inputs including 2 Toslink, 2 Coax S/PDIF (1x RCA and 1x BNC), an optional AES/EBU, and optional USB. All inputs handle up to 24/192 except Toslink which maxes out at 24/96. Outputs include single-ended RCAs and balanced XLRs. The RCA outputs sport a pair of Lundahl LL1588 output transformers, which is an upgraded option from Metrum but standard fare for US customers.
The USB input requires the installation of custom drivers for both Mac and PC users that Metrum provides on disc (which means the implementation is not USB 2.0 compliant). Once installed, you simply need to select "M2Tech HiFace" as your device in your media player of choice. I used Pure Music and Audirvana for this review running on my MacBook Pro. I also used the RCA outputs into my Leben CS300-XS as well the Hex's balanced outputs into my Pass INT-30A. I would guess that many a Hex user will couple their NOS DAC with tube-based gear (using NOS tubes) but I'll also point out that the INT-30A is a single-ended design while the Leben is P/P so you can decide for yourself which is the better theoretical match.
The Sound of Non-Oversampling
If you think a NOS DAC would announce itself in a big, bold, and obvious way, you'd be incorrect, at least in my experience. There's nothing that screams out "NOS!" in the sound of the Metrum Hex. Rather, its sound announces itself in a more subtle way. You may notice feeling more relaxed while listening and you may find yourself listening longer. And you may find yourself listening into the performance in a very natural way, as if you were hearing music performed right there in front of you, engrossed in the music-making as opposed to being fascinated by the sounds of the music being made. We can certainly theorize that this result is due to the NOS topology or to the Metrum's specific implementation thereof. I'd go with the latter since not every NOS DAC sounds alike.
I will say that the Hex DAC does not offer the most authoritative bass response through its RCA outputs. While bass is there, it is somewhat reticent sounding. So if you crave deep and boisterous bass, go with the Hex's balanced outputs otherwise I'm not so sure you'll be thrilled with the Hex. That being taken care of, in most other respects I'd expect you'd be as pleased as punch.
All music reproduction has an internal (and external) logic. This holds on a component level, a system level, and an in-room level. Some DACs offer up oodles of resolution so much so that this aspect of reproduction becomes a focus in and of itself. Other DACs may offer up a tipped up top end so music feels edgy and fleet of foot, or a thick and rich midrange for a lush and slower sound. This component-level logic then fits into your system logic, which fits into your room's logic and finally your senses and tastes go to town interpreting.
To my ears, in my system(s), in my room, the Metrum Hex offers up a very natural-sounding sound. Music feels like its made up of the stuff its made up of (I believe this is at least a paraphrase of one of my favorite writers on hi-fi, Herb Reichert). I would imagine that this has something to do with the harmonic structure of the sounds being delivered in a natural way so that each instrument's voice rings out true. Of greater importance, the inter-relationship of instruments, voices, and sounds feels natural. Organic. At no time does some aspect of reproduction shout out to shatter the magic of the musical moment. So there are no unnaturally bright violins shrieking, no sibilant singers ssss-ing, but there is plenty of tone, texture, and color. This presentation holds for CD-quality on up but it's certainly the case that better-sounding recordings sound better. It's worth noting that the Hex's frequency response is stated as 1Hz - 20 khz (-2.5dB) for 44.1kHz sample rates, and 1Hz - 65kHz (-3dB) for 192kHz sample rates.
Something like Larry Young's Unity from the HDtracks 24/192 download, mesmerizes with the stunning interplay between Young's B-3, Woody Shaw's trumpet, Joe Henderson's saxophone, and Elvin Jones' drums. The sense of time/timing is, for lack of a better word, right on. The Hex's internal logic is not machine-logic, rather it strikes me as being damn-close to pure music-logic. Things just sound right. So listening becomes extended listening and the more you listen the more you relax, taking in the music, more and more over time. This experience is much more important than individual components of sound although I wouldn't complain if the Hex offered up more authoritative bass response through its single-ended output.
Intimate music through the Hex was delivered with nearly uncanny intimacy. Mississippi Fred McDowell's self-titled 16/44.1 download from HDtracks sounded simply reach-out-and-get-touched real. The Hex delivered every ounce of emotion elicited from shaky strings and slide guitar, Fred's frighteningly robust vocals, his hacking cough, along with the people talking in the background all in perfect proportion. Acoustic music like Será Una Noche's sumptuous sounding 24/96 La Segunda from MA Recordings or the wonderful and surprising Altre Follie, 1500-1750 [Alia Vox Spain AV9844] from the ripped CD sound particularly lovely through the Hex with the voices of instruments given full voice for an emotionally charged presentation.
I took my reference Acoustic Plan DigiMaster DAC w/the upgraded linear power supply ($6,750, see Steve Plaskin's review) for a comparative spin since it shares the Hex's 24/192 capabilities and NOS approach while offering up different logic including a tube output stage. Here, through the DigiMaster's RCA output (it does not offer XLRs), bass response was clear and full and bettered the Hex's bass response via its RCA output. The DigiMaster's presentation is also more meaty and rich throughout the midrange. The overall presentation was even more relaxed perhaps due to less forward sounding upper frequencies and while I would not call the Hex forward-sounding per se, it does have a nice, crisp, and forthright top end whereas the DigiMaster's is softer and less lit up.
I also happen to have the Auralic Vega ($3,500) in for review and we really couldn't ask for a more opposite side of the DAC design coin. The Vega boasts a preamplifier, DSD playback, megahertz upsampling, and not only digital filtering but your choice of several filters. And I'd say the Vega is equally if not more involving albeit in a very different manner. The Vega digs in deep to uncover every single subtle nuance of the music being made, a micro view that sounds like you're hearing every bit in the recording. In comparison, the Hex offers up less of the micro-picture while still sounding perfectly resolute. I will talk more about the Vega in its very own review but I thought it important to point out that two very differing paths can lead you to the same place, namely musical enjoyment.
In terms of the Hex's single-ended and balanced output, while I enjoyed listening through both and will remind you that I mainly listened to the RCA output through my Leben integrated amp and the XLRs through my Pass integrated amp, bass response was noticeably better via the XLRs. I also compared the RCAs to the XLRs in the Pass and got the same results, better bass through the balanced outputs.
Does the Hex sound analog? Seeing as all DACs output an analog signal they all must sound analog. The nice thing about the Hex is it doesn't sound digital.
Natural Over Sampling
For PCM playback up to 24/192, the Metrum Acoustics Hex DAC delivers tons of musical magic. This magical aspect relates to its very natural and fatigue-free presentation that draws you into the music being made as opposed to keeping you at a distance with sound effects. While the Non-Oversampling approach isn't the only road to musical fulfillment, the Metrum Hex proves it can certainly be one way of getting there.
Also on hand and in use during the Metrum Acoustics Hex review: Mytek Stereo 192-DSD DAC, Teac UD-501, Fostex UD-501, Auralic Vega