发布时间 : 2015-12-14aune M2: Acoustically superb, but a work-in-progress

  Pros: Great-sounding, articulate DAP; simple UI and operation; automatic gain adjustment. Did I mention great sound?
  Cons: Missing basics like external DAC, digital out, cover art; gapless limited to CUE files; ALAC/AAC not (yet) supported.
  I've participated in a couple of review tours for Aune products (the B1 portable headphone amplifier and the X1S desktop DAC) in the last several months, and really enjoy the clean, neutral Aune house sound (or at least what I believe to be their house sound, based on limited exposure).

  In early October, having seen several posts and reports of the upcoming M2 DAP, I took a chance and asked if there was any chance of a review unit. As luck would have it, Aune was just wrapping up development of the hardware at that time, and they asked me if I'd like to help troubleshoot and beta test the software. I jumped at the chance, and a couple of weeks later was in possession of a lovely blue (BLUE!) pre-production base-model M2. A few weeks later I received a second M2 (pre-production Pro model this time) as a sample for a Head-Fi meet I organized in Calgary, and to send out on tour once firmware development nears completion.

  There are three different versions of the M2, as follows:
  M2 (US$349): The "standard" model, with 2x Japanese NDK 2520 ultra-low-jitter crystals.
  M2 Pro (US$499): With 2x USA Crystek CCHD575 crystals, specially tuned OP-amp combination.
  M2s (US$649): Upgrade of M2 Pro. Same spec as M2 Pro, but with different colors, and Satellite-level, Teflon PCB substrate.
  Base model M2 (Blue) and M2 Pro (Black)
  Over the course of the last couple of months I've listened to both M2 units quite a lot, and have seen the firmware go through around 10 iterations, with improvements each time. One thing that has remained constant it the sound quality of the device, which in my opinion is excellent.
  Disclaimer: While I have assisted Aune in refining the M2 firmware, I am not affiliated with the company and have not benefitted monetarily from my participation in any way. This review represents my honest, independent opinions about the M2.
  Some Reviewer Context (Click to show)
  The Aune M2
  Let's dive into the review with physical attributes and specifications.
  Unboxing & Design
  The M2 comes in an understated yet sturdy dark blue cardboard box, similar to the one that for the B1 amp. That makes sense as the two have almost identical dimensions. The first M2 I received came sans accessories, and the second (the M2 Pro) came with a USB-micro USB cable for charging.
  The M2 itself is a conservative looking digital player, with a chassis constructed from hollowed 6-axis aircraft aluminum. The anodized face-plate has a brushed metal look, with laser-engraved text, and other surfaces are a matte-finished metal. The first M2 I received has rather sharp edges (this one seemed more of a prototype) while the M2 Pro edges are less sharp (though I'd prefer a bit more curve).
  Taking a tour of the M2 externals we find:
  Front: As mentioned, a brushed metal surface with matte bevelled edges. At the top end of the unit the word "aune" is etched into the surface, and at the bottom "M2 32Bit DSD Music Player." The 2.4" IPS display takes up most of the top half of the unit, and below the screen are three buttons: Back, Play/Pause/Select, and Forward.
  Rear: The back of the M2 is flat, with a matte finish. There is no functionality here. The brand, model and serial number appear, and I would assume there will be additional standards information on production units.
  Top End: Looked at from above, we see the power button on the left. A long push turns the unit on and off, while a short push turns the screen off and on during use.
  Bottom end: Looked at from below, from left to right we have the 3.5mm headphone out jack, 3.5mm line-out jack, and micro-USB charging jack. This jack does not double as a sync port.
  Left side: The left side has no controls or design elements.
  Right side: Looked at from the right, we see from left to right a reset control opening (use pin or paper clip), TF slot (micro SD, 128 GB supported), jog dial for volume control (and back/forward control when screen locked), and a multi-function expansion port (with rubber port cover). The jog dial is etched with a grid pattern to provide texture and grip when turning; however, the etching is quite fine, so it's easy to slip when adjusting the dial (I'd prefer a slightly coarser, rougher etching for better grip and sense of control). The multi-function, Mini HDMI expansion port doesn't currently do anything; functionality will be added in future for both data transfer and digital output.

  Selected Specifications:
  Headphone Out:
  Output Impedance: 1Ω
  THN+N: 0.0009% @ 300Ω
  Peak power: 32Ω/230MW
  Line Out:
  THN+N: 0.00038% (24bit/44.1k 1KHZ 0DB)
  DNR: 117DB
  CCIF: -118DB
  Outputs:3.5 mm Headphone, 3.5mm Line
  Amplifier:ARM + CPLD + AK4490 + High Bias amplification
  Volume Control:0-100, jog-dial controlled
  Gain Selection:Gain adjust adapts to headphones/earphones used
  Storage:1x micro-SD card (up to 128GB, FAT32), no internal storage
  Supported Audio Formats:
  DSD: DSF/DSD64, DFF/DSD64, ISO Whole Track
  WAV: 16/24/32 Bit, 44.1K/48K/88.2K/96K/176.4K/192K WAV+CUE
  FLAC: 16/24 Bit, 44.1K/48K/88.2K/96K/176.4K/192K
  Battery Capacity:3500 MA
  Battery Life:7.5+ hours* (Class A, continuous play)
  Charging Time:4.5 hours at 800 MA (charger not provided)
  Power Saving:Auto Power Off, Backlight time off, Breakpoint Resume
  Dimensions:120mm x 65mm x 17.5mm
  * The pre-production unit I tested was rated at 6.5 hours battery life. I've been informed by Aune that this has been increased to 7.5 hours+ (Dec. 8, 2015)

  My Experience with the M2
  The M2 has what is (to me) an unusual arrangement of controls. I'm most familiar with the Fiio DAPs (X3ii and X5ii), which I find quite intuitive and easy to operate, and have also had experience with the Hidizs AP100 (controls sometimes screw with my head), Cayin N5 (intuitive), Shanling M2 (intuitive), and Cayin N6 (confusing physical user interface).
  It didn't take long to get used to the Aune M2 controls, and I found the uncomplicated system relatively easy to navigate. I find the UIs used by Fiio, Cayin and Shanling very similar, and indeed I've read that they share common base programming. The M2 software is clearly built from the ground up, and is also a work in progress. The system is common to the Aune desktop X5S and S18 audio transports, and I imagine as one item's software evolves, so too will the others.
  My only real issue with the UI was sometimes pushing a button or the dial (particularly the dial) a little longer than I should have for a short push, or a little shorter than required for a long one, and so making a move I hadn't intended. The Quick Start graphic below explains pretty well the different button and dial controls and combinations for various functionality.
  As mentioned, the M2 firmware is pretty basic. This makes it easy to navigate, but also means that features available with other players are not available (I'll deal with that shortly). The home screen is laid out simply, with 4 icons showing the buckets of features as follows:

  Songs: Play songs. All your songs. Set the player to random play and let rip.
  Folders: As with most other players, folder browsing by Artist > Album > Song (or however the user has their folders laid out).
  Playlists: Most Played (player defined) and Favorites (user defined).
  Settings: Controls for start-up auto-play (on/off), play mode (folder, shuffle, repeat), filter mode (Sharp SD, Sharp LD, Slow SD, Slow LD), backlight brightness (1-7), auto power off timer, backlight timer, gapless play (on/off), operation tips (on for newbie, off once you're a seasoned user and know the controls), language (Chinese, Simplified Chinese or English), settings restore, system upgrade (or update), system info and lock setting (control functions for buttons and jog dial when screen is locked).

  In terms of features, the M2 is pretty bare-bones compared to some of the other digital audio players in the same price category (like the Fiio X5ii and Cayin N5). It seems Aune have decided to keep things clean and simple, and focus attention on audio quality. My observations on M2 functionality include:
  Deep sleep/hibernation/deep standby mode: While there is no hibernation function, the M2 boots extremely quickly (due I'm sure to the very simple software). With a quick boot (less than a couple of seconds?) and starting up at the last song played before powering down, a deep sleep feature, though "missing," is forgiveable.
  Protective case: I think every DAP should come with some kind of case or pouch, especially when you get into the $250+ range. The M2 doesn't have a case, which to me is a shame. Even a simple silicon slip-on cover would help protect one's M2 investment from nicks and scratches.
  USB connection to PC for data transfer: At the moment there is no way to connect the M2 to a PC to load music as the Micro USB port is only for charging and the expansion port is purely cosmetic (for now). Not a huge deal as I just removed the micro SD card whenever I wanted to load new music or make other library changes (this is how I do it even when USB data transfer IS possible as it's faster).
  External DAC: The M2 can't be used as an external DAC. For a portable device, that's a shame. When I'm on the road (which is frequently) I like to pack a portable DAC for use with my laptop for movies or Netflix, but I'd prefer not to have to pack two devices (DAP and DAC). I can imagine gamers might also appreciate being able to use the M2 as a DAC. Considering the quality of sound coming from the M2, it's a real shame that DAC functionality isn't included.
  Coaxial Out: The M2 is clearly designed as a small-format, all-in-one audio device. There is currently no digital output with the M2. The product literature states, "With… multi-format support and superb reproduction of the original live event/recording sessions, the M2 is your mobile concert hall that liberates you from the large-size HiFi system." For my purposes, the lack of Coax is not a huge deal as the quality of sound coming from the device is very good, and I'm quite happy to connect to an external amp (like the B1) via line out, if and when necessary. For anyone wanting to use the M2 purely as a transport and hook up digitally to a desktop DAC, that isn't an option at the moment (though that will change when software is developed further and the expansion port goes live). ?EDIT (Dec. 8, 2015) Aune are currently working on an external docking connect through the Mini HDMI port, which will bring digital output capabilities (Coax, AES, I2S, etc.) via a receiver module to high end DAC.

  Playlists and browsing categories: As mentioned already, playlist support is limited to favorites (tracks can be added and deleted by the user), most played, and all tracks. There doesn't appear to be support for multiple playlists, or for imported lists. Browse by artist, genre, composer, etc. is also not currently supported.
  Album Cover Art: This is another feature that is not yet supported, and it's not clear whether it will be at some point in the future. Would be nice to have, but as with other features may be a required sacrifice/compromise in the interest of sound quality.
  Support for all music formats: Product literature lists most of the commonly supported music formats as supported. I used a variety of source files during my testing, and while most of them played fine, I did run into some issues. I had no problems with DSD and FLAC files, which sounded superb. I did have issues with AAC and ALAC files, however. With the latest firmware I tested (FW 1.10), both AAC (256 kbps and 320 kbps) and Apple Lossless tracks would only play if they were shorter than 7'30". Longer tracks were skipped, which meant I was unable to listen to most classical and a lot of my jazz and '70s prog-rock tracks. For the tracks that did play, sound quality was excellent, but occasionally songs would stop playing midway through and the player would skip to the next song. For a Mac+iTunes user, this means some music will be unplayable, unless music files are converted to MP3 or FLAC, resulting in added complication of conversion tools, a non-iTunes media player, and otherwise unnecessary usage of hard drive space.
  Gapless playback: This feature is listed in product literature as supported, but currently warrants an asterisk as it is limited to single-file albums with CUE. Most of my music consists of album folders containing individual files for each track, and in this set-up, gapless does not work. I am told that full gapless support is being worked on for future firmware releases, but for now this again means an added complication of file and format conversions.
  Equalizer: There is no equalizer control available. I don't as a rule use EQ, so for me this is not a big miss, but for those who do it could be a deal-breaker.
  Battery: The M2 is rated at 6.5 hours of playing time, which is on the short side. I didn't test this carefully, but I did feel the battery ran out rather quickly compared to my other DAPs. The M2 does get warm during extended use (though not overly so), so it's clearly using some juice! ?EDIT (Dec. 8, 2015) Aune states that the M2 is Class A music player, with voltage greater than 500MA, and that a compromise had to be made between a Class A/B device with 8 hours of playing time, and Class A with 6.5hours. The decision was made to sacrifice playing time for better sound quality. With daily charging, 6.5 hours battery life should be enough for regular use, and external power packs can be used to extend this if necessary. Several hours later I got another another update saying that the battery life on the production models has been upped to 7.5+ hours. Not clear yet if that's a FW tweak, or if they've done something with hardware.

  The M2 Sound
  If it seemed I was dissatisfied with the M2 as a product at this stage, I wouldn't be surprised and you'd be forgiven for thinking so. While there are deficiencies in comparison to some other popular, more full-functioned DAPS, and I eagerly await introduction of and improvement on features listed as "supported," I can say that I am willing to forgive some of my gripes purely on the basis of sound quality. I very much like the sound of the Aune M2 and that's what I'll look at next.
  Overall I find the M2 has great balance, detail and texture. It's sonically revealing and has a quite neutral character.
  Bass: Bass goes low, has good slam and dynamics, nice reverb and ambience. Speed is excellent, as is bass definition. Sub-bass is present, but not as deep as some other DAPs.
  Mids and Highs: Neither forward nor recessed, detailed and layered, clear, good extension and no issues with sibilance. Balance well with bass for even, natural presentation.
  Vocals: As with mids and highs, M2 is revealing of texture in vocals. Micro detail of vocal signatures portrayed accurately and articulately. Both male and female vocals shone in their balance with rest of music.
  Filters: The M2 has four DAC filter settings: Sharp SD (short decay), Sharp LD (long decay), Slow SD, and Slow LD. I didn't play around with the filter settings too much, but was able to hear differences, particularly between Sharp SD (drier, more detailed) and Slow LD (warmer, fuller). Most of my listening and all of my comparisons to other DAPs was done with the Sharp SD filter engaged.
  Detail: Definition of individual instruments and voices is excellent, with tight, textured detail and layering. Excellent clarity, no muddiness or bleed in complex, energetic pieces or passages. Transients are well defined from bass through treble, verging on dryness without being dry (with subtle warmth added with Slow LD filter).
  Soundstage and Imaging: I find soundstage depends a lot on the headphones or IEMs used, but the M2 has a good sense of space and depth and accurate positioning. Switching filters allows for some control here, with Sharp SD filter giving a drier presentation with less ambience, and Slow LD warmer and more atmospheric.
  Hiss: I didn't notice any hiss with the IEMs I used. Background is nice and dark.
  Power: When I first started playing with the M2, I was a bit concerned about its power handling capabilities as even with relatively sensitive IEMs I had the volume set to around half way (45-55/100 depending on IEM). You'll notice in the spec's that "gain adjust adapts to headphones/earphones used," and I was pleasantly surprised to find that the M2 was able to drive quite powerful headphones with some authority. I tried the M2 out with my 470-ohm Audio Technica R70x and my AKG K7XX, in addition to loaner ZMF Omni I've been enjoying for a while, and with all three I got decent volume and sound quality (though the R70x sounded just a tad thin, and with all I had to push the volume up to 100/100). Adding and stacking with the very capable Aune B1 portable amp (see @nmatheis' excellent review of the updated 2016 edition here) took things to another level, and this is a combination I highly recommend.

  Sound Comparisons with Other DAPs
  I compared the M2 and M2 Pro to a number of players, from the affordable but capable Shanling M2 through mid-tier Fiio X5ii and Cayin N5, and on to higher end Fiio X7 and Cayin N6. Comparisons were performed using my handy Line5 headphone output switcher, which allows me to volume match, synchronize music on two DAPs, and then switch back and forth while playing without having to unplug and/or plug anything in. Instant switching while tracks continue to play allows easier identification of sound differences as there's virtually no memory lag to deal with.
  One of the things I've noticed as I experience a range of audio equipment is that sound differences between DAPs are far more subtle than between headphones and amplifiers, particularly within a given price tier. Quality across the board is becoming better and better, and often differences will only become apparent with focused listening.
  Another note about sound comparisons described is that less or more of something isn't necessarily a bad thing. DAP-A could have more or better sub-bass, for example, but that doesn't mean DAP-B's sub-bass is poor. So take the comparisons with a grain of salt.
  For non-sonic differences, all the DAPs I compared can be used as external DACs, have digital out, and offer gapless functionality and ALAC/AAC support. I won't spend any more time on differences in feature sets, but sonically here's a summary of how various models stacked up (detailed listening notes appear after the summaries):
  Aune M2 (base model) vs. Aune M2 Pro: Both are engaging, with balance, detail, texture. Sonically revealing and neutral. The Pro has a hard-to-define, subtle lustre to its sound that is missing in the base model. Pro is very slightly bigger sounding, dynamic range and instrumental clarity just a hair enhanced, but other than that everything is the same.
  Aune M2 (base model) vs. Cayin N5: I didn't have a lot of time to compare these two as the M2 came in on the same day I shipped out the N5 to the next participant in that review tour. I found the M2 bass attack marginally quicker than N5, and sub-bass a touch deeper. Shorter decay in mids and highs gives better definition to notes and instruments. Vocals are cleaner, more textured and detailed With N5, bass softer-edged with longer decay, adding body and feeling of space. Longer decay in mids and highs adds ambience and contributes to warmer presentation.
  Aune M2 Pro vs. FiiO X5ii: M2 transients quicker, leading and trailing edges of sounds clearer. Bass less prominent, mids and highs more forward. Vocals brighter, more detailed, layered and textured. X5ii bass rounder, deeper sub-bass, more reverb, more impactful than M2. Mids comparatively recessed. M2 better balanced across frequencies; with X5ii bass dominated overall presentation.
  Aune M2 Pro vs. Cayin N6: M2 Pro is definitely at N6 level SQ-wise, though slightly warmer. Decay on bass and mids fractionally longer, giving fuller, more ambient atmosphere. N6 not dry, but drier. M2 Pro not warm, but warmer. Marginal differences. No real preference, but taking price into account have to say M2 Pro represents better value.
  Aune M2 Pro vs. Shanling M2: Shanling M2 is lower key, shy but capable. Doesn't draw attention, but neutral clarity and balance. Aune M2 Pro weightier and meatier, fuller. Bass tighter, deeper, greater impact… more present but not forward. More dynamic and extroverted, greater extension, warmer (but still not warm).
  Aune M2 Pro vs. Fiio X7: M2 Pro subtly cleaner and more detailed. Bass depth similar to X7, but quicker, less reverb and more plucky entry. Mids and highs very similar to both in extension and detail. M2 Pro slightly less warm, tighter and more detailed in mids. Differences are quite minimal and, to me, focused on the bass end. While sonically similar, the X7 boasts numerous features not shared by the M2 in addition to the ones mentioned already, including touch screen, wireless and streaming (albeit at a higher price).
  Detailed Listening Notes (Click to show)
  It should be clear by now that I really like the way the Aune M2 sounds (both base and Pro models, but particularly the Pro). I feel it outclasses most of the other DAPs I compared it to, particularly in speed (transients, in bass particularly but throughout the spectrum), and detail, texture and layering. I'd place the M2 (Pro anyways) at a similar level as the Cayin N6 in sound quality.
  While there's much to praise with the sound of the M2, I feel it's very much still a work in progress. Some functions that are standard in most other DAPs have been left out, or are still under development. The main unresolved issues and missing features include:
  ALAC and AAC playback not supported: Longer songs won't play, and there can be random instances where a song just stops and the player skips to the next one (for some reasons, the M2 doesn't like my ALAC rip of Fleetwood Mac's "Rumours.") To me, this makes the M2 an unsuitable DAP for anyone using iTunes as a primary music source (on PC or Mac) as converting song files to FLAC or MP3 is inconvenient and overly complicated. Aune tells me ALAC and AAC are being worked on, so this is an issue that may be resolved in future FW updates.
  Limited gapless support: Gapless works a treat, IF you're using CUE files with an entire album ripped to a single sound file. If, like me, you rip songs to individual files and place them in an album folder, gapless won't work. I tested this with DSD, FLAC and ALAC, and all of them had gaps between songs. Word from Aune is that this issue will be addressed.
  No digital in or out: If you're looking to use the M2 as an external DAC, it can't be done. If you want to use it as a digital transport hooked up to another DAC, you're also out of luck (for now). The expansion Mini HDMI port was included to provide digital transfer capabilities in the future, via some sort of docking station or receiver module, but what that will be like is currently unclear.
  Limited playlist support: Playlists are limited to favourites, most played, all songs, and albums (via folder browsing). Tagged browsing (by artist, composer, genre, etc.) is not supported, though could easily be implemented.
  As it stands, the M2 represents a superb DAP for a user with these criteria: 1) All music is in DSD, FLAC or MP3 format, 2) gapless albums use CUE files, 3) uses folder browsing only, and 4) has no need to connect a DAP digitally to other equipment.
  For anyone else, the M2 is a DAP to watch and consider as issues are resolved and functionality is added. I for one WILL be watching as I see the M2 as a DAP with huge potential, and will return to edit this review (hopefully with pluses) as the M2 develops and improves.
  My thanks to Aune for the opportunity to try out the M2 (both base and "Pro" models), and provide input during pre-production development. Thanks for reading, and I hope my feedback has been helpful.