Astell&Kern AK100 portable media player

Apple's iPod came of age in the fall of 2003, when, with the release of iTunes 4.5, the player was no longer restricted to lossy compressed MP3 or AAC files. Instead, it could play uncompressed or losslessly compressed files with true "CD quality"; users no longer had to compromise sound quality to benefit from the iPod's convenience.

Ten years later, while the current top-model iPod features a 160GB hard drive, it still can play only files with sample rates of 48kHz and below and a maximum bit depth of 16. Those of us with a growing library of high-resolution files are therefore restricted to playing them in our big rigs at home.

Enter Astell&Kern. At the beginning of 2013, this brand from iRiver, a Korean portable media company, introduced its AK100, a portable player costing a dollar short of $700 and capable of handling 24-bit files with sample rates of up to 192kHz. (A&K calls hi-rez music Mastering Quality Sound or MQS.) All the usual file types are supported—WAV, FLAC, Ogg, APE, AAC, ALAC, AIFF, MP3—but when it was first introduced, the AK100 couldn't play Apple-format files, including those encoded with Apple's Lossless (ALAC) codec. This was a major problem for someone like me, whose iTunes library consists almost entirely of ALAC files. This shortcoming was addressed with an early 2013 firmware update (v.1.30), so I arranged for a review sample. Now I could take my hi-rez files on the road!

The AK100 . . .
. . . is a small and elegant-looking device, its electronics housed in a case of black-anodized aluminum with a hairline finish. A 2.4" color touchscreen grants access to the usual transport controls, as well as the ability to navigate the player's folders and files. The playback volume can be controlled with the touchscreen, but there's also a small volume wheel mounted on the side, to the right of the screen. On the left side, three pushbuttons duplicate the onscreen transport controls, while on the bottom are a micro USB port for connecting the AK100 to a PC and, under a slide cover, two Micro-SD memory-card slots.

A long push of the top-mounted button boots up the AK100. This button also wakes up the display when music is playing; a second push and hold turns the player off. (A cute "SEE YOU SOON" message appears on the display.) Also on the top panel is a 3.5mm stereo jack for connecting headphones, which will also, with an adapter (not included), accept a TosLink optical datalink, so that the AK100 can stream S/PDIF data to an external D/A processor. A second 3.5mm jack is used to send optical S/PDIF data to the AK100, again via an adapter, so it can operate as a standalone DAC.

When music is playing, touching a hexagonal bolt-head icon on the bottom right of the screen brings up a menu. The Info icon shows you the filename and type, the bit and sample rates, and the file size. The Equalizer icon brings up an On/Off button for the equalizer; pressing it on brings up 5 EQ bands that can be boosted or cut with a simple swipe of the finger; pressing on each band brings up a submenu that lets you choose that band's center frequency—the choices are 62Hz, 250Hz, 1kHz, 4kHz, and 16kHz—and further adjust the boost or cut in 1dB steps from –10dB to +10dB . A + icon allows you to add files to a new playlist. And with two more icons, you can select loop or shuffle play.

The AK100 comes with 32GB of internal flash memory. While this may sound like a lot of storage, remember that hi-rez files are large. Whereas a lossless-compressed CD album in ALAC and FLAC formats needs 200–300MB of space, its 24/96 equivalent will need 800–900MB, its 24/192 version 1.2–1.5GB. Fortunately, the AK100's internal storage can be supplemented with one or two 32GB Micro-SD memory cards (the SanDisk and Transcend brands are recommended by Astell&Kern), to give a maximum possible storage of 96GB. Each card appears as a separate drive when you connect the AK100 to your computer and you drag'n'drop music files to it in the usual manner.

The AK100 is said to be the first portable audio device to use Wolfson's WM8740 DAC chip. The WM8740 is a two-channel sigma-delta part that supports data input word lengths from 16 to 24 bits and sampling rates up to 192kHz, and includes digitally controllable mute and attenuator functions. According to its datasheet, the WM8740 offers a reconstruction filter with sharp or slow rolloffs, but Astell&Kern don't specify which they use in the AK100.

Setup
Transferring music to the AK100 is straightforward. PC users can run the iRiver plus4 software, which can manage multiple multimedia files and save them to the AK100, as well as automatically upgrading the player's firmware. Macintosh users connect the Astell&Kern to their Mac with the supplied USB cable and choose "Connect Removable Disk" when that's shown on the touchscreen. ("Charge Battery" is also an option.) The AK100 appears as an external drive on the desktop, and the message "USB Connected" is shown on the touchscreen. Music files can be dragged'n'dropped into the pre-existing "Music" folder. (Unless these are WAV files, the metadata and cover art travel with the audio.) When the AK100 is disconnected, choosing "Settings" on the touchscreen, then pressing "Advanced," then "Auto Library Scan" sorts the files by the associated metadata. Hi-rez files are also added to the MQS playlist during this process.

When you navigate to the Artist folder with the onscreen menu, you can select either each album, in which case the songs will play in the original order, or "All Songs," to play all of the songs on all that artist's albums in alphabetical order. The touchscreen has a little more latency in its response than iPads and iPhones, I found.

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As the AK100 isn't supplied with a battery charger, it needs to be connected to the user's PC for its internal battery to be charged. It takes just over five hours for the battery to be completely charged, and the battery life is specified as "up to 16 hours." (A&K say that playing hi-rez files takes more power than CD-quality files.) Playing all sorts of files, I got between 12 and 16 hours per charge.

The AK100 has a Bluetooth function, hwich I assumed would allow it to act as a Bluetooth DAC, though one limited to sample rates of 48kHz and lower. It proved easy to pair with my iPhone 3GS via Bluetooth. However, for some reason, I wasn't able to stream audio to the AK100 from my phone. Astell&Kern subsequently let me know that the Bluetooth function isn't for streaming, but to allow users to hear their phone ringing.

A question of impedance
The Astell&Kern website lists five headphones that they recommend for use with the AK100: the Denon AH-D7100, Audio-Technica ATH-W3000ANV, Beyerdynamic DT1350, and Shure SRH840 and SE425. I don't have any of those, so to assess the AK100 I used the four sets of headphones I regularly use: two over-the-ear models, the Sennheiser HD650 and Sony MDR-7506; and two in-ear monitors, JH Labs' JH16 Pro and Ultimate Ears' 18 Pro. As you can see from the measurements sidebar, the AK100 has an output impedance of 22 ohms. This is significantly higher than iPods and iPhones and will be on the high side for many headphones. With a headphone having an impedance of the same 22 ohms, for example, the output voltage will be halved. And the frequency responses of headphones whose impedance varies with frequency—all of them!—will be modified by the interaction between this high source impedance and those of the headphones.

For example, if you look at the measured impedance of the JH16 Pro, this averages 13 ohms in the lower midrange and bass, rising to 35 ohms at 2kHz, then dropping to 12 ohms at 7kHz. This will modify the headphones' frequency response by –2.6dB in the bass and midrange, +1.8dB in the low treble, then –3dB in the high treble. Similarly, the Ultimate Ears 18 Pro average 18 ohms in the lower midrange and bass, and 11 ohms between 8 and 10kHz, rising to 30 ohms in the low treble. With both of these in-ear monitors, this variation in impedance will shelve down the lower mids and bass compared with the low treble when driven by the AK100—but more so with the JH16 Pro than with the 18 Pro, where the bass and lower midrange will be suppressed by 0.9dB and the low treble emphasized by 1.2dB.

Company Info
Astell&Kern, Korea
US distributor: iRiver Inc.
39 Peters Canyon Road
Irvine, CA 92606
(949) 336-4540
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Comments
gld3gld3's picture
"Ten years later, while the

"Ten years later, while the current top-model iPod features a 160GB hard drive, it still can play only files with sample rates of 48kHz and below and a maximum bit depth of 16. Those of us with a growing library of high-resolution files are therefore restricted to playing them in our big rigs at home."

Not exactly. There is at least one option for those wanting to play higher sample rate/higher bit depth files on their ipods.

My 5th gen ipod is currently playing "Happlessness Blues" by Fleet Foxes at  sample rates of 96kHz  and bit depth of 24 (FLAC). How? Rockbox. 

John Atkinson's picture
Rockbox

gld3gld3 wrote:
My 5th gen ipod is currently playing "Happlessness Blues" by Fleet Foxes at  sample rates of 96kHz  and bit depth of 24 (FLAC). How? Rockbox.

Rockbox does indeed add a lot of functionality to iPods (and other portable players). However, it can't overcome the hardware limitations of the player. Even if you can play a 24/96 files, as you report, the iPod's DAC chip is limited to sample rates of 48kHz and below, so the file will be downsampled.

In additon, while Apple's AAC and Lossless codes will preserve 24-bit resolution, the iPod's DAC is a 16-bit part, so will truncate the word length by the 8 LSBs. For example, if you look at the spectrum at www.stereophile.com/content/ipod-classic-160gb-1khz-90dbfs-16-bit, this shows my 2010 iPod Classic playing a dithered 16-bit tone at -90dBFS. Everything is correct, and the noisefloor is free from harmonic spuriae.

By contrast, the spectrum at www.stereophile.com/content/ipod-classic-160gb-1khz-90dbfs-24-bit shows what happens when the iPod plays back a 24-bit dithered tone at the same level encoded with the ALAC codec. The appearance of the odd-order harmonics is due to truncation of the 24 bits to 16.

So yes, it may be possible to play 24/96 files on this iPod running the Rockbox software, but the file will be downsampled and truncated. The AK100 doesn't suffer from these hardware limitations.

John Atkinson
Editor, Stereophile

jcborges's picture
iPod 24-bit

How were your measurements of the iPod taken? I ask because looking at the specs of the iPod DAC at http://cirrus.com/en/products/cs42l55.html it is definitely capable of 24-bit, although it is limited to 48khz sampling frequency.

Why would Apple put a 24-bit capable DAC in the iPod and have it truncate to 16? And how could it be done? And couldn't Rockbox bypass that to access the full bit depth available on the chipset?

Thanks and regards

John Atkinson's picture
Re: iPod 24-bit

jcborges wrote:
How were your measurements of the iPod taken? I ask because looking at the specs of the iPod DAC at http://cirrus.com/en/products/cs42l55.html it is definitely capable of 24-bit, although it is limited to 48khz sampling frequency.

I took a 24-bit AIFF file representing a dithered 1kHz tone at -90dBFS and prepared from it WAV, ALAC, and AAC at 320kbps files. The WAV file wouldn't play in either my iPod Classic 160GB or my iPhone 3GS, but from the other three versions I expected to get a spectrum that looked like the red and blue traces in fig.5 at www.stereophile.com/content/astellampkern-ak100-portable-media-player-measurements.

Instead, from both players and with AIFF, ALAC, and AAC versions of the data, I got the spectrum shown at www.stereophile.com/content/ipod-classic-160gb-1khz-90dbfs-24-bit. The characteristic pattern of odd-order harmonics indicates truncation of the 24-bit data to 16. By contrast, the AK100 playing both the AIFF and ALAC versions of the data gave the correct spectrum shown in the review.

jcborges wrote:
Why would Apple put a 24-bit capable DAC in the iPod and have it truncate to 16?

I have no idea. This was not what I was expecting, as the new "Mastered for iPod" protocol recommends using a 24-bit master to prepare the AAC version, which in turn implies a 24-bit datapath..

jcborges wrote:
And how could it be done? And couldn't Rockbox bypass that to access the full bit depth available on the chipset?

That is possible, but as you point out, the chipset is still restricted to sample rates of 48kHz and below, contrary to what the original poster claimed.

John Atkinson
Editor, Stereophile

jcborges's picture
iPod DIGITAL playback

First, thanks a lot for your answers. But you left me wondering about something. 

If understood correctly, your measurements were made in the analog domain. What about digital?

If you play a 24bit/48khz ALAC file through a digital dock, what is the format of the PCM stream that comes out? 24-bit or 16-bit truncated? Have you ever checked or know someone who has, for example by connecting it do a DAC that shows the input format?

In the other words, if you have 24/96 files that you want to put in an iPod to playback digitally, should you just resample them to 48khz or should you apply TPDF dither to avoid truncation? Does it matter? 

 Thanks and regards

John Atkinson's picture
Digital Output

jcborges wrote:
If you play a 24bit/48khz ALAC file through a digital dock, what is the format of the PCM stream that comes out? 24-bit or 16-bit truncated?

If you play a 24-bit AIF or WAV file, the AK-100's digital port outputs 24-bit data. But I will check with ALAC data.

John Atkinson

Editor, Stereophile

John Atkinson's picture
A clean bill of digital health

John Atkinson wrote:
jcborges wrote:
If you play a 24bit/48khz ALAC file through a digital dock, what is the format of the PCM stream that comes out? 24-bit or 16-bit truncated?
If you play a 24-bit AIF or WAV file, the AK-100's digital port outputs 24-bit data. But I will check with ALAC data.

Okay, played back a dithered 24-bit 1kHz tone at -90dBFS, encoded both as AIFF data and as ALAC data. In both cases, all 24 bits were active in the AK100's optical output and the digital-domain spectra of the two codecs were identical, with a noisefloor at the 24-bit level.

So the answer to your question is yes, when the AK100 plays a 24-bit ALAC file, the digital output is true 24-bit as it should be.

John Atkinson

Editor, Stereophile

JohnnyR's picture
What a Bargain

*eyeroll*

So having a player that has crappy high output impedance and doesn't have gapless playback is a good thing? For the money I would expect a lot better. What's wrong with using FLAC in other players that cost less and have gapless playback to boot?

Another laughable product that Atkinson likes.cheeky

John Atkinson's picture
Re: What a Bargain

JohnnyR wrote:
So having a player that has crappy high output impedance and doesn't have gapless playback is a good thing?

As I wrote, the most recent firmware upgrade allows gapless playback. I examined the effect of the 22 ohm output impedance at length in my review - "crappy" is a mischaracterization of the relatively small changes in frequency response that result from this impedance, especially when you consider that many headphones and in-ear monitors have boosted low frequencies.

John Atkinson

Editor, Stereophile

JohnnyR's picture
Quit Defending High End Crapola

For $699 you should get a lot better response and features. Sorry but changes from 1 to 2 dB would be scandalous in a preamp or power amp but since it's this THING then it's "okay". oh brother.blush Seriously, lower priced players don't have this problem so it's a FLAW easily avoided if they had designed it right.

ChrisS's picture
Because Uncle Joseph says so!

Comrade,

There must be something about your Stalinist upbringing that makes you believe that the perfect audio product exists somewhere out there, when really that kind of perfection only exists in your imagination.

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