Mastering Quality Formats and Audio Testing
The AK100 plays MP3, WMA, and OGG files, but that would be missing the point. Its real purpose is to play FLAC and WAV files, the former of which features lossless data compression that's inaudible to the human ear as a result, and the latter of which have no compression at all. As a result, for this player to make any sense, you'll need to purchase high-definition audio tracks.

To that end, Iriver has teamed up with HDtracks, a long-standing digital store selling uncompressed audio tracks from major labels. In addition to 24-bit lossless FLAC files at 44.1kHz or 48kHz, you can also buy ones recorded at 96kHz and even 192kHz—letting you take advantage of the mixing engineer's dream of direct-to-consumer files recorded, mixed, and mastered on Pro Tools HD or an equivalent high-resolution system.

That said, don't expect a comprehensive catalog at HDtracks. Many classic rock acts, as well as jazz and classical labels, are well represented. But if you like newer (as in the past 30 years) music, there's not a lot there. To cite just a few examples, there's nothing from Radiohead, The Police, Metallica, The Cure, Depeche Mode, or Nine Inch Nails. There's nothing from The Beatles or Pink Floyd either. There's one Muse and one Sting album. Almost everything I looked at required full album purchases, with no option to buy individual tracks, and at $16.99 to $24.99 a pop, albums are expensive. You can sync media with the AK100 either as a USB mass storage device, or with the bundled, PC-only Iriver software.

The AK100's Wolfson WM8740 24-bit DAC is well known in audiophile circles, as it's the same kind used in high-end Linn stereo components. And audio quality with the right material, as you can expect, is amazing on the AK100. Duke Ellington's "In a Mellow Tone" by Jackson, Hazeltine, Reedus, and Gill sounds like they're playing right in front of you in a small jazz club; each instrument sounds full, round, and, well, real. There's a beautiful sense of space in this 24-bit, 88kHz recording. "Hotel California" by the Eagles sounds almost ethereal and otherworldly at 24-bit and 192kHz. The soundstage is wide and deep, with plenty of air around the various guitars and Don Henley's voice. The acoustic guitar strings and snare drum grace notes in "Roundabout" by Yes are clearly audible at 24-bit and 96kHz.

Here's the thing, though: These recordings also sound fantastic as 16-bit, 44.1kHz lossless files ripped from regular CDs. You'll hear a pretty massive difference going from 256Kbps to lossless 16-bit, 44.1kHz, but much less of one going from lossless 16-bit, 44.1kHz to 24-bit, 96kHz or 192kHz. Classical and jazz recordings have the most to gain from increased resolution. But even with those, you'll hear much more of a difference when changing headphones, speakers, concert halls, mixing engineers or mastering engineers on particular recordings, even external DACs—just about everything else matters more than whether a lossless recording is 44.1kHz or 96kHz, especially when you're talking about the final mixdown format, and not individually recorded instrument tracks in a digital audio workstation.

That said, even regular lossless files at 16-bit and 44kHz sound great on the AK100, thanks to the quality of the Wolfson D/A converter. There's also a five-band graphic equalizer; many audiophiles sneer at using one, but it's useful in some situations. It's not parametric—you can't control the EQ points, but they're quite musical, at 60, 170, 310, 600, and 16,000Hz. The AK100 supports stereo Bluetooth streaming, so you can listen to music wirelessly, albeit at slightly compromised sound quality. Iriver recommends sticking with 44.1kHz or 48kHz files in this case, as larger ones will strain the wireless connection.

The 2,000mAh battery lasted for a solid 10 hours, 5 minutes on a continuous playback test while driving the Sennheiser HD 25 II headphone pair. But without an AC adapter, the player takes almost six hours to charge via USB, which is disappointing.