Listening in: With headphones
I then tested all the products using headphones in order to evaluate each unit's combination of DAC and headphone amplifier. Listening through the JH13 in-hear headphones, I came to similar conclusions in terms of sound quality and relative rankings, although the differences between products weren't as pronounced as they were through my stereo. I did notice looser bass in the D3, DACport, and Explorer, likely due to the relatively high impedances of these units in combination with the relatively low impedance of the JH13. However, these changes didn't alter my overall preferences.
The AKG K701 is less picky about impedance than the JH13, but more picky about amplifier power and quality. Most of my comments in the previous section hold, though the DACport and rPAC's better headphone amplifiers assert themselves with the K701 — both made these headphones sound more lively, offering tighter bass and better definition in the mids and highs. With the K701, the DACport and the rPAC best even the Explorer — keep this in mind if you're using full-size headphones that are difficult to drive.
As I mentioned, the Explorer has an optical output, which can be useful for bringing an optical-audio output to computers (such as the MacBook Air) that lack it — or, from the opposite perspective, for adding high-quality USB compatibility to other DACs that lack this feature (or have older, lower-quality USB inputs). I used the Explorer to ferry USB audio into my full-size DacMagic's optical port, and I then compared the result to a direct optical-to-optical connection between my Mac and the full-size DacMagic. I detected no difference in audio quality between the two configurations. In contrast, I find the DacMagic's USB input lacking compared to its optical port, so I also compared the full-size DACmagic's built-in USB input to the Explorer/DacMagic combo: In this configuration, the Explorer handily bested the full-size DacMagic's mediocre USB input — USB audio interfaces have improved substantially in the last five years! If you have a use for it, the Explorer's USB-to-optical bridge feature is a great way to ensure that a high-quality but USB-deficient DAC remains useful.
Phones and tablets
Finally, I tested the MicroStreamer (running the new, lower-power firmware) with an iPad mini, an iPad mini with Retina Display, an iPhone 5, and a third-generation iPad using Apple's iPad Camera Connection Kit and Lightning to USB Camera Adapter. All three sounded great when listening to high-quality audio files, handily besting the devices' built-in audio circuitry. And the MicroStreamer is portable enough that I've made extensive use of this capability. I was even able to use iTunes Home Sharing to stream 96 kHz/24-bit files from my Mac to the iPad mini under iOS 6. (Unfortunately, under iOS 7 — at the time of this writing, iOS 7.0.4 — playback stuttered, which I'm assuming is a bug in the OS.) I did notice that the MicroStreamer's power draw had some impact on battery life: I lost a couple hours of playback time on my iPhone when using the MicroStreamer in this way.
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