Say you want a truly portable high-rez system. The real difficulty you'll run into running a portable DAC with your iPad is power — or lack of it. The iPad 2 doesn't supply sufficient power for most of today's inexpensive DAC/headphone amp combinations — which otherwise make great traveling companions for the high-fi tablet.
You can easily interface with a powered desktop setup like our Musical Fidelity pair, or you can choose a battery-powered DAC/headphone amp combo. There are quite a few around these days — HeadAmp's Pico is a sought-after choice — though few of them do 24/96 (I haven't yet had a chance to check out Furutech's new Cruise in this application, but it looks promising.
If you want the widest range of compatibility (and in particular if you're looking for something that's both portable and supports 96 kHz playback over USB, or to make a unit you're already using on the desktop do double duty), you'll want to find some way to supply additional power over USB along with your data stream.
We used NuForce's uDAC-2 ($129, nuforce.com), and CEntrance's DACPort ($399.95, centrance.com), two tiny DAC/headphone amp combos, settling on the CEntrance for the majority of our testing. The relatively inexpensive, pint-sized unit is USB-powered, supports 24/96 playback over USB, and includes a good-sounding and surprisingly powerful Class A headphone amplification stage — a nice traveling companion all around. The uDAC-2 is a good desktop performer, especially at the price, but connection with the iPad/CCK was a flaky for us, especially under iOS5. NuForce, of course, makes a dedicated product for iOS as well (which we'll discuss further on).
Sure, you could use a powered USB hub in line, but that'd tie you to a wall socket, which makes your rig a lot less portable — transportable, sure, but you won't be using it on the bus. If you want to take your high-rez source on the go, the answer is a USB battery pack. These things have become more popular lately among travelers given the short battery lifespans of powerhouse smartphones, so it's not too difficult to fill the gap. Be advised, however — not all of the packs on the market will pass USB data, so depending on what you end up using you may have to pick up a USB Y-adapter (you may remember these things from the days when "portable" hard drives required two USB connections — one for data, one for an additional 5v of juice).
While a CyberPower CP-MBC ($59.95, cyberpowersystems.com)did just fine with a Y-adapter, we had the best luck with a pair of units from Tekkeon, recommended by CEntrance's Michael Goodman: the inexpensive TekCharge MP1550 ($24.95, tekkeon.com), which uses 4 rechargeable AA cells of your choice to generate 1 amp of output, and the slightly pricier MP1860A ($69.95, tekkeon.com), which uses an onboard battery to supply a bit more juice (2.1 amps; enough to charge the iPad should you find yourself in need). Both of the Tekkeon chargers offer USB data throughput (they're basically functioning as single- or double-port battery-powered USB hubs) as they dish out power to your DAC; it's definitely a tidier setup when you're on the go.
The setup I've been using — iPad 2 driving the DACport via the CCK, with a Tekkeon MP1860A in line as a power source — is easily luggable. It's not something you'd want to use on your commute, but on a longer bus, train, or plane trip during which you might be working or reading on your iPad anyway, it makes perfect sense. I experimented on a recent NYC-Boston bus journey, pocketing the power supply and DAC, enjoying high-rez playback while I worked on some upcoming S+V pieces.
It's not as compact a high-rez solution as, say, the HiFiMan HM-801, nor does it have the balanced outputs or storage space of a dedicated device like that player — but the point is that if you've already got your iPad with you (and who doesn't these days), you don't necessarily need to carry a separate device, even if your tastes run to high bit- and sample-rates.
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