For audiophiles who like to listen on the go—and while using Apple devices—these are heady times. In the last year or so a new category of consumer electronics has hit its stride. These products take a stream of digital audio data directly from an iPod, iPhone, or iPad and convert it into an analog signal capable of driving headphones. Five such devices became available in 2013, and of this crop I'm particularly impressed by the Theorem 720 by CypherLabs, with its rugged build and astounding 18-hour battery life. Introduced this past September, the Theorem 720 will even recharge your iPod or iPhone as it plays music: It stores so much juice that after a 5-hour listening session, it'll leave your phone or music player with more charge than you started with.
As do the competing products, the US $900 Theorem 720 combines, in a small chassis, a high-performance digital-to-analog converter with a comparable headphone amplifier. To get the most out of these devices, you need to feed them lossless audio files, encoded, for example, in Apple's Lossless Audio Codec (ALAC) format. Listening to such files on one of these devices and with a pair of great headphones is a musical experience that's far superior to what you get from an iPod or iPhone alone, and it can even rival the quality of a much more costly home audio system.
To understand why these units are a big deal requires a little history. For many years, Apple's i-devices didn't have a port for outputting digital sound data to an external digital-to-analog converter. This kind of port, used to access digital audio data—typically in a format called S/PDIF, for Sony/Philips Digital Interconnect Format—is a feature of personal computers and many disc players. But in keeping with its closed approach, Apple resisted making the digital data available (although an optical S/PDIF connection has long been available on Apple desktops and laptops). So the only way to listen to music stored on, say, an Apple iPod was through the iPod's own built-in digital-to-analog converter (DAC).
The iPod's DAC is, of course, a cheap commodity circuit that sounds somewhat better than nails on a blackboard. Audiophiles dreamed of getting access to the digital audio data, because that would let them bypass the internal DAC and run the data through a high quality DAC instead, which would give much better sound quality. The fidelity you could get would be limited not by Apple's choice in DACs but rather by the size of your checking account, which is just how audiophiles like it.
The standoff was finally resolved in 2005, when Apple unveiled its Made for iPod program. Apple began making the digital audio data available through the 30-pin connector then at the bottom of iPods, iPhones and iPads. (The company has recently begun replacing the 30-pin connector with its new Lightning connector.)
The first product to make use of the newly available digital stream was the Wadia 170iTransport. The Wadia was relatively clunky, and it did little more than take the stream from an i-device and turn it into an S/PDIF stream that any stand-alone DAC unit could use. But that was enough: Audiophiles rejoiced, and the iTransport quickly became a hit.
Now, in 2013, after five years of evolution, we have a growing assortment of portable units that can take the digital signal all the way to high-quality audio. Of these, the Theorem 720 struck me as unusually sturdy and well-engineered. I like its heft and simple, minimalistic design. On the front panel there's an on-off/volume knob, balanced and mini-USB inputs, and a headphone jack. The back panel has a 9-volt charging port, a gain selector, and an analog output jack.
Inside is a DAC based on the AKM4396 chip, from Asahi Kasei Microdevices Corp., that can work with a digital audio stream consisting of samples as large as 24 bits and a sampling rate as high as 192 kilohertz. The amplifier section is based on ADA4897-2 op amps, from Analog Devices. There's a total of four, one each for the headphone and line-out amplifiers and two for the balanced output.
I wanted to hear how the unit sounded through great headphones, which I don't happen to own. So I took the Theorem 720 to Lyric HiFi & Video in Manhattan, a New York City institution that supplies audio gear to people for whom money isn't really an issue. The store's owner, Leonard Bellezza, graciously put many treasures at my disposal. When I told Bellezza I had a new portable DAC-headphone amplifier, he shrugged his shoulders wearily and said, “Every manufacturer I deal with has some revolutionary new DAC."
I started with a pair of Grado PS1000 headphones, which retail at $1700. I fed the Theorem 720 from my iPod Classic, on which I'd put tracks ripped from CDs using the ALAC lossless format.
Let me preface my impressions with the admonition that if your hi-fi ideal is the sound that comes from McIntosh tube amplifiers, or from Mark Levinson amps playing through planar speakers (as it is for many of Lyric HiFi's customers), you might not love the Theorem 720. You'll probably find the unit to be too “bright and edgy," says Robert “Analog Bob" Herman, a sales associate at Lyric.
Here I should mention that the Grados, supremely great as they are, have a reputation of being a tad on the bright side themselves. And when I played the Theorem 720 with my own Etymotic in-ear phones, the edginess was much reduced.
To me though, even with the Grados, the sound was precise and detailed and clean. In songs like Ray Charles's “That's Enough" and Kathleen Edwards's “One More Song the Radio Won't Like," I heard texture and detail in the singers' voices that I'd never heard before, and the bass was tight, firm, and authoritative. On the Kate Bush song “Cloudbusting," which has orchestral sections, the sound was full and remarkable.
I was particularly struck by Bush's song “Mother Stands for Comfort," a challenging piece full of jangling, percussive crashes and howling vocals. With the Theorem 720 playing through the Grados, the attack was strong and transparent, supplying plenty of punch and heartrending vocals.
Herman then suggested a little comparison experiment. He dug up a Pure i-20 iPod dock (about $125), which, like the iTransport, extracts the digital signal from an iPod or iPhone and gives you an S/PDIF signal suitable for any DAC. We fed the S/PDIF data to a Moon model 340i integrated amplifier ($5000), which included a DAC within its chassis. Once again, we listened through the Grados.
Herman preferred the sound of the Moon, and I had to admit it was smoother and, for lack of a better term, more lush. But I missed some of the fine details I'd heard with the Theorem 720. And, of course, it's not really fair to compare a $900 portable unit with a $5000 home component. And yet, the fact that we were doing so—and that the Theorem 720 did so well, in my opinion—was a testament to the fidelity of this little powerhouse.
Amplifiers Made for iPod/iPhone DAC Headphones
|CEntrance||HiFi-M8||US $699||Works with iOS and Android|
|CypherLabs||Theorem 720||$899||Has 18-hour battery life; works with iOS and Android|
|Fostex||HP-P1||$699||Has an S/PDIF digital-output port enabling connection to a home hi-fi setup|
|Furutech Alpha Design Labs||X1||$479||Works with iOS and Android|
|Sony||PHA1||$599||Successor model PHA2 just released in Japan|
|V-Moda||Vamp Verza||$598||Works with iOS and Android|
|VentureCraft||Go-DAP TT||$810||Amplifier section uses subminiature vacuum tubes|
|VentureCraft||Sound Droid Typhoon||$645||Offers users the ability to select the sample rate and word length to match the digital audio data file|
Glenn Zorpette is editorial director for content development at IEEE Spectrum. A Fellow of the IEEE, he holds a bachelor's degree in electrical engineering from Brown University.