ADL X1: This gizmo will pump up the sound of your iPhone

Over the past year or so we've seen a new product category emerge: the portable digital converter/headphone amplifier. Of course, no one "needs" such a device -- phones and iPods already have converters and amps built-in -- and sound perfectly fine with average headphones. The sound is good enough, but your phone's converter and amp share space and battery power with the phone's electronics. A separate converter and amp, about the size of a phone, has only one mission: improved sound quality. So if you upgraded to a high-end in-ear headphone, like the $399 Logitech UE 900, or a full-size Hifiman HE-400, you'll be in a position to hear what the ADL X1 converter/amp brings to the party. It will make your headphones sound like better headphones.

The ADL X1 handles 16-bit/48kHz digital audio via USB cable when connected to an iPod, iPhone and iPad, or 24-bit/192-kHz audio via USB with Macs or PCs (the X1 doesn't work with Androids or other phones). The small LEDs on the ADL X1's case indicate the digital sample rate -- 44.1, 48, 88.2, 96, 176.4, or 192kHz -- of the music being played. Connectivity options are good, in addition to the 3.5mm headphone jack, you get a 3.5mm analog input, iDevice USB input, USB mini B, and a 3.5mm optical digital audio output. The ADL X1 measures a trim 2.6x4.6x0.7 inches and weighs 5.2 ounces. The case is metal and plastic, and the X1 has a volume control knob, not up/down buttons, so it's easy to quickly set exactly the volume you want. The ADL X1 uses a 3.7 volt/2,600mAh lithium ion rechargeable battery. The ADL X1 comes with a USB-to-30-pin Apple cable -- that's how I hooked it up to my iPod Classic -- and starting with units shipped after July 8, an Apple Lightning cable will be included.

Setup was a simple plug-and-play routine -- and I noted sound improvements in clarity, bass power, definition, and soundstage spaciousness -- compared with listening to the iPod Classic solo. The Raconteurs' "Broken Boy Soldiers" packed a bigger wallop over the ADL X1; it makes headphones sound like better headphones. Of course, it doesn't make sense to team the ADL X1 with a set of cheap or even moderately priced headphones -- definitely buy better ones first, but if you've already invested in a high quality headphone, the ADL X1 can take the sound to the next level. I listened with Cardas EM5813, Logitech UE 900, and Jerry Harvey JH 13 in-ear headphones, and Hifiman HE-400 full-size headphones. All were improved by the X1, but it really clicked with the HE-400. The iPod Classic on its own could not come close to matching the ADL X1's dynamic kicks.

When I compared the ADL X1 to the $598 V-Moda Vamp Verzadigital converter/headphone amps they sounded similar, but the ADL X1 was a tad softer and sweeter sounding with my Jerry Harvey JH 13 in-ear headphones. The ADL X1 pulled way ahead of the Vamp Verza with the Hifiman HE-400. That said, the Vamp Verza's big advantage is that it works with iDevices and Android phones; the ADL X1 is just for iDevices (or Macs or PCs).

I heard a small amount of noise/hiss from the ADL X1's amplifier over my headphones, which was also true with the ALO and V-Moda Vamp Verza amps I've tested. I didn't find the noise all that intrusive, but other listeners might be put off by the noise.

The ADL X1 is available online for $479.