Can a portable amplifier improve an iPod's sound?

I've written about better-than-iPod-sounding music players before, but this is my first portable headphone amplifier review. The Todd The Vinyl Junkie Portable Slim amplifier ($349) really works and will take the sound of an iPod, or any portable player, to the next level. The amp's sleek chassis (4.2x2.7x0.4 inches) is nearly the same size as an iPod Classic. It's built in Dallas, and the case comes from Canada.

The Slim's rechargeable lithium ion battery provides 15 hours of playback time. It recharges in about 2 hours from a USB port or a USB wall charger; you can also operate the amp while it's plugged in. The Slim hooks up to an iPod's multipin connector, or other types of players' headphone jacks.

The amp uses a microprocessor analog-stepped rotary volume control. Each of the 32 steps changes the volume level by a precise 2dB. The volume setting is indicated by the color of the front-panel LED (red at low volume, green at moderate volume, then light blue, then dark blue, and so on). There's also a switch to select 0dB, 10dB, or 20dB of gain to accommodate all sorts of headphones.

The Slim's optional USB digital-to-analog converter ($100) adds a Texas Instruments/Burr-Brown USB DAC to the amp, which I used hooked up to my Mac Mini computer at home. In other words, the Slim can be used as a home headphone amp.

To evaluate the Slim's power I started with a pair of headphones that have made many an iPod roll over and die, my Hifiman HE-5s. Oh, yeah, the sound was a giant step closer to what you get from a full-size home headphone amp. The deep bass organ notes from Philip Glass' "Koyaanisqatsi" album left no doubt, the Slim could make a big sound. The album's orchestral power was fully developed in ways that no iPod has ever matched.

I next plugged in my Ultimate Ear Reference Monitors in-ear headphones, which really do sound spectacular with the Classic, and proceeded to switch back and forth between the iPod's headphone jack and the Slim amp. I used a great-sounding percussion recording, "Alternesia" by Jon Iverson, to evaluate the sound. The mighty drums sounded bigger and deeper with the Slim, and the bells and chimes were "airier" and clearer. That said, a bigger difference could be heard in the way the Slim better communicated the instruments' dynamics; switching back to the iPod, the sound was compressed and flattened.

Rocking out with Los Lobos' "Live at the Fillmore" the differences were even more dramatic: the Slim totally clobbered the iPod, which sounded downright pathetic by comparison! The Slim also scored points on quieter, more ambient music, like Brian Eno's "Small Craft on a Milk Sea." The iPod blunted much of the music's texture and nuance. Sure, the iPod on its own sounds fine, until you hear something better. Crosby, Stills and Nash's vocal harmonies had a richness over the Slim that disappeared on the iPod. I could also more easily hear their individual voices over the Slim.

Questions? Please direct them to Todd of Todd The Vinyl Junkie. He also sells a nice selection of high-end gear, including turntables, headphones, cables, and accessories. Most are sold with a 30-day return guarantee, and he doesn't charge sales tax.