I wasn't expecting much from the Hifiman Express HM-101; it's just a $39 outboard USB digital-to-analog converter and headphone amplifier. Well, this tiny USB-powered (it doesn't need batteries or an AC power supply) device definitely pumped up the sound of my Audio Technica ATH-M50 headphones! They sounded significantly better with the Express than they did plugged directly into my Mac Mini's headphone jack. Sure, the Mac's sound is perfectly acceptable--until you compare it to something better.
The Express is a lot better.
Before we go any further, the Express isn't just for headphones, it also has a line-out 3.5mm jack you can run to a set of desktop-powered speakers, like myAudioengine 2s. DAC resolution isn't specified, but it's probably 16-bit/48-kHz.
Switching over from the computer's headphone jack to the Express, the first thing I noticed was that the Express could play a lot louder. That's great, but when playing drummer Ginger Baker's "Going Back Home" CD at equal volume levels from the computer and the Express, the Express unleashed more of the drummer's hard-hitting dynamics. The computer squashed his sound, especially Baker's mighty bass drum. Wow, the little thing delivers. If anything, the Express errs on the side of too much bass fullness, which isn't such a bad thing.
It's not just power; I heard a difference when I played Gillian Welch's gorgeous "The Harrow & the Harvest" CD; her vocals and banjo were clearer over the Express. The background noise level was lower, so I felt closer to Welch's performance.
The Express is good, but for more money you can do better, and moving up to the $400 Centrance DACport left the Express in the dust. It dramatically clarifies the sound, tightens up the bass, dynamics punch harder, and it has superior high-frequency detailing. That's the way it goes, but for $39 the Express gets you 70 percent there.
So if you're still plugging your headphones or speakers into your computer's analog connectors, take the plunge and get an Express.