Portable audio players traditionally have poor quality amplifiers built in. Because you're used to it, you probably don't notice that the louder it gets the worse it sounds. Static gets introduced, instruments blend together, and frequencies get distorted. Ipods are known to have among the worst audio quality in the portable music market. A headphone amplifier takes the burden off of the player and does all of the amplification. It also processes the sound, which I'll get into in a minute.
So who needs a headphone amp? Ideal candidates are people who have high end headphones, or even just big headphones. These phones need more power to drive them, so they tax the player even more than usual. Examples are the Etymotic Research and Shure E5 headphones. Another time when it's essential to have an amp is when you're trying to split the audio between two pairs of headphones, as this doubles the power needed from the player.
Have you ever noticed that wearing headphones for a long time is uncomfortable and makes you feel restless? There's a reason for this. When we hear audio in real life, it's never only coming in through one ear. Even if someone's yelling at you directly from your left, your right ear gets a little bit of that through your bones and skin as well as from reflections of the sound. This is called crossfeed. When you listen to headphones, there are some sounds that only come in from one ear. This fatigues your brain because the sound seems so unnatural and it can't place its location. A good headphone amp will automatically crossfeed a bit of audio from each channel to the other ear, resulting in a much more pleasant and relaxing listening experience. This sounds like a bunch of bull, but the difference is obvious once you have an amp.
The headphone amp market used to be pretty bad. Good amps cost thousands, while cheaper amps weren't very high quality. Still, I bought one in the middle of the road, and it was amazing. I brought it on road trips, plane rides, and even just walking around listening to music. Then my friend stepped on it and I had to get a new one.
Enter www.fixup.net, a rather unprofessional seeming site made by a rocket scientist (yes, literally) named Xin Feng. I'd actually e-mailed back and forth with Dr. Feng in the past because he had created a way to power laptops with camcorder batteries. Long story.
Feng took it upon himself to build the ultimate headphone amp at a ridiculously low price. The device is fanatically over engineered, somehow incorporating features not found on amps ten times its price. I bit the bullet and bought one, hoping it would live up to the hype. These amps have a lot of different features that I won't pretend I understand. Words like op amps and triple buffers confuse me. Truthfully, I don't care how he does it as long as the end result is good.
And it is - I bought the SuperMacro (pictured), but in retrospect I probably would have bought the middle sized one. It's not too big, it's just that I'm obsessed with buying small things. Right out of the box, the sound quality is insane. Instead of feeling like the music is playing inside my head, it seems as though the artists have assembled themselves in a wide formation in front of me. The bass is richer, the trebles are crisper, and instruments and vocals aren't muddled together. Honestly it's astounding how good it sounds.
I notice that whenever I listen to music with the SuperMacro I hear more than I had before. I hear the bow brush across the strings before a note is played. I hear the singer take a breath before delivering a verse. Slight nuances are still slight, but they add up to make the listening experience richer and more enjoyable.
I should mention that I'm not an audiophile. I appreciate good quality audio, but only to a point - I'd rather hear a good artist through an 8 track player than a mediocre one through the best setup. At only a few hundred dollars, though, the SuperMacro lets me get a lot more mileage out of my favorite songs.
One last interesting point. To get a top of the line home audio system is impossibly expensive, costing possibly hundreds of thousands of dollars. However, between a $350 headphone amp and a $200 pair of Etymotic Research headphones, you can literally have the best portable audio in the world which rivals those hundred thousand dollar home systems. When done on a small scale, audio is a lot easier to manage.