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I. I have never heard of such thing before. The amp in the good'ol days that I know of is used for car audio system and home audio system. I used to amp my car stereo when I was young. It was much fun. But amp for a little set of headphones??? Are you kidding me.
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II. Some questions for you guys:
1) - Is headphone amp same as external DAC? DAC can act as amp and amp device can act as DAC too? All these fancy names out there confuse da hell outta me.
2) - Doesn't amp just inhibit more "distortion" when you use it with aftermarket IEM? or is it the opposite? For example, say you have a Etymotic HF5 and you hear a distortion with iPod Touch at volume at 70%. Will the amp remove distortion you hear at volume 70?
These are just a few of the questions that first bring to mind....
I. Let's say you have a typical car stereo receiver that according to the box makes 200w, but actually it makes 25w per channel, with only one channel driving a 4ohm load. Your speakers are at 90db and have to overcome the noise in a cabin, especially since you chose to buy speakers instead of a set of Yokohama AVS dB tires. Without going into the specifics of mathematics here, you can be sure that not only are each of your speakers getting less than 25w, they're probably lacking current and have too much THD - so even if they can hit 91db where your ears are, but you can't hear much difference between softer notes and louder notes.
Now you have an iPod that makes 5mW at 32ohms. You have headphones, as a more extreme example, like the Sennheiser HD650 which has an efficiency of well over 90dB but is 300ohms, or an AKG K702 that is 56ohms but is only at about 86db efficiency. You can run either with just the iPod at or near max volume setting, but even IF you actually think that's loud enough, you're still lacking the same requirements for THD and current.
II.
1) DAC - digital to analogue converter. A device designed purely as DAC does just that; as far as headphone equipment is concerned, some can be built to have a headphone output too, and the design varies from one device to the next - some can have the simplest headphone out, some can have a serious headphone amplifier and even separate power supplies or capacitor banks for the DAC section and the headphone section. Some are designed primarily as DACs and the are better at that, some can be good enough a separate headphone amp can yield only marginal gains if any, some are actually amps built with a simple DAC section (usually USB). Why all the variations? Depends on the needs of the buyers, but mostly the reason why they try to cram as many functions as possible into one box is to save on space. Unlike traditional speaker rigs, headphone systems now tend to be built around a computer or a portable player, which meant that less boxes are better than separates. Imagine having a separate DAC, preamp and amp on your desk. (Ask someone else why they'd strap four difference devices together and listen to all that on the go where most of your mind is supposed to be paying attention to what stop it is already or if the next guy looks like a pick-pocket; I haven't gone that far.)
In any case you can think of these devices as similar to "receivers" in speaker audio - you have a huge box with a digital input, and the DAC, preamp and amp are in the same box, you just hook up the speakers; these come in stereo (like the Marantz PM75, or the latest HK's and Onkyos), or surround as on home theater receivers. In cars, this is a lot like when you integrate an analog X-over into the amplifiers, and they apply the cross curves to minimize bass freqs to the front speakers and minimize anything that isn't bass for the subwoofer.
2) It cannot inhibit more distortion unless
a. The source feeding the amp is feeding it a distorted signal, and the amp is amplifying an already audible distortion level that makes the sound, to some, unbearable (or pointless enough for using the amp in the first place)
b. The amp is so badly designed it introduces more distortion than the source
In something like the example you stated, the problem is the iPod through its headphone output may be distorting already, which is why you need to use the lineout function, with a cable specifically designed to take that signal out from the 30-pin dock connector and into a 3.5mm plug (or 2xRCA plugs, depending on what inputs an amp uses).
Do take note that not all systems are alike, and in this case, I have to point out a couple of things. First, the Touch at least as far as my experience with one was concerned produced a "lineout" signal that was noisier than my Cowon S9 set to max volume (in practice I used it at 75%), and some even believe the lineout on it is just the same as the signal/circuit going out the headphone port but is set to max level by default. I have no idea if this was settled and confirmed, but regardless I didn't use its lineout. In other devices, such as redbook (Sony's CD) standard CD players, the circuits have a dedicated DAC chip and then an analog output stage (opamp, HDAM, or tube) that takes the signal from that and produces a fixed 2v lineout signal, or routes that signal into a built-in headphone amp. Portable players with the exception of the S:Flo (which is basically a miniaturized CD Player) tend to have one or more of these functions integrated into a single chip, so don't expect a mass-market audio device to be as good as a serious CDPlayer. The second is that Etymotics tend to be more transparent than most other brands. While other brands may still need more power or more likely, current, to sound their best, they can at least sound loud enough and yet not be too honest about the tendency of some devices to sound shrill with the treble or just generally too thin.
Personally, instead of looking for the "right" amp for your player and earphones, you might have an easier time just replacing the earphones. That way you won't have to deal with the amp not getting you the results you wanted, or even if it did, that you have to worry about how to put those in your pocket, listen and move without the cables moving, being able to whip out the device to change tracks...I could go on and on about this, but generally if you don't think you'd be up to lugging around another device strapped to that player, best leave that option for those who can live with those realities. Although if you are at least just going to listen when seated somewhere, then that's a lot of other issues out of the way, and all you have to deal with is whether the amp can improve on your gear.
