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WalkGood has written up a good summary about portable amps, and what they can or cannot do in the Why bother with portable amps? thread. It's important as people seem to have the mistaken impression that an amp and expensive LOD will give them some kind of audio nirvana, when a better pair of IEMs is really what they need.

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Originally Posted by WalkGood View Post
I’ve been using a portable amp over 2 years now and in June of last year I decided to post my opinion on abi about “What Can Portable Amps Accomplish?,” below is the post quoted from abi, I have tweaked it a little over time, so if you can offer up some suggestions please feel free …

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Some people are confused about what portable amps may or may not do, so I decided it would be a good idea to post up a few thoughts. For starters, I am addressing the higher end portable amps and not the inexpensive ones. Does that mean you have to spend a fortune on an amp, not necessarily. Just do your homework and don’t fall for the placebo effect as portable amps do not improve present poor sound quality. Keep in mind that many or most portable headphones do not necessarily need an amp and in my honest opinion a higer spend on headphones/iem's will deliver the most obvious improvement.

Some players sacrifice the sound output (amp power) for size, battery life or other reasons, thus making portable amps a possible option. A few important things to keep in mind that affect the overall sound quality are the audio encoding scheme* file format you use to encode your music, the quality of the original studio recording and the headphones you use, which can make a bigger difference in performance than many admit. Either way, a good portable headphone amp should transform weaker players into more powerful portables that deliver a richer, fuller sound quality at lower listening levels. Even though I still believe; if your headphones/canalphones are bad, you should first look to invest into a decent set of headphones, rather than spending money to amplify the bad headphones louder, thus "an amp is not a good fix for bad sound."

What can amp’s do:
  • Amplifies the signal for increased volume levels
  • Drives higher impedance headphones / can also help with low impedance iem’s that are too sensitive and produce hiss. A lower cost solution for this would be to use an impedance adapter
  • "Possibly improve the stereo crosstalk (which might improve perceived soundstage/stereo separation) and you'd need a good amp for that, cheap ones like the FiiO E5 have enough crosstalk on their own," thanks Martin
  • Depending on which amp, it might add a new dimension or color the sound
  • Improves or increases the dynamic range from high to low, which can allow lower listening levels
  • Increases bass response, punch or thump, but this can also depend on the quality of phones to a high degree as well (see bad phones comment above)
  • Bad or meager sounding headphones or DAP will not benifit or improve sound quality if you amp them
  • Placebo effect can be a factor, be careful and don't let yourself be fooled

Edited 11-12-2009
* correction above - when I said "file format" I realy ment that hi-fi transparency demands data rates of at least 192kbit/s (VBR) or higher and yes this may vary from listener to listener.