Line level vs headphone level

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Bit of a newbie question this, but I know you guys will be able to help me out.

I'm building little portable soudsystems with lepai TA2020 based amps. Through experimentation I've found some sources to be much louder than others. Notably my tonium pacemaker (a little mp3 mixer thing) is much louder from the line out than the headphone socket (which is about as loud as a phone headphone output).

Can anyone explain why? Or link me to an article explaining why? Is there any way to mod the amp so it works louder with headphone level input?
 
It's simple really; the line output has a higher voltage than the headphone output.

The headphone output is capable of higher current output but has a limited voltage for multiple reasons (one being to cap the headphone volume level).

If you want the amp to play louder with the headphone output then you need to raise the gain of the amp.

If you want to learn about gain, read this article......

http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html
 
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On older gear a headphone impendance of about 600R was the target.
Today 32ohm phones are common to help deal with the low voltage batteries of portables.

Line level inputs are very unlikely to be anything less than a few tens of thousands of ohms.
 
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