Difference between line out and headphone jack?

[CoolDragon]

Hi,

As talked in the forum, if you have a Port Replicator II, the internal speaker is still on while docking on PRII, the solution is to minimize the volume (to zero) with the IBM volume control key but not mute it.

But if I attach a headphone to the line out of the PRII, the sound quality is worse than if I attch it to the headphone jack of the laptop, though it is so loud that you have to make the sound level very small in Windows.

There must be difference between the two outputs, anybody know what to connect to the line out?

[snife]

you should attach speakers that have their own amp/power supply to the line-out

[CoolDragon]

I read a little bit on the net, and now I know the difference is that headphone output is pre-amplified while line out is not. So you should connect a headphone to the headphone out and connect a self-amplified external speaker to the line out.

What I still don't understand is that why a pre-amplified headphone out is much lower in sound level than a non-preamplified line out? How can this be possible?