A headphone amp is a miniaturised version of your main amp - your main amp drives big speakers, a headphone amp drives very little ones.
Speaker size has nothing to do with it, they can just both drive different loads.
True, but you're not going to pump a 100W amp through your cans are you?
Most amps - except perhaps at the very high-end - use a series of step-down resistors to reduce the amps output
It's usually just one resistor and the headphone coil then completes the potential divider circuit, the resistor just drops a proportion of the output voltage.
I wouldn't personally know whether it is one or several but that is what I recall reading somewhere.
to provide your cans with a manageable load.
? the cans are the load.
Misphrase perhaps, but again you're not going to wire your cans in to your apeaker terminals are you?
So the obvious theory is that a dedicated headphone amp will give a more "pure" sound than one from where the output has been steadily reduced through a series of resisitors.
I must be missing the obvious theory, are you saying the dropper resistor somehow colours or distorts the sound?
Not my "obvious" theory, but others express it elsewhere.
Some CDP's - such as my Marantz CD6003 - have their own headphone jack and will drive some cans like Grado's with ease but fail utterly with K's.
They are op-amp based like a Cmoy or RA1, some parallel up op-amp stages to improve the current drive but rarely.
However, even running your cans through your main amp is not going to have the same sound signature as the one you have through your main speakers in any case so I don't see the argument.
So you don't agree that amplifiers have a sound signature ?
Of course they do, but whatever cans you use you're not going to have the same sound signature of your main speakers are they?