QUOTE(lottie @ Apr 13 2008, 02:42 PM)

What's the difference between an 'amp' and a 'PA system' and why are there different amps for different instruments (and which do you use for a violin?)??
*clueless*

... but interested
Here's my idea on the difference - A PA (Public Address) system will usually have inputs for microphone, instruments and other sources (eg a CD player) all going into some sort of 'mixer' to balance the sources. This mixed signal is then sent to an amp (amplifier), maybe via an equaliser (to modify the tone) and then onto the loudspeakers. These are usually separate to the amp so that they can be optimally placed within the hall. An 'amp' (in this context) would usually have some of the above 'components' but all contained in one box. Typically, an amp would have one to four inputs and an output of perhaps 10 to 100 Watts.
A typical PA system would have, perhaps, 8 or more inputs and an output which could go up to 1000s of Watts.
Now why have 'different' amps. In general, amps are optimised to the intended instrument they are going to be used with; hence a bass amp for low register instruments (big speakers to move lots of air but relatively slowly). A guitar amp would be designed to to cover the middle to high register effectively and may include some 'effects' that are usually used with guitars (distortion, overdrive etc). Finally, there is the keyboard amp - this, surprise, surprise, has a fair response across all the register. It is usually very 'clean' (not much distortion or noise) and, typically, has an additional mic (microphone) input for soloist to sing along with the player.
So, what to use with a violin - I think the general consensus is to use a keyboard amp (or maybe a bass amp) for this instrument. But, to be honest, it is best to try before you buy - preferably in the intended environment.
For a practice amp or small room, you'll need something with a power output of about 10 Watts, going upto 30 or 50 Watts for a small hall.
Hope that's of some use -
Best Wishes -D-