I am confused about 15A and 20A current.

Unless your wiring is a minimun of 12 gauge (14 gauge is smaller) you cannot just change the breaker to a larger size. If you have 14 gauge wiring on that circuit try replacing the existing breaker with the same brand 15 amp breaker as breakers can wear out over time. If you still have a problem after the replacement, you're going to have to find another circuit with more reserve capacity or install a dedicated circuit.

You need to have a circuit that's capable of 20A as well as a breaker otherwise you'll end up defeating the point of having a breaker. While I'm pretty sure that standard 12/3 Romex is good for 20A, I'm not qualified to say for sure.

Do not just replace the breaker! In order to use a 20 Amp breaker you need a minimum of 12 gage wire. If the current breaker is 15 Amp then most likely the circuit is wired with 14 gage. I do not recommend you do anything to this circuit. To be honest if you don't understand the current rating you should not be messing around with the circuit. Not trying to be mean spirited, just trying to save your house and/or your life! As for the amp tripping the breaker, it if is the only item on the circuit there is something wrong with the amp. If there are other things on the circuit you should either use another exisiting circuit or have an electrician put another. preferably dedicated, circuit.

I never stopped to think about the possibility of an older home (etc) with old or small gauge wiring. One needs to be certain that the wire is at least 12-3. The posts above this post are correct and bring up some excellent points that I failed to mention. Another reason that I really like Audiogon :-)

DO NOT REPLACE A 15 AMP BREAKER WITH A 20 AMP BREAKER!!! Alexanderj is right! Remember the old "penny in a fuse box" trick, in the days before breakers? Unless you can confirm that your circuit will adequately handle 20 amps, you run the risk of an electrical fire that can consume your entire house...like the fuse box situation that I just described (saw that happen one time, although the house wasn't lost, there was electrical damage done, before the main 50 amp buss fuse blew!) Funny, I just had a conversation about circuit breakers the other day. They can oxidize over the years, and should be replaced every few years (for your audio circuit). Your Rotel's 380 watts should not trip a breaker when you power up. Assuming a doubling peak at start up (760 watts), and even with a low power supply to your circuit (110 volts), you will only draw about 7 amps at power up. Are there other high amperage appliances on your stereo circuit? This could be your problem, but I would bet on an old cruddy breaker. Or there might be a problem with your amp, but usually, the internal fuse(s), will blow, NOT the circuit breaker.