In older days we had large cumbersome valves which were used for making amplifiers, though the output from them used to be crystal clear, were very inefficient as far as power consumption and power output levels were concerned.
Then came the transistor era, which immediately replaced the valve and tube gadgets and instantly became a massive HIT, as now audio amplifiers could be made much smaller, lighter and power efficient.
Use of transistor made it possible to manufacture huge amplifiers with monstrous power outputs. Even today transistor amplifiers are very much in use, along with their MOSFET counterparts.
Today integrated circuits have simply replaced the above type of amplifiers and what's more, these ICs are even designed to handle stereo amplifier outputs which makes them absolutely a piece of cake as far as assembling stereo amplifiers are concerned.
These IC also incorporate many internal built-in protection circuitry.
One such simple single chip stereo amplifier circuit is discussed here. AS the diagram shows the IC requires just a very few number of external components and therefore can be assembled within minutes and yet is capable of providing a good 25 watts of stereo music into a couple of 8 Ohms speakers.