Home audio amps (amplifiers) are in every stereo system. Here are the different types of amplifiers, and recommendations for buying...
In the simplest system, there'll be maybe a Compact disc player, an amplifier, and two loudspeakers. The job of the amplifier is to boost the very small electrical signal coming out of the CD player into a signal strong enough to drive the loudspeakers. Simple!
A while ago, most amplifiers had loads of knobs and switches and fiddly dials. These let the user play around with the music until his heart's content.
But over the years people have realized that the more components a signal has to travel through, the lower the sound quality is.
So it's much more common today for high-end home audio amps to look plain and boring, with only an on/off switch and a volume control. Don't be scared by the simple look - it's for a good reason!
One way to divide them is between solid-state amplifiers (which use transistors and other electronic bits and pieces), and valve amplifiers (which use vacuum tube technology).
Although valves are older technology, a lot of audiophiles think (or hear!) that valve amplifiers have a warmer sound.
Solid state amplifiers are usually smaller and run cooler than valve amplifiers.
Another way to divide home audio amps is between integrated amplifiers (where everything is in one box), and separated amplifiers (where the pre-amplifier and the power amplifier are in separate boxes).
The pre-amplifier deals with the tiny signal that comes from the CD player. It lets you control the volume and a few other functions.
The power amplifier is the stage that really boosts the signal into a loudspeaker-driver. This is usually in a plain box without any knobs, switches or dials!
The reason for separating the two amplifiers is that the power amp can cause distortions in the pre-amplifier. Separating them physically stops these interferences.
A lot of amplifier manufacturers recommend keeping the CD player (or source) close to the pre-amp and putting the power amplifier next to the speakers. Because of this some systems even have two power amplifiers, one for each loudspeaker. Now that's cooking with gas!
The logical progression from this is to put the power amps into the speaker cabinets. This creates what you'd call an active loudspeaker. There are a few benefits to this approach, on top of getting the power amp away from the pre-amp.
There are also much smaller headphone amplifiers, which are specially designed to increase the power and therefore quality of headphone sound.
There are literally hundreds of amplifier manufacturers, with a huge variation in quality of sound, quality of construction, and of course price!
Budget (or entry level) amplifiers are almost always integrated (all in one box), and have solid state components.
While there are many firms with good products, I'd point you to NAD as having a solid reputation at the lower end of the market.
At the top end, there are a surprising number of competing companies. All of them have solid claims to an awesome sound. Some of them are Linn Products, NAIM Audio, Quad, Meridian, and Krell.
The middle ground is somewhat blurred though. This is party because several of the Top Guns also have entry level products, and partly because some budget amplifiers can sound great if they are matched with a high quality source and good speakers.
How much do you need to spend? Only you can decide, by auditioning different systems and seeing how they sound to YOU.
As always, the most important part of a home audio system is the source material, be it a Super Audio CD, a Studio Quality digital file, or an old vinyl record (see how to convert an LP to MP3 format). Budget carefully and remember that amplifiers are Number Two in the hierarchy of importance!
Related article: stereo preamplifer primer.