Reliability power amplifier is often only seen as a bit to produce a loud sound. Pick out an amplifier between the choices made when you might be a happy crowd, an empty room difference.

Sometimes, you choose your amplifier two main considerations. How much power you need within a frequency range of what is acceptable.

How much power?

This is the question most people will begin with. When we talk about the amount of power we need you have to consider the audience you are playing to. As a rule of thumb you can use the following:

Medium bands: 3 to 4 watts per audience member.

Large bands: 5 to 6 watts per audience member.

These are only basic estimates but they will usually suffice for most occasions. Just use your common sense, 200 watts is not going to be enough for 200 people in a football stadium. Here's a few examples:

Home stereo: 150w is plenty

Rock or heavy metal music in a stadium: between 5000w and 25000w

What amplifier?

The range of the amplifier depends on two things. Is the instrument going to be amplified separately from everything else, a guitar amp for example, or is the amplifier to be used as the end amplification.

A guitar amplifier is specifically for guitars because of the range of frequencies it works best at and the extremely high gain that is required. A good (read expensive) end stage power amplifier should give a good response right across the desired frequency range.

If the amplifier is only to be used on a voice microphone then there is no point paying out on an expensive full range PA amplifier.

Again, common sense should be used here and the manufacturers/suppliers are usually very helpful when it comes to technical queries regarding music type and frequency range.

A good point to make that the beginner is often not aware of, is that you cannot just plug anything you like into a power amplifier. The inputs will often need some kind of conditioning of the signal. This will often involve either a mixer or a pre-amplifier to get the signal up to the desired levels.

This a good reference table so that you can check the amplifier has the desired range:

Singing voice: 60Hz to 1.2kHz
Wood winds: 30Hz to 3kHz
Drums: 70Hz 300Hz
Guitar: 65Hz to 1kHz

These are only a rough guide. Individual instruments can stray wildly from their typical frequency ranges.

I've given you a very basic guide to amplifier selection and obviously there are more in depth guides and tutorials but everybody has to start somewhere and armed with this information you should be able to request the desired amp.

Related reading: Active Speaker Systems  Professional Power Amplifier