An amplifier turns low voltage signals from your stereo into a signal with more power that is then sent to your speakers. Think of the term 'amplify' and that's exactly what it does: it allows for more powerful drivers to be able to play louder volumes.
It all depends on your car stereo system. So, if you're buying everything new, it's a good idea to get your car speakers and subwoofer first. When you have everything, check the full wattage, and whatever your speakers and sub take up, you have to match that at least.
You only need one if you want louder and clearer music. You should definitely get one if you want louder sound without distortion, because an that's basically an amp's job. It will allow you to increase volume without increasing distortion and the sound will be unbelievably clear.
There's no out-and-out best brand when it comes to amplifiers, or any other car audio component. As with everything, much of it is down to personal taste. Of course, there are many high quality and not so good brands, so it's important to do some research before you buy an amp.
5-channel amps are a good option if you want to run four speakers and a subwoofer. Many people already have an amp installed when they decide to run more speakers so you have to decide whether it's cheaper to get a second amp, and if you have the space. Both options are great and will push some power as long as you get good quality components.
It all depends on your vehicle and what room you need. Most people install the amp in their trunk, because that's usually the biggest free space. If you have a subwoofer box in your trunk, it's a good idea to put your amp next to that. If you don't have much space, you install it under or behind a seat. It all depends on you and the space you have.
There are different types of amplifiers and the two most common are Class A/B and Class D. They're both excellent in their own way, but A/B requires more power and they will get hotter than Class D amps. So, if you're tight on space, like if your amp has to go under your seat, it's probably a better idea to get a Class D amp.
It depends on your set up really, but generally It's not easy, but it's not difficult either. As long as you have all the right equipment, and follow my step-by-step article for how to install an amplifier, and maybe watch a video, you can't go wrong. it's important not to skip any steps, and only get good quality equipment and fittings, because that will ensure you get the best out of your amp for a long time.
The old Class A/B vs Class D amplifier debate will go on and on. The fact is there is no out and out best, and it really depends on what you're looking for. Class A/B amps offer the best of Class A and B and offer power efficiency and good sound. Class D amps have the highest efficiency but aren't quite as high-fidelity.
The impact a car amp will have is a huge improvement in your music's clarity. Without one even good quality car speakers will sound thin, because an amplifier helps boost the sound to overcome road noise. While you will always get external noises, a car amplifier will definitely stop you thinking about it.