Transcript: One question I am frequently asked is, "Do I need an amplifier?"
Most marine stereos have an amplifier built in and some of them are surprisingly powerful. It is very possible to get good quality sound without an external amplifier. For most people, I encourage them to try their stereo without an amplifier first, because you can almost always add an amp later.
Of course, there are circumstances where you will definitely need an amplifier.
The first is obvious. If you need a lot of volume or you are very picky about sound quality an amplifier can help improve both. For example: A party boat where you plan on hosting a lot of people and dancing will definitely need an amplifier. Human bodies can really dampen sound, so the more people, the more power you will need from your amp.
If you want a lot of thump and you plan on using a subwoofer, then you absolutely need an amplifier for it. Subwoofers require a lot of power to sound good. If you don't provide adequate power to your sub, you might as well not be using one at all because you probably won't be able to tell the difference.
Most stereo head units will only safely power 4 speakers. If you want to use more speakers, then an amplifier can help. Most amplifiers can handle 2 speakers per channel wired in parallel. So, for example, a 4 channel amplifier can power 8 speakers.
On larger boats, you may want to set up separate listening zones where you can adjust the volume independently. Say you want the music in the cabin to be at a different volume than in the cockpit. A zone amplifier comes in handy for that purpose.