Making amplifier for for 5 watt speaker?

Okay, so i have a 5 watt speaker that i want to install in my dirt bike helmet and i want to play music with my iphone 5. There's only one problem, if i plug in the speaker straight to my iphone it won't play music loud. I need an amplifier but i can't have anything to big because this speaker and the amp has to fit into my helmet. I also i have no idea what to make the amp out of or what kind of amp i need to buy.

Uh… Look into some earbuds.

Google pocket amplifier. There are plenty of construction plans but it seems to me that something small enough to put in to an helmet should be already made. See what they do at motorcycle shops.

Do not use buds. That on a dirt bike is asking for jammed into the ear troubles.

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