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no problem. i suggest looking at things like this. http://www.aliexpress.com/item/5V-Bluetooth-Receiv...really simple to wire up and has : 2x3w amp (pam8043) Bluetooth 2.1 compatible receiver chip. Fm radio, IR remote, USB mp3 decoding player and AUX input as well. and quite cheap set if u consider it has it all. U can most likely run it off a single 18650 cell or any other single cell li-po or Li-ion battery. just remember to use protected batteries always and get a micro USB-to-one cell charging board to complete your boombox needs. Just add evenly charged cells in parallel to get longer battery life. if protected no need for any sort of balance charger, just use one charging board for the set.
Just look for them in aliexpress.com or DX.com. search for "5v audio amplifier bluetooth" and gonna get a lot of choices. They are all basicly The same AMP in diffrent form. Allmost all are pam8043 + similiar bluetooth chips on most. Some have remotes even. And radio + USB decoding on The same board. I recommend you choose one of The panel type amps with The remote and all that. :) hope this helps. (Also, EBay) :)
nice build. just wondering about the amp. it might be a bit overkill since the 5v to 12v adapter puts out like 500mA * 12v = 6w of power. so you could get as much "umpf" with a lot smaller amp. like pam8043 chipamp, and would not need the boost converter at all. it is a 3 +3w 5v amp. There are also boards with built-in bluetooth and this chip. I have used i quite a few of those chips and now building something way bigger. found your instructable while looking for a suitable amp for my project :) but anyway, good work. keep tinkering.