Have you taken your subwoofer out for the winter or just have a spare one and want to hook it up to your computer or home theater? Well no problem, it's really simple and surprisingly inexpensive. First you need 12V power like in your car to power the amp so you need a cheap computer power supply aka PSU. The only thing you have to worry about is if it provides enough power for your particular amp.
See the next step for those details. Luckily every PSU has a large sticker on the side listing its max current ratings in amps for each voltage rail. See the attached image for an example. First find the max wattage rating of your amp. Let's say it's a Watt amp. Now divide that by 12 as in 12 volts and you get 25 amps.
So at 12 volts it needs 25 amps to reach max power. The example attached to this step wouldn't be sufficient with its pathetic 18 amp peak power. It's so low because they split the normal 12 volt rail into two. Older power supplies tend to fail a lot and ebay sellers don't always list the max current ratings so if you want to be sure, get a cheap, brand new power supply from newegg.