So i want to have a play with a sub that will be powered through a 12v car amp and i want to know if it will be happy with a signal from the computer, computers speakers are around 8 ohms arent they?
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PCs / Soundcards (now adays) don't have "speaker out"*, just line out, which is what amps will be expecting. It just may not be 2V or 4V peak to peak, it wil probably be the "normal" 1V p-p.
* apart from a few older soundblaster and clones (ISA slot) and a few odd-ball motherboards (one with a vaccum tube amp!). Exceptional exceptions.