|A 250Watt amplifier @ 12V draws 21amps.|
If your powersupply has multiple +12V rails they are commonly limited to 20amps each. Your car amp would overload the powersupply as soon as a load is applied . eg: when the Stereo is turned on and calls up the amplifier. It would not matter if you had a 600W or 1000W computer PSU if the +12V rail connected to the stereo amp can only deliver 20amps or less
Check the label on the side of the powersupply. It will list the amp ratings for each voltage rail..
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