With the amount of amperage your amplifier draws, you will be fine without a second battery or capacitor. This is assuming you are driving a car with a decent sized stock alternator. Some of the imports have small alternators. Check your manual for the amperage rating on your alternator.
I personally run much higher wattage amplifiers in my truck drawing a lot more current on an alternator rated at 95 amps I've not had a single problem.
If you start to notice lights dimming or if you put a voltage meter on it while stereo is playing and you are noticing voltage dips, then I'd worry about a cap or second battery.
-Jesse
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2001 Ranger XLT Off Road
Sony CDX-C910, Sony CDX-U303
Sony XA-300 USB->unilink adapter
In dash 5.8" LCD
Infinity Beta 3 way component set
4 - OZ Audio 250L 10" subs
2 Sony XES-M1's, Sony XES-M3
Esoteric Audio wiring
Toshiba 7020CT laptop with DVD docking station.
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