You will need a LOC (Line-Out Converter) to convert the high-level speaker signal to low-level RCA signal to the amp(s).
If your total system power is 1200 watts RMS or more, you will need to:
1. Upgrade the alternator to a high output alternator
2. Upgrade to a yellow top battery
3. Install the "Big 3" with at least 2 AWG wire.
Failure to do this will result in the early death of the alternator as it will eventually fail.
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A second battery is never a good idea as it's an added strain on the alternator to charge two batteries AND supply power for the car. The ONLY time a second battery is useful is if the audio system is to be used with the car off.
A capacitor is only good so far as the audio system isn't trying to pull too much from the electrical system. A cap doesn't provide more power, it's designed to 'stiffen' the voltage to the amp, nothing else. If the current isn't there, a cap won't help.
The alternator must have at least 20% more amperage power than the entire vehicle and sound system combined for a cap to be of benefit which is ironically the same requirements for an amp to be efficient.
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