2 amplifiers, one capacitor, not enough power??


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I have just recently purchased a 93 civic ex coupe. I upgraded the sound system with 2 (6&1/2 inch) and 2 (5&1/2 inch) sony Xplods and 2 Power Acoustic 12" subs (700 watt rms. 1800 watt max) all speakers are powered by 2 rockford fosgate amplifiers (601s and 501x) each at 1800 and 1500 watt max power respectively with a combination of 3300 watt max. I have also installed a 3 farad capacitor.
The problem: The capacitor reads below the standard 12V supposed to be powered, all the way down to 9 at times (when the engine isnt running) and when the engine is running the voltmeter on the capacitor reads around 10-11 Volts. The sound bangs, but the subwoofer cannot produce the low bass frequencies (which it should be able to do) and the lights periodically are dimming. I beleive that the problem is the lack of power that is being supplied to the amps, so im thinking of upgrading the alternator and battery (if neccesary) and am considering either a 135 amp or 150 amp alternator. Is this the right choice??? Is there something else that I should do, and if a new alternator is required (which it probably is) how many amps should it be at?? and if a new battery is recomended, how many cold cranking amps should it be supplying??

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 3088
Registered: Dec-03
you need a higher output alternator.
it's correct to use max ratings of the amps for capacitor sizing, in that you need about 1Fd for every 1Kilowatt of max-rated power, since the capacitors are only really helpful for short bursts.
The problem is that your alternator isn't supplying enough current to handle the continuous (RMS) current draw of your amplifiers.

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 3089
Registered: Dec-03
oh, and the upgraded batteries will only help for when the engine isn't running, and ideally you'll then want to go with a battery isolator, and yellow top deep cycle batteries in the back just for the audio system while keeping a high cranking amp battery for the starter.

in regards to alternator size, find the stock rating of your factory alternator first, probably by calling a shop like Advance auto parts and asking them.
then find the combined RMS amplifier wattage, and divide that by 12. for the 4 channel amp (class AB) add another 40% of it's total, and for the class D sub amp, add another 20% of it's total.
take that sum and add it to your factory alternator rating for an idea of the size your alternator should be.

2 amps.. 1200wRMS for subs, and 300wRMS (75x4) for mids/highs.

1200w/12V = 100A
100A + 20% = 120A

300w/12V - 25A
25A + 40% = 34A

that's 154A of current draw for the combined amps at full output. Now, if your stock alternator is 50A, you'd want an alternator in the 200A range, ideally. Then keep the capacitor to increase transient response times, and help out at peak momentary demands.

for simpler math, to add 20% multiply by 1.2, and to add the 40%, total*1.4

If the alternator size you get is too big for your wallet, you can usually get away with adding the amplifier total to about 60% of the stock alternator's rating insteat of the full rated output. Most cars only use about 60% of the stock alt's current anyway, but I hate to go by that because I've seen as little as 15A (200W amplifier) cause a car to stall out completely on the stock alternator.. the factory cut the rating that close to the bone.
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