Do I need to buy a capacitor for the system in my car? - diyAudio
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Old 18th November 2011, 12:36 AM   #1
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Talking Do I need to buy a capacitor for the system in my car?

I have A kewood Amp which, Bridged at 2 ohms peaks at 500w and A 12" MD Sound Sub, the sub is 800w RMS and 1800w Peak with dual 2ohm voice coils all connected to a 1997 GMC Jimmy. My question is, do I need a capacitor? because when I turn up the volume the head lights, int. lights, and dash lights all dim with the music even while the vehicle is running.

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Old 18th November 2011, 12:50 AM   #2
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What would a capacitor do for this application and where would you put it?

The problem that you are experiencing is that your alternator likely isn't able to supply the power required to run your sound system and the rest of your car when you turn the volume up.

I would recommend purchasing a bigger alternator.. something around 130-140 amps.

Your amplifier is probably consuming around 42 amps of power assuming that it is drawing 600 watts at full load (since it is not 100% efficient). If your alternator is only producing 80 or 90 amps and the rest of your car requires 60-70 amps, you will be bumping up against the limits of your alternator.

To find the amperage drawn by your amplifier, divide total power in watts by voltage.

Finally, I would recommend getting a smaller sub woofer or upgrading your amp since it is not providing enough power to power thus sub. You should get an amp that can provide more than the RMS power specified by the sub. I have a smallish 8 inch subwoofer in my room that draws about 250 watts and it is plenty powerful for a 14x12 space so 800 watts is probably overkill imo.
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Old 18th November 2011, 01:08 AM   #3
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Originally Posted by techbiker View Post
What would a capacitor do for this application and where would you put it?

I Don't know, I was told that a cap would store temporary power so the alternator would not need to produce as much and the amp would get an even amount of power and run more efficiently (or something to that extent).
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Old 18th November 2011, 01:38 AM   #4
kaos is offline kaos  United States
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Geez my factory alternator is rated for 130 amps. Thatís not that much by todayís standards. Should be thinking more like 200 amps+ if you have problems like dimming lights. Keep in mind that full alternator output isnít available at idle, but at around 6,000 RPM alternator shaft speed (around 2,000 RPM engine speed). Thatís why you need one thatís rated for considerably more than what your total electrical load is.
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Old 18th November 2011, 01:43 AM   #5
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Originally Posted by familyguy9877 View Post
I Don't know, I was told that a cap would store temporary power so the alternator would not need to produce as much and the amp would get an even amount of power and run more efficiently (or something to that extent).
Takes a big, expensive capacitor to do what you want. A higher powered alternator may cost less.
Before you put money in either, make sure the wires to your amp are super fat (like #6 or better), long thin wires at 12V can eat lots of juice that the amp never gets.

The speaker wires for a two ohm load, unless quite short, should be at least #10 or so to keep damping (punch factor) good.
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Old 18th November 2011, 01:54 AM   #6
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The standard big car audio caps are pretty much useless. Their ESR is too high to be useful as a true capacitor, and they don't store enough energy to be useful as a power source.

Most of the energy store in the cap is never even used. A 1F cap charged to 14V is 98 Joules of stored energy, yet if your system voltage drops to say 12V under load, you are only ever using 2 Joules of that 98. See how pointless the cap is now?
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Old 18th November 2011, 02:34 AM   #7
18Hurts is offline 18Hurts  United States
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I have a few thoughts

The amp bridged into 2 ohms puts out 500 watts...but you have a dual 2 ohm voice coil subwoofer? If you connect both voice coils together + to + and - to - that is a 1 ohm load. Your amp will attempt to drive that and suck amps as hard as it can to do it. If your voice coils are connected in series, that would be 4 ohms so your electric draw would be considerably less.

Bridging a "regular" amp into a 2 ohm load is a very inefficient way to drive a sub--it draws max amps and your efficiency goes way down as it generates a ton of wasteful heat. I would connect the sub this way to see if your pulsing lights go away.

Amp + to + on one voice coil
The - on the first voice coil to the + on the second voice coil
- on the second voice coil to the - on the bridged amp

That will give you 4 ohms and cut your electrical draw by at least 50 to 60%. If it does not dim anymore and you like the output--done!

If you feel the need to run 500 watts--get a digital amplifier for that function. They are much more efficient than Class A/B and won't eat the amps your electrical system can't provide. If you feel the need for more than 500 watts (because it sounds cool to say 1,000 watts) run a second sub to gain the +3dB which is the gain from 500 to 1,000 watts you would get running one sub.

It is not the watts, it is the output. Taught a friend of mine that and he blew the glass out of his car at "only 400 watts". The sub box weighed 325 pounds and was HUGE... but it sure was efficient!
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Old 18th November 2011, 04:39 PM   #8
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Thanks for all the feedback I will try wiring my sub differently.
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Old 18th November 2011, 04:56 PM   #9
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I would do a good battery first, the big 3, make sure you have good wire (4 gauge or thicker, i like 0 gauge then a dist block to all your amps) going to the amp, THEN do the alt if you don't have enough power. I think that just the alt. without the other stuff won't help much.
after all that, if you still have a dimming problem, step up to a larger class battery, or add a second one, again, with good thick wire and good grounding points.
knuzconceptz has great prices on all the installation stuff you need.
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Old 18th November 2011, 05:19 PM   #10
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Good electrical wiring is a given, however you won't get anywhere unless you find out what your alternator is rated at. If it's only producing 90 amps, then no capacitor will solve your real issue. I believe that when you upgrade your car, it's important to do it correctly. Adding a second battery just masks the root problem... which is likely a weak alternator.

For safety sake I will try to find out what your alternator is rated at before you do something foolish. Electricity close to fuel in a closed environment is no joke. One spark could be disasterous.
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