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Capacitors FAQ

What's a capacitor? What they do and when to use one

Installed capacitor

C

apacitors store up power from your battery, then release it to your amp during peak demand for more consistent bass. They are often used as a buffer zone between your amp and your car's electrical system to prevent the car's lights from dimming during loud playback. But how does a capacitor work? Do you really need one? 

» What's a capacitor?

» What's a capacitor used for?

» Even if my lights don't dim, won't a capacitor still improve my system's low end response and overall sound?

» What size capacitor should I get?

» How do I hook up a capacitor?

» My capacitor has a third terminal. What's that for?

 

Q: What's a capacitor?

A: A capacitor, or cap, is an electronic component that can take up, store, and discharge electrical energy. Because they can do all that quickly, capacitors are used to filter or buffer any sudden changes in a circuit's voltage, smoothing the ensuing signal.

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Q: What's a capacitor used for?

A: In car audio, large outboard capacitors, sometimes called stiffening caps, are used to prevent lights from dimming when loud bass notes play. They accomplish this by supplying the amplifier with a quick jolt of power.

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Q: Even if my lights don't dim, won't a capacitor still improve my system's low end response and overall sound?

A: Not really. A cap prevents the sound from deteriorating due to under-voltage, but doesn't actually improve the sound. It supports the amplifier by feeding it the power it needs for short bursts. So, while not improving sound quality directly, a cap does make it easier for the amp to perform its best.

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Q: What size capacitor should I get?

A: The rule of thumb is to put in 1 Farad of capacitance for every 1,000 watts RMS of total system power. But there is no electronic penalty for using larger value caps, and in fact, many see benefits with 2 or 3 Farads per 1,000 watts RMS. The larger the cap, the more charge is available for the amp when it needs it.

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Get everything you need

Give your subwoofer amp the boost it needs with a capacitor. We also have all the installation hardware you need to get your capacitor and amplifier up and running.

Q: How do I hook up a capacitor?

A: If you don't have the instructions that came with your capacitor, you should know first off that a cap can be dangerous. It can charge and discharge so much power so very quickly that it can weld metal objects, like tools and jewelry, and melt its own insides out.

A new cap comes completely discharged, so it's safe. A resistor or wired light bulb usually is included in the package. Wiring the bulb or resister across the cap's terminals allows the capacitor to discharge slowly and safely. The same bulb or resistor, wired differently, also gets used to charge up the cap safely.

As in all car electronic installations, start by disconnecting the ground cable from the car battery. In this installation, also take out the in-line fuse on the amp's power wire next to the battery.

A capacitor should be mounted as close to the sub amp as possible using the shortest wires possible. This is so the extra charge doesn't have far to go to get to the amp quickly. Make sure the cap gets mounted securely and won't become a dangerous flying object in the event of an accident.

A capacitor has two poles: a positive and a negative. They should be clearly marked on the capacitor. The positive connects to the same positive power lead that goes to your sub amp's positive, 12 volt, connection. Use the same gauge wire as the amp uses for its power. This can be accomplished with a distribution block. Or, sometimes, the cap comes with multiple connection terminals that make it easier to wire it into your system. The multiple terminals act just like a distribution block so, for instance, the power wire coming from your battery can connect directly to the cap's positive terminal while a short cable connects from there to the amp's positive power connection. The negative pole of the capacitor connects to your chassis ground, just like the amp. The best practice is to use the same bolt the amp uses for ground. Make sure all the paint is scraped off around where you put the chassis ground and the connections are clean and tight.

Next, you need to charge up your capacitor. If done too quickly — it could "pop," destroying the cap.

If you don't have the original charging/discharging resistor or light, you'll need to get one. An automotive 12-volt test light, the kind with a bulb, not a small LED, will do nicely. Otherwise, you can use a high-wattage, low resistance resistor, available at most electronics parts stores. The exact value doesn't matter, but get one with a value of 10-1,000 ohms along with a rating of 1-20 watts. The lower the resistance, the higher the wattage should be.

Take the test light or resistor and connect it to the two terminals of the amp's in-line fuse holder (where you took the fuse out earlier). Re-connect the car battery's ground cable. The resistor will get hot, or the bulb will light up, while the cap charges. After 10 to 30 minutes, the bulb will fade out, or the resistor will start to cool. Remove the light or resistor carefully — they can get very hot. As you replace the fuse, you may experience a small spark — that's okay, but should remind you of how powerful the electric forces involved are. Your capacitor is now installed.

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Q: My capacitor has a third terminal. What's that for?

A: Some caps come with a built-in meter that displays the voltage. If a meter were to stay on constantly, it could drain the car's battery. So caps with meters often have a remote turn-on lead connection, just like car amplifiers, so the meter turns off with the system. A thin, 18-ga. or so, wire should run from the cap's turn-on connection to the amplifier's remote turn-on terminal, or any other switched 12 volt power source.

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T-Spec capacitor

T-Spec 3.0-farad capacitor

  • Chris from Central Valley

    Posted on 9/21/2021

    Hey Buck, I recently installed an NVX 1 Farad capacitor with my NVX 1200 rms 5 channel amp and 2 JL Audio 10" subs. I charged the cap per the instructions but it seemed like it took awhile to charge it up. The voltage meter worked initially but now isn't working at all. Should I discharge the cap and recharge it? Replace it? Or how would I see if it's holding any power? My headlights don't dim at all when at high volumes and high bass demands. I have a 2019 Toyota Tundra. Thanks for your help!

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    Buck Pomerantz from Crutchfield

    on 9/27/2021

    Chris, It sounds like you should take the cap back to where you got it and see if they will replace or repair it. If your lights don't dim on big bass hits, you won't need a cap anyway.
  • Jim from Detroit from Detroit

    Posted on 8/16/2021

    I own a classic hot rod (1991 Camaro Z28) and have rewired the entire sound system. I have installed a Rockford Fosgate R600x5 and wanted to add the 1 Farad Rockford Fosgate Capacitor. My question is, I don't drive it very often and want to have the power there when I want to turn it up. If I disconnect the battery for storage, what will my issues be? If I leave the battery connected and don't run it for 3 weeks, what will my issue be, if anything? Thanks in advance. -Jim

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    Buck Pomerantz from Crutchfield

    on 8/17/2021

    Jim, When storing a car for a while, it's best to use a battery maintainer device to keep the electrical system and battery charged, so it won't lose any computer settings or functions. A disconnected capacitor will eventually lose its charge, so to reconnect, you'd need to use the resistor or light bulb used to charge the cap when first installed.
  • Graham L Thick from Hickory

    Posted on 7/15/2021

    What would be the wiring sequence to add an additional 3 Farad T-Spec capacitor to the current 3 Farad T-Spec capacitor.

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    Buck Pomerantz from Crutchfield

    on 7/16/2021

    Graham, You install a second capacitor the same way the first one was installed - negative pole of the cap connects to ground and the positive pole to the positive power wire near the sub amp, after charging the cap through a light bulb or resistor before finishing the connection.
  • Ray from Wheat Ridge, CO.

    Posted on 6/22/2021

    I run a circuit breaker in place of a fuse holder. How can I charge my capacitor in this case? I really like the idea of circuit breakers vs. fuses and am not really willing to change back to fuses. There are reasons why most electrical circuits use them now instead of fuses.

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    Buck Pomerantz from Crutchfield

    on 6/23/2021

    Ray, You charge a capacitor the same way whether the power line is protected by a fuse or a circuit breaker.
  • Rob from Melbourne, Australia

    Posted on 5/19/2021

    My system is in a 12v Kemworth semi truck. I have a set of jl audio c2 splits running off a xd400/4 and sealed 12w7 with hd750/1 and 8ga and 4ga OFC power and ground wires respectively. The problem im having isn't with headlights dimming, it's that at anywhere up near max the system drops in volume significantly. While this is happening Sometimes the light on the hd amp flickers green/red , sometimes it stays green. Will a capacitor fix this?? Should also add that originally I had another sealed 12w7 with a alpine pdxm12 which also had the same problem with loosing volume and didn't take long to blow that first 12w7, I always assumed maybe I put too much power to it with the m12 which is why I went with the hd750 this time but its still having same problem of loosing power all the time. I'm really hoping a capacitor can sort it out

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    Buck Pomerantz from Crutchfield

    on 5/20/2021

    Rob, You are describing exactly what happens when a JL Audio HD amplifier is over-driven. Here's their explanation: "Should a JL Audio amplifier equipped with Advanced Rollback Protection reach an unsafe operating temperature, a special circuit rolls back peak power output, without changing gain, until the amplifier cools down to a safe operating temperature." What this means to you is the amp's gain control is set incorrectly, allowing the amp to overload and go into protect mode. Lower the gain setting so no distortion or overload can ever happen.
  • Evan Iles from Findlay

    Posted on 5/6/2021

    I currently have a 2 farad capacitor for my 1200watt amp. I drive a hybrid and when it switches from the electric drive train to the gas engine I get this surge/swell sound out of my subs. It also happens when the engine is running but there is plenty of batter (engine battery) power: I.e, the engine is cold and still warming up or the heat in the car is on. Would getting bigger (or second) capacitor help with this? If a second is the way to go, what's the wiring sequence? Thanks

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    Buck Pomerantz from Crutchfield

    on 5/7/2021

    Evan, A capacitor will provide a boost when there's a voltage drop, it won't mitigate a power surge, so adding another cap won't solve this issue. Perhaps consulting your vehicle's dealership service department can help you find out whether this is a solvable problem, like automatic noise cancellation, or a design feature inherent to the vehicle's electrical system.
  • Haleem Shaffiq from Derby

    Posted on 5/4/2021

    Hi can you ground a car power capacitor through a ground distribution block that has the 2 amps grounded to it. It has 4 gauge wiring tonone 0 gauge wiring connected to the car chassis. Thank you

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    Buck Pomerantz from Crutchfield

    on 5/6/2021

    Haleem, Yes, you can ground your capacitor to the same spot as your amps.
  • Benny from New Jersey

    Posted on 4/21/2021

    What happens to the capacitor when car is switched off and parked? Does it slowly discharge? In that case should it be charged again as per the safe procedure using resistor/light bulb or once safely installed in the car we can forget about charging it more or less as it happens automatically? My concern is constantly charging it carefully after car is parked for quite sometime.

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    Buck Pomerantz from Crutchfield

    on 4/21/2021

    Benny, A properly installed capacitor connects one pole to ground and the other to constant +12-volt power from the battery. When the car is running, the voltage of the electrical system and capacitor's charge will be approximately 14.4 volts. When the car is switched off, the cap's charge will drop to match the battery's 12 volt charge. As long as the battery has charge, the cap will too.
  • Brendyn

    Posted on 4/17/2021

    When replacing the resistor with the online fuse, will the capacitor drain its charge before the fuse is back in? 4farrad capacitor.

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    Buck Pomerantz from Crutchfield

    on 4/19/2021

    Brendyn, When removed from power, a cap keeps its charge for a while. Certainly long enough to change a fuse. Some power capacitors, especially in radios, TVs, and guitar amps, can hold a dangerous charge for years.
  • Joshua from Warren

    Posted on 3/25/2021

    How do I know if my capacitor is bad ? Like is there a certain reading I should get from the capacitor itself ? Cause it has a led reading display on the face of it think said 12.9 last time I saw it . I ask this cause seems like my amp or car has to be warm or outside has to be warm before it will either power up or to we're I hear the amp actually working ?

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    Buck Pomerantz from Crutchfield

    on 3/29/2021

    Joshua, If the display shows over 12 volts, the cap is probably okay. A way to tell for sure is to disconnect it from the battery and measure the stored voltage with a multimeter to see if it holds charge.

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