Capacitors FAQ

What they do, and when to use one


Buck Pomerantz

Buck Pomerantz was born and raised in Philadelphia. His parents bought their first television set when he was born. He figured out how to run it by the time he was two. Besides athletics, his formative interests included electronics, amateur radio, music, and stage crew work. He got his BA in writing from Brown University. Then he joined a rock 'n roll band as their soundman and moved to Charlottesville, Virginia. After that venture failed, he spent time in Boston, New Orleans, and Berkeley. He worked in a music store in Austin manufacturing, installing, repairing, and operating sound systems for recording studios, clubs, and bands. He moved back to Charlottesville, ran a little recording studio and finally joined Crutchfield as a copywriter. He has 2 grown children and 3 grandchildren, but after a good nap he can still rock out.

More from Buck Pomerantz

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.

back to top

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.

back to top

Q: Even if my lights don't dim, won't a cap 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.

back to top

Q: What size cap 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 faster it gets ready for the amp's next big hit.

back to top

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," destroy 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.

back to top

Q: My cap 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.

back to top

T-Spec capacitor

T-Spec 3.0-farad capacitor

  • K. Hardy from Calgary, Alberta

    Posted on 4/25/2015 5:15:59 PM

    Can capacitors be mounted on the firewall under the hood?

  • K. Hardy from Calgary, Alberta

    Posted on 4/25/2015 5:30:26 PM

    Does a powered subwoofer also need a capacitor connection?

  • Buck Pomerantz from Crutchfield

    Posted on 4/27/2015 10:01:07 AM

    K, A capacitor can certainly be used with a powered subwoofer. A capacitor should always get mounted as close to the subwoofer's amplifier as possible, never under the hood in the engine compartment.

  • John from New york

    Posted on 4/27/2015 7:43:07 PM

    Can you put one cap on two amplifiers ?

  • Buck Pomerantz from Crutchfield

    Posted on 4/28/2015 10:26:21 AM

    John, A capacitor gets wired with one wire running from its negative terminal to ground and one wire from its positive terminal to the positive 12-volt power line of an amplified system. It does the most good to connect a cap closest to the subwoofer amplifier in a system, but it won't hurt to connect it elsewhere, like at a distribution block.

  • Cory Dzbinski from Oceanside, CA

    Posted on 5/14/2015 2:50:08 AM

    Hey there, good article. I have a question that seems obvious to me but I haven't see it asked or explained anywhere. Won't the cap slowly discharge when it's off? What if you don't drive your car for a couple weeks. Won't the cap be empty and be at risk to "pop" and destroy the cap if it then recharges quickly?

  • Buck Pomerantz from Crutchfield

    Posted on 5/14/2015 9:56:27 AM

    Cory, As long as the vehicle's battery maintains a charge, it will keep the cap charged. If the battery loses its charge or gets disconnected, then the capacitor will lose its charge and need to be re-charged slowly, like it was newly installed.

  • Lee

    Posted on 7/12/2015 9:11:21 PM

    I have had great success with stiffening caps in-line with A/B amps. Will a cap help with a class G amp?

  • Buck Pomerantz from Crutchfield

    Posted on 7/13/2015 4:03:11 PM

    Lee, A capacitor's function is unaffected by the technological Class of an amplifier's internal circuitry.

  • Nick Duval from Canada

    Posted on 7/18/2015 1:14:11 AM

    My capacitor has recently starting making a high pitched screeching noise what could cause this? It doesn't happen all the time, it happens randomly.

  • Buck Pomerantz from Crutchfield

    Posted on 7/20/2015 8:50:02 AM

    Nick, I have never personally heard of such a thing, but it probably is not good. I have seen capacitors fail by springing a leak, developing a hole where internal gases have pushed through the casing. Maybe that's what's happening to your cap.

  • Amanda

    Posted on 7/25/2015 9:58:24 AM

    Hi! I just want to ask if an audio capacitor has fumes? And if it has fumes in it, can it be dangerous to our health? Is there any chance that the fumes will leak? Hope you could reply to my email asap. Thank you!

  • Buck Pomerantz from Crutchfield

    Posted on 7/25/2015 4:52:09 PM

    Amanda, There are no fumes or gas inside capacitors. Capacitors are made of two foil sheets wrapped together with a chemical called a dialectic in between them. Like in a car battery, this substance may contain some acid, but you'd have to rub it in your eyes, roll in it, or eat a bunch before it would do you any harm. That being said, I think it is possible that when a capacitor fails catastrophically the electric charge could vaporize some of the dialectic and release a small puff of gas that if breathed in probably won't be good for you. If you or a friend think you've been exposed to such a thing, I recommend flushing the area out with water, and contacting your doctor for advice.

  • sr5573 from bhubaneswar

    Posted on 7/30/2015 2:13:10 PM

    what is the working principle of a capacitor?

  • Buck Pomerantz from Crutchfield

    Posted on 7/30/2015 4:01:38 PM

    SR, Check out this 5-page article How Capacitors Work. It ought to answer a lot of your questions about capacitors.

  • pelonomi from gaborine

    Posted on 8/4/2015 2:53:05 AM

    hi, amplifier(1200W), subwoofer(1000W), 6x9 speaker (350W), twitter(300W) and capacitor connected between amp and power source, my question is there is disturbing noise when car rev or on drive what could be the cause of that, and most of the time fuse (connecting amp with power source) are blown

  • Buck Pomerantz from Crutchfield

    Posted on 8/4/2015 12:08:25 PM

    Pelonomi, The number one cause of engine noise in a car audio system is a loose or intermittent ground connection. That could also explain the blowing fuses. Carefully check your amplifier's power and ground wiring and correct any faults you may find. Make sure the ground wire makes a clean, tight, and paint-free contact with your vehicle's chassis.

  • Mike from Santa Rosa Beach, FL

    Posted on 8/5/2015 1:48:29 PM

    How do you determine the MAX AMP INPUT/CHARGING AMPS a Capacitor can handle??? I would LIKE to connect a capacitor directly to the ALT. Output(My Stock one is 130AMP), to isolate the sub-woofer and any other amplifier circuit I add in the future. I have found that the MAXWELL Super-caps (2.7VOLT 350-500F), with 6 of them bundled together, 16.2VOLTS and 3000F, would make a great substitute for the car battery, as long as they don't drop below 10-11VOLTS or go without being charged for more than 2 days... What worries me, will the 130AMPS coming from my ALT destroy the caps??? THANK YOU SO MUCH in Advance!!!

  • Buck Pomerantz from Crutchfield

    Posted on 8/6/2015 10:23:00 AM

    Mike, The capacitors made for car audio application are all made to handle the high current that flows when the cap charges and discharges. You'll have to find out the current-carrying capacity of your super caps from the manufacturer.

  • Justin Stefko from Newton

    Posted on 8/6/2015 12:03:25 PM

    Can power capacitor be rebuilt or repaired? I have a 30 figured that doesn't work

  • Buck Pomerantz from Crutchfield

    Posted on 8/6/2015 4:48:51 PM

    Justin, I'm afraid not. Capacitors are replaceable, but not repairable.

  • Kel from Nashville

    Posted on 9/17/2015 1:55:57 PM

    I am about to change vehicles, and was wondering if I should replace my capacitor. I have used the capacitor for a little over 3 years and didn't know if replacing it was necessary?

  • Buck Pomerantz from Crutchfield

    Posted on 9/17/2015 2:11:55 PM

    Kel, It is not absolutely necessary to replace your capacitor, but it may help if your car's headlights dim a little when your music plays loud.

Products in this article

Find what fits your car