Capacitors FAQ

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 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.

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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.

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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.

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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.

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