Strong musical impact from drums and bass require large energy storage and quick discharge. Measured in farads, car audio capacitors are called upon to supply this stored energy. The cap houses charge supplied by the vehicle's charging system, and quickly discharges voltage as needed by the connected amplifiers. Five-farad capacitors can in theory support any number of amplifiers, with the total wattage draw being the important factor.
Role of Capacitors
Car audio capacitors range from .5-farad units for smaller basic car stereos to units providing five farads and more. These are used to mitigate the effects of large current draw from powerful car stereos on a standard vehicle charging system: dim headlamps, sluggish vehicle response and weak sound. As strong musical notes place increased demands on an amplifier, the amp will pull current as needed from the vehicle to meet that demand. Capacitors supply voltages in excess of the standard 12 volts required of car audio equipment to operate, alleviating the strain on the vehicle's charging system.
All car audio amplifiers require a minimum of 12 volts to come close to the specified wattage claims of the manufacturer. Since very few vehicles stay at 12 volts, given electrical demands from other accessories, sound can suffer from a lack of current. If you had three amplifiers connected and not playing audio, they would all draw roughly 12 to 14 volts at standby. The amount of electrical draw from each amplifier is therefore a function of the musical frequencies each amp is playing, rather than the physical quantity of amplifiers. In order, most electrical demands are from low bass and high treble, with mid-range frequencies placing less of an overall strain on the system.
Farads vs. Wattage
A capacitor in a powerful car stereo rarely meets its voltage output limitations. This voltage overhead is useful in case multiple amplifiers require high amounts of voltage at once. The general rule of thumb is one farad per 500 watts of amplifier draw. Remember that most amplifiers do not reach their operating peak. The important factor therefore is total wattage output, measured as RMS or "root mean squared." This translates to average power output.
Amperage is an important factor for evaluating the stated wattage of an amplifier. Remember that there is roughly a 10:1 ratio between amperage draw and wattage. You can confirm this by looking at the fuses on the amplifier. A single 30-amp fuse means that the amplifier can produce 300 watts of power at most. A rating of 500 watts or more with a 30-amp-rated amplifier refers to peak power, attainable for very short periods before the fuse blows. Consider this as you build your system and select your capacitor.
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