The Basics: Car Amplifiers
Sometimes, you just need to crank up the volume. Car audio systems must overcome a good deal of ambient noise--a vehicle traveling down the highway in the company of other loud engines is surrounded by a high background noise level. Factory-installed head units have a modest amount of amplification (generally about 10 watts per channel or less), and many aftermarket head units have at least twice that. The more power the amp included with your CD or cassette player delivers, the cleaner the sound from the speakers. But be careful--a receiver rated at 50 watts maximum power probably has no more than 20 watts of clean power. For that reason, serious mobile listeners eschew the modest amplifiers built into head units and instead opt for powerful external amplifiers.
How to Shop
For the easiest and typically least expensive installation, look for a single amplifier to power all the speakers in a car system. This would be the number of channels listed on the amp, as a single channel is only capable of powering one speaker. Moreover, look for an amplifier that contains its own crossovers; these circuits split the music frequencies so (for example) the highs go to the front speakers while the lows go the rear speakers. This helps maximize power delivery to the speakers and improve sound. When shopping, look at the way an amplifier lets you control these crossovers. Some use rotary controls, while others have less accessible inset controls requiring a screwdriver for adjustment.
You could consider amplifiers with one, two, or four channels, but you'll need a channel for each speaker; this soon increases the complexity and cost of the system. Also, if you'll be adding a subwoofer for killer low-ends sound, you'll usually need a separate amplifier. Remember, every speaker will use one channel of amplification. Four-channel amps are, by far, the most popular.
Amplifiers don't really come with a massive array of features, but there are a few to consider. Soft-start circuitry eliminates the thumps and thuds that can be annoying and potentially damaging to car speakers. Depending on your application, the number of speaker-level and line-level outputs and inputs need to be considered. Make sure you have enough for the setup you are currently installing, while leaving room for at least one future upgrade.
Amps generate a significant amount of heat that affects their efficiency. Numerous methods of dissipating this heat are used--everything from metal fins that create more surface area for air cooling to a few systems that actually use liquid coolant to keep the temperature down. Read reviews and specs to choose between hype and facts.
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