Car audio head units in all basic factory sound systems have small, built-in amplifiers to power a handful of speakers. Meanwhile, premium factory-installed sound systems usually use larger, more powerful "outboard" amplifiers that are separate from the head unit to power more speakers and achieve better sound quality.
Since an amplifier is typically buried in the car, you'll rarely see it in an OEM (original equipment manufacturer) sound system like you would a head unit and speakers. But amplifiers are integral components that provide power and volume to your car tunes, and they play an important part in the character of the music you experience in your car. Without an amplifier, you could never experience high-quality music reproduction in your car.
Before the Boost
An amplifier boosts the low-level audio signal generated by the head unit so that it's powerful enough to move the cones of the speakers in the system and create sound. But before the signal can be amplified, it has to be processed by a preamplifier or "preamp."
There can be more than one preamp component in a complex car audio system. Preamp refers to any stage in the audio path in which the signal is processed before being amplified. The preamp inside a head unit takes the raw signals from the various sources in the head unit, such as the CD player or radio tuner, and sends the low-level output (also called line-level output) to the amplifier.
The preamp stage is also where such tone controls as bass, treble and equalization manipulate the audio signal that will ultimately adjust the sound. When the signal reaches the actual amplifier, its internal-input preamp stage processes it further.
Part of the preamp processing typically involves circuitry known as a crossover. It electronically divides the full-range audio signal that's fed to the amplifier into separate frequencies. Think of a crossover as an audio-signal traffic cop, directing specific frequencies so they can be reproduced by specialized speakers, such as woofers and tweeters.
Many audio systems use two types of crossovers. An active crossover is a preamp component that divides the full-range, line-level signal before amplification so an amplifier or an amplifier's separate channels handle only a certain frequency range for specific speakers. A passive crossover accepts an audio signal after it's been amplified and siphons off frequencies for specific speakers.
Passive crossovers are found on "two-way" speakers that have a pair of speakers mounted on a single frame. A two-way 6-by-9-inch speaker that's found in many stock audio systems incorporates a large midrange and small tweeter. In that case, an audio signal will first pass through a small passive crossover that separates the frequencies between the two speakers.
A drawback of passive crossovers is that they cause an inherent power loss by attenuating certain frequencies. That's why many car amplifiers have built-in active crossover circuitry. It allows the amp's channels to be specialized for specific speakers and also to operate more efficiently.
Power and Heat
Following the preamp processing, the amplifier creates a high-power alternating electrical current that works in conjunction with the speakers to create sound. Vacuum tubes were once used to amplify electrical signals, and they are still used in some high-end home systems. But tube amps are susceptible to heat and vibration and so are not ideal in an automotive environment.
Electronic transistors typically amplify the audio signal in a car audio system. Using electronic components such as capacitors and resistors, an amplifier boosts an inaudible line-level signal from your head unit so it's powerful enough to move a speaker's cone back and forth to create sound.
During this boost, amplifiers generate heat, so heat management is a major part of an amp's design. Small, low-power amplifiers that are built inside of head units can be crammed inside the dash without fear of overheating. But outboard amplifiers produce significantly more power, and therefore a lot more heat.
Part of an amplifier's exterior is made up of what is known as a "heat sink." Similar to a car's radiator, the heat sink has ridges or "fins" to create more surface area to radiate the heat that's generated inside the amplifier.
Amplifier Power Ratings
Since we listen to most music in stereo, amplifiers generally have at least two channels — left and right. Amplifier power output is measured in watts per channel. A higher wattage rating for an amplifier means more power output to the speakers, and in turn louder volume.
The industry standard for obtaining amplifier wattage ratings is RMS. This stands for "Root Mean Square" and is a mathematical equation for measuring the magnitude of the output signal. While you don't need to calculate this formula when you're shopping for a stock car audio system, you should be aware that some aftermarket electronics manufacturers take liberties with wattage ratings to make their amps appear more powerful. (With OEM audio, however, amplifier power ratings are usually legitimate.)
One tactic that aftermarket car audio companies use is to post "maximum" or "peak" power ratings instead of RMS or continuous-power ratings. But just as you don't continuously drive your car at its maximum speed, you'll rarely achieve the maximum power output claims made by some aftermarket companies. Look for RMS ratings to be sure you're comparing apples to apples when you're shopping for aftermarket amplifiers.
The Consumer Electronics Association trade group administers a voluntary car audio amplifier power measuring standard called CEA-2006-A. Many aftermarket car audio brands adhere to the standard and include a CEA-2006-A logo on their packaging so that you know their power measurements are legit.
Don't Blame the Speakers
While almost any amplifier will produce around twice its RMS power in peak mode, it's usually with very high distortion. This occurs when the amplifier is pushed to its limits and creates severely distorted sound that can damage a system's speakers.
When an audio system suffers from distortion, your first impression may be to blame it on defective speakers. But in reality it's more likely caused by distortion from the amplifier. The small amplifiers built inside of head units — or even some outboard amps in OEM sound systems — are not always up to the task of cranking out tunes at very loud volumes.
In fact, most amplifiers will become audibly distorted at about three-quarters of a system's overall volume. If you have to crank up the volume to that level to hear your music the way you like it, your amplifier is simply running out of steam. If you only replace the speakers in an underpowered system, you're bound to be disappointed.
Out of Sight, Out of Mind
Since they are usually out of sight and out of mind, amplifiers are probably the most misunderstood part of a car audio system. But these crucial components make the difference between good and great sound in an audio system, and knowing a bit about amplifiers can help you get great sound in your own car.
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