Car audio units consist of small, built-in amplifiers which are used to power speakers. The amplifiers are buried inside the car; it cannot be seen in an OEM (original equipment manufacturer) sound system like the head units and speakers. The car amplifiers are integral part of car stereo systems as they provide power and volume necessary for tuning music inside the car.
Without an amplifier, the high-quality music reproduction inside the car can never be noticed. An amplifier is generally used to boost the low-level audio signals and make them powerful enough to move the speaker cones and create sound. It is always necessary that the signal be processed by a preamplifier before the signal is amplified.
The preamp which is inside head unit generally is used to take raw signals from the various sources such as CD player or radio tuner, and send the low level output to the car amplifiers. The preamp stage controls bass, treble and equalization which manipulates audio signal that ultimately adjusts the sound.
Let us discuss what preamp needs. It generally involves crossover circuitry. It divides full-range audio signal that's fed to amplifier into many separate frequencies. The two types of crossovers are an active crossover that divides line-level signal before its amplification. A passive crossover takes an audio signal after amplification and draws off frequencies for specific car speakers.
The amplifier creates high-power alternating electrical current that works in combination with the speakers to create sound. Vacuum tubes are used to amplify electrical signals, and are used in many high-end home systems. The tube amps are susceptible to many heat and vibration changes and are not so ideal in an automotive environment.
Electronic transistors amplify audio signals in a car audio system. The use of electronic components capacitors and resistors, help an amplifier to boost an inaudible line-level signal from head unit so it's powerful enough to move a speakers cone back and forth to create sound.
Amplifiers produce heat, so the heat dissipation process is a major part of its design. The small low-power amplifiers are built inside head unit and are crammed inside it to prevent overheating. Its exterior is made up of "heat sink." These have ridges or fins similar to a car radiators, to create more surface area to radiate the heat that's generated inside amplifier. These have generally two channels - left and right. Its power output is measured in watts per channel. A higher wattage means more power output to the car speakers, and turn it into louder volume.