Favourite Car Electronics

The power amplifier is the main element of the sound system. This device receives a low-level signal from a line-out of head device and amplify its voltage and current to the necessary quantities sufficient for the normal work of the loudspeakers. In this article we will tell you about arrangement of amplifiers, their classification and their place in the automobile stereo system.

Classification Of Amplifiers

The amplifiers can be divided symbolically into four main parts: the AC power adapter of amplifier, signal processing unit, the driver and output shaping unit.

The Power Unit is a group of electrical circuits that create and regulate voltage for feed various parts of the amplifier.

The Signal Processing Unit compares the input signal received from a preamplifier of radio-recorder with an output signal of amplifier for the adjustment in order to remove distorshions arising at amplification. In addition, the unit increases the input signal to the level necessary for its subsequent amplification in other parts of the amplifier.

The Driver separates the signal into two heteropolar signals (phase separation) and amplifyes it for onward transmission in output shaping unit.

Finally, the last stage of amplification is output processing unit (it correctly to identify it as output cascade or terminator), which is basically defines amplifier class.

Amplifiers are divided by classes, depending on their performance (efficiency) and the level of distortion of output signal:

• Class A. Amplifiers of this class have low efficiency, but give a very "pure" signal. The majority of "Class A" amplifiers have efficiency that is equal 20 - 30%, that is, when consuming of 100 Watts from a car battery, it gives an output signal to the dynamics of power in 20 - 30 Watts. The remaining power is lost in the electrical circuit of amplifier, becoming warm. High-quality amplifiers of A class is rarely used in car audio because they have little power at very high prices. Valve amplifier of Class A can be found only in the very expensive audiosystems of Hi-End level.
• Class B. The efficiency of this class of amplifier almost twice higher then efficiency of the amplifier Class A. However, the distortion of the output signal is very high, that makes this class of amplifiers unacceptable for the car audio.
• Class C. Amplifiers this class have efficiency that equal to nearly 75%, that makes them very effective, but with gain of efficiency distortion increases dramatically. These amplifiers are not suited for amplification the sound in the Hi-Fi stereo system.
• Class AB. Most Hi-Fi amplifiers belong to this intermediate class. They incorporated the possibility of amplifiers of Class A - relatively "pure signal" with relatively good performance (slightly lower than in class B).
• Class D. This is the most modern class of amplifiers using digital signal processing. Amplifiers of Class D are very compact, that in the future will give them an advantage on the market of automotive sound system. Currently, digital car amplifiers are much rarer than popular analog amplifiers of Class AB.

Total Harmonic Distortion (THD)

Audio signal consists of multiple frequencies and halftones. Harmonics is halftone of the original note (basic frequency), that is responsible for the character of its sounding. An audio signal can be presented as a complex combination of oscillations of precisely interrelated sine waves (harmonics).

In the process of amplification, passing through the various blocks of amplifier, the sound signal is distorted, "accumulating" by unnecessary harmonics. The increased number of harmonics in amplified signal, expressed as a percentage, is the Total Harmonic Distorsion. In the specification of the amplifier there are several THDs indicated for various frequency bands, output power levels and the resistance load. The lower the ratio, the higher quality of amplifier.

Channel separation (Stereo Separation)

This index shows the level of isolation of two amplification channels (left and right) from each other. Their interaction is due to the existence of a common power unit in the amplifier. This indicator is expressed in decibels and characterizes the level of intensity of the left channel relative to the level of "leaking" in it right channel and vice versa. The higher the figure, the better the amplifier. To avoid "leakage" you can replace one stereo amplifier with two separate mono amplifiers. In the high-end class, the problem is solved with installation of two power supplies units in a single stereo amplifier.

Damping Factor

Damping factor of the amplifier is defined as the ratio of resistance of loudspeaker to resistance of amplifier. The lower loudspeaker resistance, the lower damping factor.

Valve amplifiers due to design features have low damping factor that causes the "soft" bass in the sound picture. Producers of transistor amplifiers are trying to improve the damping factor for reproduction "hard" bass, as if you want to make bass "softer" you nee place low-frequency speacker in wood box.

Connection and setting up amplifiers

Scheme with a sinle 2-channel amplifier

On the picture you see the scheme, using a single two-channel amplifier which each channel have two component speakers connected (two front and two rear). This is the most simple and cheap scheme of amplification without use of an active crossover. Please note that a pair of rear speakers are connected to the main front pair in parallel. Parallel connection of speaker reduces their resistance twice. If the amplifier ha impedance load equal to 4 Ohms, the parallel connection of two 8 Ohms speakers is quite acceptable. The main thing when connecting speakers is to correctly calculate their total resistance. Do not make it less than the load resistance of amplifier.

Scheme with a two 2-channel amplifier

On this picture you see the scheme, which enhance the low-range audio signal separately from middle and high frequency bands separated by electronic crossover. Because the subwoofer has efficiency less than tweeters, it consumes more power from the amplifier than the last for making equal sound pressure. Amplifying in one amplifier, bass frequencies take most of the power and virtually nothing left for middle and high frequencies, which have become badly emerging in the sound picture. The increase of volume for "stretching" middle and high frequencies cause distortions in the low frequencies. Sound picture become completely bad.

If, however, increase the bass frequencies separately from the rest, we have an excellent opportunity to make medium and high frequencies loud enough and bright, not distorting the low-frequency component of signal. Sound picture becomes clearer, and the effectiveness of the system significantly increased.

For example, if we have an amplifier for the subwoofer output of 60 W, then for good sound in the car for medium-and high-frequency you can yse separate amplifier with only 20 Watts of power. If the crossover is configured correctly, each amplifier receives its own portion of frequency range and the potential sound pressure level (SPL) of this system will be equivalent to power 150 W, rather than 80 watts (60W + 20 W).

Bridge connection

When bridged connection is used then positive wire of speaker output of the one channel is combined with of speaker output of the second channel. Combining so left and right channels, we get a much more powerful mono channel to connect to the subwoofer. Its power is four times more then power per channel before using bridge mode because power is a square of the voltage divided by resistance that is remains unchanged.