Simply put, an amplifier (amp) is a device for increasing the power of a signal. The “head” unit (such as a radio or CD player) in a car has limited power and typically does not have the power to operate additional or larger speakers. When adding aftermarket components, an amplifier is often necessary to provide additional output to the speakers and subwoofers. Basically the amplifier acts as the power source for the car sound system by modulating the power generated by the car battery to allow the speakers to reach their full potential. The “gain” of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels (dB).
Because of the large amount of power required to drive subwoofers and produce the low frequency sound, car subwoofers must have at least one amplifier to provide enough power for the subwoofers to operate as designed. If space is a concern, powered subwoofers are available in which the enclosure has both an amplifier and a subwoofer. The performance capabilities of the amplifier and subwoofer must be matched to avoid damage to the system.
Class D Amplifiers
In simple terms, a Class D amplifier is an electronic amplifier where the power devices, usually MOSFETs, are operating as binary switches, being either fully on or fully off. The speed of the switches must be much higher than the desired frequencies of the input signal. The amplifier uses Pulse Width Modulation (PWM) to convert the input signal to a series of pulses, which in turn drive the power devices, which deliver a changing voltage into a fixed load such as a subwoofer. The high frequency switching produces undesirable frequencies which are removed by a low pass filter, leaving only those relevant to the input signal. Class D is sometimes misinterpreted as being synonymous with “digital,” but that is not the case. All Class D amplifiers are not digital, but the term is often used for Class D amplifiers with significant amounts of digital processing in them.
Class D amplifier are noted for their very high efficiency, so they are often used where a high level of power is required, such as driving large subwoofers. Practically, efficiencies of over 90% are achievable, thus reducing the amount of heat produced. Because of their efficiency, they require a smaller heat sink which reduces size and cost. Class D amplifiers are used in many applications, including powered subwoofers, powered speakers, mobile applications (to save battery power), and bass amplifiers. While early Class D amplifiers were used mainly for subwoofers, advances have produced high power, low distortion units covering the entire audio band.
A midrange driver is a speaker that produces sounds in between the low frequencies of a woofer and the high frequencies of a tweeter. Typically the frequency range is from about 300 to 5000 Hz. They are often used in three-way multi-driver speaker systems to complement the low frequency woofers and the high frequency tweeters. As it turns out, midrange drivers produce the most significant part of the sound spectrum, containing most of the fundamental sounds of musical instruments and the human voice. These are the sounds most familiar to the human ear. Mid-range speakers are often found in televisions, where talking is of paramount importance. Since these frequencies are so important, the human ear is readily able to detect any distortion or lack of clarity. A mid-range driver should be capable of low-distortion reproduction of sound with adequate volume. Fortunately, because the ear is so sensitive to these sounds the power needed to drive a midrange driver can be low. Mid-range drivers come in different types, but are usually cone types or, less commonly, dome types, or compression horn drivers. Typically the cone is made of paper, although a wide range of other materials is also used.
A complete discussion of MOSFET is quite complicated and involves some rather ugly mathematics, so we’ll stick to the essentials relevant to car audio. Basically, MOSFET is the acronym for a “metal oxide semiconductor field-effect transistor” designed to handle significant power levels. A MOSFET can be considered to be a voltage-controlled switch. They are typically used in Class D audio amplifiers where there is rapid switching off and on in response to the input signal from a head unit such as a radio. The switching rates are very high, far above human hearing, and the amplified signal is filtered to remove the high frequency part of the signal to maintain the fidelity of the output signal. MOSFET has the property of having good efficiency at low voltages (less than 200 V). In the context of amplifiers, a MOSFET amplifier can drive speakers to kilowatt power levels with good clarity and fidelity, which is especially useful for power hungry speakers such as subwoofers. Because of the efficiency of MOSFETs they produce less heat and require less cooling, which allows reduction in size and cost due to smaller heat sinks to handle the temperature. MOSFET is not confined to amplifiers, and MOSFET circuits are the design of choice in most microchips manufactured today.
Power transferred from an amplifier to a speaker is measured in watts. Power ratings are an indication of the power the amplifier can supply. Amplifier power can be measured in different ways. For example, there is peak power output, which represents the amount of power which can be produced for a very brief period of time. However, for amplifiers the general convention is to measure the RMS (“root mean square” – see below) power. This is the maximum output of the amplifier using a continuous sine wave input signal at the onset of clipping. Clipping is basically the level of power at which you cause an arbitrary amount of total harmonic distortion (THD). RMS power ratings are sometimes referred to as “continuous power ratings.” Since music signals are not sine waves, the RMS power is actually only an approximation of the power, but it is a reasonable way to compare amplifiers. The RMS power rating should be viewed as the maximum power produced by an amplifier within an acceptable range of THD in a particular band of frequencies, typically 20Hz to 20kHz for music amplifiers.
Root mean square (RMS). Okay, now for the mathematically inclined. The root mean square is a statistical measure of the magnitude of a quantity which can vary in value. It is particularly useful when a varying quantity can be either negative or positive, as is the case with sine waves. To calculate the RMS value you square the value of each quantity, or each value in a continuously varying quantity. The squared values are then added and the mean (for practical purposes the arithmetic average) is calculated. The square root of the mean is then the RMS value. In essence it is the square root of the mean of the squares. Aren’t you sorry you asked?
A subwoofer (or sub) is a loudspeaker used for very low-frequency sounds with great power, frequencies known as the “bass.” The typical frequency range for a subwoofer is about 20–200 Hz for consumer products. Subwoofers are intended to augment the low frequency range of loudspeakers (see “midrange speakers” and “tweeters”) which are designed to cover higher frequency bands and which typically do not handle low frequencies well. The combination of speakers provides greater depth to the overall sound of the music. Subwoofers are often quite large, with 103 to 153 subwoofers common. For best performance typically subwoofers are placed in a loudspeaker enclosure capable of resisting deformation while withstanding the air pressure caused by the power of the low frequencies.
A tweeter is a speaker that reproduces only higher audio frequencies. The name is derived from the high pitched sounds made by some birds, especially in contrast to the low woofs made by many dogs after which low-frequency drivers are named (woofers). The average tweeter outputs frequencies from approximately 2,000 hertz (Hz) to 20,000 Hz, which is considered to be the upper limit of human hearing. Because a tweeter is limited to high frequencies, it is combined with speakers that reproduce other frequencies to produce a full range of audible sound.
Typically tweeters use a voice coil suspended in a fixed magnetic field. When current from an amplifier is applied to the coil, a varying magnetic field is generated. The coil works against the magnetic field of a fixed magnet which is part of the tweeter. As the coil is forced to move it moves a diaphragm attached to it. The mechanical movement of the diaphragm is determined by the signal supplied by the amplifier. The movement of the diaphragm vibrates the air, which causes audio waves which we hear as high sounds.
The material used in the tweeter effects the type of sound produced. Generally, tweeters made of soft materials, like polypropylene, textile blends, or silk will give a sound that’s refined and somewhat mellow, whereas hard materials, like metal, ceramics, or graphite, produce highs that are bright and snappy.
Dispersion is the extent to which the tweeter yields sound over a given area; that is, the area in which the sound is best heard. Dispersion is a major concern in tweeters because high frequency sound is much more directional than the low frequencies produced by woofers. Piezo (piezoelectric) describes the ability of certain crystal lattices to respond by flexing in proportion to the amplitude and frequency of an incoming signal. Piezo tweeters are very efficient drivers. A diaphragm is the sound-producing element in a tweeter and is the surface that produces the sound. It can be driven by several technologies, including piezo or conventional dynamic diaphragm.
Cone tweeters are noteworthy for being the most economical but having a limited dispersion pattern. They are similar in style to woofer cones but adapted for the production of high frequency sound. They are infrequently used now, having been largely replaced with domes.
Dome tweeters are the most common type. They are more accurate than cone tweeters and have low distortion and a much wider dispersion pattern. Typical dome materials can be soft or hard, and include metals such as neodymium or titanium for extended high frequency response, plastics such as Mylar, or even silk for smoother sound. Combinations of materials are also used. They can be also powered by a magnet and coil diaphragm, or by a piezo driver. All have relatively low mass and high power handling capabilities.
A bullet tweeter is a type of dome tweeter in which there is a large passive, bullet-shaped device above the center that expands the dispersion angle of the sound so that it covers a greater area with a relatively small driver.
Horn tweeters have a relatively small emitter at the apex of a “horn” shape. This is an effective and powerful way of radiating high frequency, but they tend to be very directional and may lack the extended range of the dome tweeters. They are also powered by a magnet and coil diaphragm, or by a piezo driver. Some versions with wider dispersion can be used in car audio applications. High frequency drivers in horns are often referred to as “compression drivers.” This type is more efficient than other types and can be used to reduce the amount of thermal stress on a voice coil.