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Car Amplifiers Glossary
Definitions of common audio terms
Click on a letter below to jump to that section of the glossary.
Your vehicle's alternator is the ultimate source of electrical power for the vehicle itself and any aftermarket system you install. You can figure on about 40% of your alternator current capacity (amperes) as being available to power an aftermarket system.
The unit of measure for current or electrical "flow" through a circuit — the amount of electrical charge passing per second. It is commonly abbreviated as "amp" and should not be confused with the word "amplifier," which is also commonly abbreviated as "amp."
An amplifier is an electronic device that increases an electrical signal. A car amplifier boosts the input signal's voltage and current capability, so the signal's power also increases. Amplifiers are often called "amps," but you should not confuse them with amperes, which are also called "amps."
American Wire Gauge (AWG) is the standard for measuring the diameter of electrical wires and cables. The smaller the AWG number, the larger the wire. Because of their high current demands, car amplifiers need large power wires to get enough energy from the battery to operate correctly. The more wattage an amplifier has, the larger the wires that power it need to be. 8-gauge and 4-gauge wires are commonly used sizes for powering car amplifiers.
A bandpass filter allows frequencies that are within a certain range to pass through, but stops those frequencies that are higher and lower than that range. A bandpass filter actually employs both a high-pass filter to block low frequencies and a low-pass filter that blocks high frequencies. Bandpass filters are often used with midrange speakers to keep very high and very low notes away.
The lower part of the audio spectrum, stretching from the lowest audible note at 20 Hz to around 100 Hz, is considered bass. It takes much more amplifier power to produce bass notes than other frequencies because of the large size of the bass sound waves and the large amount of air movement required for them to make sound.
Increases the output of low frequencies. Usually centered somewhere between 40 and 90 Hz, many amps have variably controlled circuits that allow you to increase the bass level in dB increments (ie. 0-12 dB at 45Hz). Some tunable bass boosts allow you to adjust the center frequency, changing the character of the bass.
If you do choose to boost the bass, you will then need to re-adjust the amp's gain, to compensate for the boost and prevent the amp from clipping and distorting.
Bi-amping, or bi-amplification, is when each component in a speaker system — a tweeter or woofer, for instance — is driven by its own separate amplifier channel. When bi-amping, crossovers must be employed to keep low notes away from the tweeters and high notes from the woofers. Bi-amping (also called "running active") allows you much more control over your system's power and tone than with a "passive" system, where each pair of tweeters and woofers are run by a single amp channel.
When you bridge an amplifier, you combine the power output of two channels into one channel. Bridging lets you drive one speaker with more power than the amp could produce for two of the same speakers. Because of this high power output, bridging is an ideal way to drive a single subwoofer.
If your amp is bridgeable, the owner's manual will have directions that tell you how. Usually, an amp is bridged by connecting the speaker leads to the positive (+) terminal from one channel and the negative (-) terminal from the other channel. However, you should be sure to consult your owner's manual before attempting to bridge your amp.
Also, keep in mind that most amplifiers need to see a minimum of a 4-ohm load when bridged, otherwise they may overheat and shut down. If you want to bridge your amp, you should use one 4-ohm speaker or, if you prefer multiple woofers, wire two 8-ohm speakers in parallel. (Again, consult your manual before operating your amp in bridged mode.)
Crossovers consist of both a high-pass and low-pass filter. Often used to keep high-frequencies from reaching a subwoofer, a low-pass filter allows only frequencies below the crossover point to be amplified. A high-pass filter allows only frequencies above the crossover point to be amplified — useful for keeping low bass away from small speakers, so they can play more efficiently. Crossovers are usually listed as variable or selectable. Continuously Variable means the crossover can be freely adjusted to any frequency between the listed end points. Selectable means that you can choose from several preset crossover points.
A capacitor is an electronic component that stores and releases electrical charge. Heavy-duty capacitors are often used as a buffer zone between your amp and your car's electrical system to prevent the car's lights from dimming during loud playback. They store up a reservoir of power, which can supply the amplifier's peak demands (like a kick drum beat) without having to get additional current from the battery. All amplifiers have built-in capacitors, though high-performance amps use larger, more effective ones. External capacitors connect to the power cable just before it reaches your amplifier.
On May 28, 2003, the Consumer Electronics Association published standard CEA-2006, "Testing & Measurement Methods for Mobile Audio Amplifiers." This "voluntary" standard advocates a uniform method for determining an amplifier's RMS power and signal-to-noise ratio. Using 14.4 volts, RMS watts are measured into a 4-ohm impedance load at 1 percent Total Harmonic Distortion (THD) plus noise, at a frequency range (for general purpose amplifiers) of 20 Hz to 20,000 Hz. Signal-to-Noise ratio is measured in weighted absolute decibels (dBA) at a reference of 1 watt into 4 ohms. This applies to both external amplifiers and the amplifiers within in-dash receivers.
CEA-2006 allows consumers to be able to compare car amplifiers and receivers on an equal basis. Manufacturers who choose to abide by the new standard are able to stamp their products with the CEA-2006 logo that reads: "Amp Power Standard CEA-2006 Compliant."
Classes of Amplifiers
An amplifier is classified according to its circuit design and the way its output stages are powered. Although some may assume that for every portion of the input signal there is corresponding 100% output from the amplifier, power dissipation (in the form of heat) and distortion of the audio signal are two key factors in determining the efficiency and fidelity of an amplifier. Each class has its own performance characteristics and advantages.
- Class A amplifiers are desirable for the high quality of their sound, but, because of the configuration of its transistors, a pure class A amplifier is inefficient and runs very hot. This is because even when there is no audio signal, the output transistors always have current running through them. The current flowing through the output transistors (with no audio signal) causes the amp to heat up unnecessarily, and "waste" input energy. Most car amplifiers that boast "Class A" circuitry are really Class A/Class AB hybrids.
- The output transistors of Class B amplifiers actually turn off for half of every signal cycle. This improves efficiency and saves energy, but introduces distortion during the switching periods.
- By far the most common car amp design, Class AB amplifiers also allow current to run through the output transistors when there is no audio signal, but at a much lower level. A class AB amplifier runs cooler, and therefore, more efficiently than a class A, with low distortion and high reliability.
- Class D amplifiers use output transistors as switches to control power distribution — the transistors rapidly switch on and off at least twice during every signal cycle. Class D amps boast higher efficiency, produce less heat, and draw less current than traditional Class AB designs. Class D amplifiers produce higher distortion than AB designs due to the high-speed switching on and off of the transistors, but this distortion occurs at frequencies above hearing, and is easily removed by a low-pass filter.
Clipping occurs when an audio circuit's maximum capacity is exceded. The circuit won't reproduce the excessive parts of the signal, "clipping off" the round tops and bottoms of the musical waveforms, leaving them square, and introducing a huge amount of distortion into the output signal. Clipping can be heard as a crunching sound on musical peaks. A clipped signal, especially one that has been amplified, can be very destructive to your speakers and subs.
The rate at which the crossover attenuates the blocked frequencies. Slope is expressed as decibels per octave. A 6dB per octave crossover reduces signal level by 6dB in every octave starting at the crossover point. This means that every time the frequency of the audio signal is changed by a factor of 2 (one octave), the level of the audio signal will change by 6dB. For example, if your low-pass filter is set at 80Hz with a 6dB slope, you'll see a drop in level of 6dB at 160Hz. With slopes of 12dB and higher, you'll hear little output beyond the crossover point.
Current, measured in amperes, describes the amount of electrical charge passing through a circuit per second. In order to put out the amount of power (watts) needed to successfully drive speakers or a sub while restricted to the low voltage (12 volts) of a car electrical system, an amplifier will draw and put out large amounts of current. The high current present in car amplifier power wiring is dangerous, so using the proper size wire and fuses is crucial to the safety of you and your car.
Damping is the ability of a amplifier to stop the speaker cone from continuing to vibrate after the signal stops. A system with good damping plays sound cleanly with no hint of unintentional echoes or reverberations. The higher the damping factor, the greater the accuracy.
The standard unit of measure for expressing relative power or amplitude differences. A decibel describes the ratio between the value of a measurement and a reference point. With audio, it often represents loudness, or sound pressure level (SPL). One dB is the smallest change in loudness most people can detect. A 1 dB difference is barely noticeable, but a 10 dB difference is big — a speaker playing at 10 dB higher volume will sound roughly twice as loud.
Another dB fact: For any given set of speakers, each 3 dB increase in sound level (SPL) requires a doubling of the amplifier power.
Discrete Output Devices
There are three basic types of output devices found on car audio amplifiers — integrated circuits, bipolar transistors, and Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). An integrated circuit (or IC) is found only on relatively low-wattage (20 watts RMS per channel or less) amplifiers called "bridged transformerless" amps. An IC can not pass enough current to work on a more powerful amp and is not considered a discrete output device.
Bipolar transistors and MOSFETs are found on the output stages of high-powered amplifiers. They are fast enough and can handle enough current to send wattage greater than 20 watts per channel to your speakers. Both of these types of transistors are considered discrete output devices. Usually there are two per channel, but some amps feature as many as four per channel.
Distortion is the unwanted addition to an original signal that changes its sound. Increasing and decreasing volume doesn't count. Typical distorted sounds include audible hums, pops, tones and voices sounding scratchy, and drum beats crackling. Distortion is most often introduced by a system being played too loud.
An amplifier's efficiency is how much it puts out compared to how much it consumes. A typical Class AB amplifier putting out 100 watts RMS will be pulling 200 watts RMS of power from its source, resulting in a 50% efficiency rating. Class D amplifiers generally are more efficient than AB amps — often reaching 75% or 80% — which allows them to be more compact, so they can fit in more places in a vehicle.
In audio, "frequency" is another word for "note," or a tone's pitch. An audio device's frequency response is a measure of how well the device handles all the notes in the audible spectrum, from lowest to highest. Human hearing reportedly has a frequency response of 20 to 20,000 Hz, but the high end rolls off with age.
Fuses exist to save your wiring, your car, and your life from burning up in the event of a short circuit or other catastrophic overload. You should always fuse your main power cable as close to the battery as possible. Replacing a five-dollar fuse is a lot cheaper than having to replace your car. Insurance companies probably will not cover damage due to the kind of negligence not installing a fuse demonstrates.
Gain, as it applies to car amplifiers, refers to the input sensitivity adjustment necessary to match an amplifier's input to the receiver's output. Properly setting the gain prevents an amp from "clipping," distorting the signal due to being overdriven, which can damage speakers.
Ground is the reference point (zero volts) for measuring voltage, power, and signal. It completes the system's power flow from the battery, through the amplifier, and back to the battery. In automotive electronics, the vehicle's metallic frame is ground. That's what the negative terminal of the battery is directly wired to. In car audio, loose ground connections are responsible for more trouble than anything else, by far. Make sure your amplifier is grounded tightly to the main chassis of your vehicle, with all the dirt and paint removed where contact is made.
Headroom is the distance between a normal listening level and the loudest level the amp can deliver quickly and cleanly — when the music demands it, for instance. Having little headroom means your amp can't reproduce sudden bursts of energetic music very well. The more powerful your amplifier, the more headroom it will have.
A high-pass filter is an electronic circuit that removes all the notes above the filter's frequency setting. A high-pass filter set at 100 Hz, for example, will allow all the signal above 100 Hz in frequency to pass through, but block all those under 100 Hz. High-pass filters are primarily used to block bass notes from distorting through car speakers and tweeters.
The total opposition to the flow of alternating current in an electrical circuit at a given frequency. Impedance is measured in Ohms. Although car audio manufacturers label the impedance of most car speakers and subs at 4-ohms, the impedance of a speaker is actually not a constant. It's actual impedance changes with frequency and can vary greatly. Therefore, though 4-ohms is the standard impedance in car audio, this standard is more of an average impedance for speakers and amplifiers when driven within the part of the audio spectrum for which they are designed.
An amplifier sees the speaker or sub it drives as a work-load that puts up a resistance to its output. The higher a speaker's impedance, the larger the load the amp perceives. Amplifiers are all designed to work with specific loads. Too heavy a load, and the amp bogs down and can't deliver its full power to the speaker. Too light a load, and the amp runs amok, stealing power from the electrical system, and overheating.
Loudness is a subjective perception related to sound pressure level (SPL), but no one knows exactly how. Doubling the power of a signal, giving it a 3 dB gain in SPL, doesn't make it sound twice as loud. A 10 dB gain in SPL will sound about twice as loud.
A low-pass filter is an electronic circuit that removes all the notes below the filter's frequency setting. A low-pass filter, set at 100 Hz for example, will block the notes above 100 Hz and allow those under that to pass. Low-pass filters are primarily used to keep high notes from subwoofers.
Mono (or monaural) amplifiers are single channel amps, well suited for low-frequency applications since the human ear cannot distinguish stereo in the extreme bass range. Also, since mono amplifiers are stable to 2-ohms, you can connect them safely to two 4-ohm woofers (wired in parallel).
The unit of measurement for impedance or resistance. It tells you how much a device will resist the flow of current. If you take two signals of exactly the same voltage strength and send one to a 4-ohm speaker and the other to an 8-ohm speaker, twice as much current will flow through the 4-ohm speaker. In other words, the 8-ohm speaker will require twice as much power (wattage) to play at the same volume.
Polarity describes how the positive and negative power or speaker wires are hooked up. Having the proper polarity means the battery is connected correctly — positive to power, negative to ground — and the speaker cones move forward on positive electric pulses and rearward on negative pulses.
- A speaker is driven with too much power, beyond its rating, and it overheats.
- The amplifier is driven into clipping, producing square wave distortion that destroys the driver.
Preamp outputs let you pass the preamp signal to additional amps, saving you from having to run more long patch cords to your receiver or to use unreliable y-adapters in order to hook up all your amps.
Originally, the Radio Corporation of America (RCA) developed these unbalanced pin connectors, also called phono connectors, for the radios and televisions they manufactured in the 1930s. Now, they've become the industry standard for connecting consumer electronic components together.
RMS Power vs. Peak Power
The amount of continuous power, measured in watts, that an amplifier produces is called RMS power. RMS ("Root Mean Square") is a mathematical way of obtaining a useful average when discussing power capabilities. The higher the RMS figure, the louder and cleaner your music sounds. When choosing an amplifier, the RMS rating is the power rating you should pay most attention to.
Also, keep in mind that some manufacturers calculate the RMS power ratings of their amplifiers at different input voltages. For example, an amplifier rated at 100 watts RMS at 12 volts can produce considerably more power than an amp rated at 100 watts RMS at the more typical 14.4 volts.
Stereo manufacturers often display peak power ratings on the face of their products. The peak power rating tells you the maximum wattage an amplifier can deliver as a brief burst during a musical peak, like a dramatic drum accent. The RMS figure is more significant and more indicative of the amp's long-term performance.
RMS Power at 2 ohms
This spec tells you how much more power your amp delivers when presented with a 2-ohm stereo load. You can achieve a 2-ohm load by using parallel wiring or by using 2-ohm speakers.
Theoretically, amp output should exactly double as the impedance drops from the usual 4 ohms to 2 ohms. However, amp makers use different degrees of regulation on power supplies, which can restrict the actual increase in output.
Less regulated power supplies come closer to doubling their output into 2-ohm loads. An amp with little regulation can achieve higher wattage into lower impedances. An amp with stiffer regulation maintains rated output from your amp as other electrical accessories demand voltage from the battery.
Sound Pressure Level (SPL)
SPL is measured in dB — an acoustic measurement of sound energy. One dB SPL is the smallest audible difference in sound level. 0dB SPL is the threshold of human hearing, while noise measuring 120dB can damage your hearing.
Also called an infrasonic filter, a subsonic filter cuts off extremely low bass (below the range of human hearing) that many speakers cannot effectively reproduce, thereby making the amp's power supply and output devices, and the speaker, more efficient.
Sometimes called Dual Mode, this setup powers a pair of stereo speakers and one subwoofer simultaneously from the outputs of a single 2-channel amplifier. It's an affordable way to drive a subwoofer. It requires an external Tri-Way adapter that is connected in-line between your amplifier and your speakers.
|A stereo amp with Tri-Way capability can power a pair of stereo speakers and a single subwoofer.|
A unit of electrical force, analogous to pressure. Voltage pushes the current through an electrical circuit, and is equal in value to the current, measured in amperes, multiplied by the resistance, in ohms.
The subjective perception of how loud a sound is, or how well it fills the listening space. Volume and loudness are subjective, because everyone reacts to sound differently. A sound's pressure level (SPL), or intensity, is the objective measurement used to compare different-volume sounds.
A unit measurement of power, or how much work something does. The wattage of an amplifier or sound system is used as a rough measure of how loud it can play. In electrical circuits, power, measured in watts, equals the voltage multiplied by the current, measured in amperes.