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Car Amplifiers Glossary
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The unit of measure for current or electrical "flow" through a circuit. It is commonly abbreviated as "amp" and should not be confused with the word "amplifier," which is also commonly abbreviated as "amp."
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.
When you bridge an amplifier, you combine the power output of two channels into one channel. Bridging allows you to 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 4-ohm load when bridged to mono operation. 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.)
Bass Boost Circuitry
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). Variable 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.
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 high frequencies that are typically 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.
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 volume level 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.
Gain, as it applies to car amplifiers, refers to the 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.
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.
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.
- 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.
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.