Car Audio Amplifiers
The purpose of a car audio amplifier is to take a low level signal from the source unit and change it into a high level signal for driving the loudspeakers. Amplifiers range in power from about twenty watts per channel to over one thousand watts per channel. The price range can be anywhere from fifty dollars to several thousand dollars depending on features, power output and quality. An amplifier may have as little as one channel of output to as many as eight channels at the time of this writing. The most common amplifiers are two and four channel models although mono subwoofer amplifiers are also very popular, especially the class D type.
Amplifier Power Ratings
Extra features may also be built into a power amplifier. These features include built-in active crossovers, equalizers, signal processing and speaker level inputs. When shopping for an amplifier consider that all power ratings are not created equal. Some of the low-quality brands will exaggerate or even outright lie about the power output of their amplifiers. This is a good reason to stick with the well known manufacturers. The only true measure of an amplifier's power is its continuous power rating or R.M.S. rating. R.M.S. is an acronym for root mean square and refers to the amplifiers average power output. An even better method is the CEA 2006 standard but not all amplifiers adhere to this testing method. Basically the test is performed at 14.4 volts with a four ohm impedance load. The entire audible frequency range, 20-20kHz is tested with a distortion level of 1% T.H.D.
One thing to note is that doubling the amplifier power does not double the sound output. A doubling of system power adds only 3dB of SPL (volume) to the output. SPL works on a logarithmic scale so it takes 10dB of change for the sound to be "doubled". This works out to a little over three "doublings" of power. So if you have 100 watts of power you would need to double that 200 watts, double that to 400 watts and then double that to 800 watts. That would give you a 9dB increase. To truly get the 10dB increase you would need 1,000 watts or a ten fold increase in power.
Car Amplifier Quality
Also keep in mind the quality of the amplifier. A generally good indication of quality build and the power output is the size and the weight of the amplifier itself. Better quality amplifiers will usually have a heavier and larger heatsink versus a low quality amplifier of the same power rating. Do not accept size as a definite indication though. I bought an amplifier at a yard sale for two dollars that was two feet long, eight inches wide and two inches tall that had a five inch square circuit board inside of it. This amplifier was so poorly built that I could actually hear the music I was playing coming from the circuit board itself. However there are many top quality amplifiers that do not follow this rule. The ever growing Class D (and its variants) amplifier is much more efficient and there does not produce as much heat or use as much power. This means a smaller power supply and a smaller heatsink. Because of the high efficiency design of those amplifiers a heavy heatsink is not required.
Also a good indication of an amplifier's true output is the size of the fuse used. True two hundred watt amplifiers do not use a ten amp fuse. Use your head when buying and keep in mind the brand's reputation for quality. A quick and dirty way to check the true power output of an amplifier is to take the fuse value of the amplifier (or the sum for multiple fuses) and multiply it by 6 for class A/B amps or 10 for class D amps (higher efficiency). This is by no means an accurate way to judge power but it will tell you if your amp is even close to it's specifications. For example, if a class A/B "1,200 watt" amplifier has a fuse value of 25 amps then you can take 25*6 and get 150. So this "1,200 watt" amplifier is more in the range of 150 watts.
If you find yourself on a budget or lack space for many components then the economical thing to do is buy a multi-channel amplifier with the built in features and processors that you desire. This can save a lot of room and several hundred dollars in added component and installation cost. The quality will be a little less as compared to outboard processors but will probably not be noticed. Also, by minimizing the number of components the chance of noise entering into the system is lessened.
Some features and aspects of amplifiers to consider are:
Bridgeable: This feature allows a pair of amplifier power channels to be combined into one channel of greater power. This is usually used for driving a subwoofer although it will work with any other type of speaker as well.
Channels: A channel is one power (speaker) output of an amplifier. The more channels an amplifier has the greater the installation flexibility it will have. Especially in terms of options, future add-ons and upgrades.
Class: This refers to the way the amplifier operates. The three types that are most likely to be encountered are A, A/B, and D. Class A amplifiers are the least efficient in terms of power consumption, staying on continually, but also have better sound in general than A/B amplifiers. They are very, very rare in car audio. Some argue non-existent but in any case don't expect to see any. Class A/B amplifiers are more efficient than the class A design and are the most common type. Almost all amplifiers in the car audio market are of the A/B design. Class D amplifiers are usually reserved for high power subwoofer amplifiers and can reach efficiencies in the 80%+ range. This design can therefore be smaller, uses less current and produces less heat than the other classes. However there are some full range Class D amplifiers becoming available.
Connectors: This is the method of attachment used for wires that are connected to the amplifier, including speaker and power wires. The most common kind is the screw terminal strip. This is a series of screw connectors that can be removed and replaced without compromising the amplifier. The other main type of attachment is the "Molex" type connector. This method involves a wire harness that plugs into the amplifier after the power and speaker connections have been made with a crimp or solder connection. If the amplifier is installed in more than one system these wires can get pretty short over time and become more difficult and even dangerous to work with. A variation on the two is a harness that the power and speaker wires screw into. Then the harness plugs into the amplifier. This is probably the most convenient of all connections. Virtually all amplifiers use the screw terminal strip though many older amplifiers used the Molex or straight wire connections.
Crossover/Filter: A built in crossover can be useful, especially if it is many frequencies of adjustment. A filter is a crossover that only affects one channel, not actually splitting frequencies but simply reducing a range of them. Most amplifiers that have built in filters will have the option for either Low Pass (LP) or High Pass (HP). If you see AP on the filter that means "all pass" which basically means the filter is turned off and passes all frequencies.
Distortion: This is often given as T.H.D. or total harmonic distortion. It is the measure of how much an amplifier will change a signal from the input signal it is given. Figures below 0.1% are negligible and will probably not be heard. Usually the figure can be in the 3% range without being heard but virtually all high quality amplifiers will have a T.H.D. below 0.1%.
Efficiency: This is the ratio of of power input (from the battery) to power output (to the speakers). A 100 watt amplifier with an efficiency of 50% would take in 200 watts of power from the battery and output 100 watts of power to the speakers. The other 100 watts of power would be wasted as heat. The higher the efficiency of an amplifier the better. Most class A/B amplifiers are around 50-60% efficient and Class D around 80%. Amplifiers are generally less efficient at low power and more efficient at full power so this number varies in actual use.
Power Output: The rated power output of an amplifier should be given into a four ohm load, all channels driven from twenty to twenty thousand hertz (20Hz-20kHz). Keep in mind that while the low end amplifiers are exaggerated in their power output, many high end amplifiers are under-rated in their power output. These are sometimes called "cheater amps" because they allow a car audio competitor to compete in a lower power class while in reality having a larger amplifier. This under-rating may be three times less than the actual power output. Look for the CEA 2006 standard when comparing amplifier power.
Power Supply: The two most common types are the IC chip and the MOSFET supply. The IC chip is what is used in most source units (head units) and are only capable of producing about eighteen watts per channel. MOSFET is the more common design and has a smoother sound than the chip design.
Pre-amp Inputs: This is a set of jacks (usually RCA jacks) that will accept a low level pre-amp signal from a source or processing unit.
Pre-amp Outputs: This is a set of jacks (usually RCA jacks) that pass on a low level pre-amp signal to another amplifier or processing unit. These will sometimes be filtered outputs.
Separate Gain Controls: This allows the gain of each channel of the amplifier to be set independently of the other(s). This allows you to more evenly match the amplifiers channels.
Speaker Level Inputs: For source units that do not have pre-amp level RCA outputs this feature may be used to take the signal from the speaker leads of the source unit. The signal will not be as clean as a pre-amp level output but will be adequate for most factory upgrade applications.
Stability: The measure of how low of an impedance load an amplifier can handle (in ohms). Any good quality amplifier will be two ohm stable while a rare few will go as low as a quarter of an ohm. Ideally an amplifier should double its power each time the load is halved. For example, a one hundred watt amplifier (into a four ohm load) should produce two hundred watts into a two ohm load and so on. This is most useful when running multiple speakers off of a single amplifier or in sound off competitions that are classed by total power output.
Tri-Mode Output: This feature is available under different
names but is the ability of an amplifier to run a stereo pair of speakers
and a mono subwoofer (or center channel) from only two channels of the
amplifier. Personally, I would not recommend doing this. Instead buy a
good quality four channel amplifier and bridge two of the channels for
the subwoofer.
Tube Amplifiers: These are the least common amplifiers
and are also the most expensive. Rather than the traditional solid state
components they use old fashioned vacuum tubes. They are said to produce
a warmer sound and a smoother midrange than solid state designs. For most
systems the standard design will be more desirable.
The Car Audio Help DVD catalog includes five different videos covering many areas of car audio installation and custom fabrication. Topics range from basic system installation (head units, CD changers, amps, speakers, etc.) and mobile security (car alarms and remote start) to subwoofer box design and fiberglass fabrication. If you're interested in custom fabrication and car audio installation be sure to check out what we have to offer.
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