The purpose of a car audio amplifier is to take a low level signal from the source unit (head unit, radio, etc.) 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.
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. Mono amplifiers are typically reserved for subwoofer amplifiers. The three amplifier classes (internal design) 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 rare in car audio because of their high power drain and their low efficiency. Most amplifiers labeled "Class A" are not true class A on the output. Class A/B amplifiers are more efficient than the class A design and are the most common type. The majority of 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. Class D designs are almost to the point where they are suitable for high frequency and subwoofer use. Most do not have the fidelity (yet) needed for anything above the bass range but they're getting there.
Extra features may also be built into a power amplifier. These features include active crossovers, equalizers, signal processing and speaker level inputs. These additional features can add a lot of value and save a lot of space in an installation. The quality of the components will not be as good as an outboard signal processor but will be more than adequate for all but the most serious enthusiast. Crossovers and speaker level inputs are the most common features and should be a buying criteria if they are needed in your system design. Note the number of adjustments and processors on the amplifier below. This is a five channel amplifier and is capable of powering a complete four speaker and subwoofer system.
Amplifier with built-in Signal Processing
Image courtesy of JL Audio
When shopping for an amplifier consider that all power ratings are not created equal. Some of the low-quality brands will exaggerate or outright lie about the power output of their amplifiers (see this article). 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.
Other power ratings that manufacturers may specify are peak and maximum power. These are completely meaningless and should be ignored in my opinion. Always compare R.M.S. power ratings when shopping for an amplifier. Among the good brands the peak or maximum power will simply be twice that of the R.M.S. rating. In actuality the R.M.S. rating should be 70.7% of the maximum rating but "go along" marketing has forced good manufacturers to use inflated peak ratings in order to compete with the lower brands which started the trend years ago. Since most consumers have no idea what the difference is between power ratings they mistakenly go by the "maximum power" rating often stamped on the heatsink. Some of the real bottom brands use a rating that is jokingly called I.L.S. (not a real rating). These will often be seen on the flea market EQ/Boosters that are sold for under $20. They'll claim 300 watts and will be packaged in a small under dash unit with a three amp fuse. The only way they could produce 300 watts is if they were struck by lightning, hence the term I.L.S. (If Lightning Strikes).
If you've ever checked the sensitivity rating on a speaker you will usually find that it is in the 90 decibel range. This means that at a distance of one meter the speaker will put out 90 decibels of sound with only one watt of power. You may not realize it but that is pretty loud. You're not going to set any records but it's still loud. OSHA, the government agency that regulates work place safety in the US, states that 90 decibels is the maximum level an employee can be exposed to over an 8 hour day (constant level). For a non-occupational setting that same level is the maximum you can listen to safely for one hour (in a 24 hour day). If you have an older home stereo system with a power meter you may have noticed that the power doesn't exceed 3 watts for most comfortable listening. Combine that with the additive effect of multiple speakers and you easily reach the 100 decibel level in an automobile.
The next question you have to ask is how loud do you want it. If you're trying to set an SPL record then you're going to need lots of power and probably lots of woofers. But if you just want your system to pound out the bass notes then you probably don't need a system bigger than 500 watts. Acoustic power operates on a logarithmic scale. This means that past a certain point you're only going to get a small increase in the sound level even though you're increasing the power input enormously. As an example, with 100 watts of power you will increase the sound pressure level by 20 decibels. Add this 100 watts to a speaker with a 90 decibel sensitivity and you'll be at the 110 decibel level. But if you add another 100 watts (200 watts total) you'll only increase the sound level by 23 decibels (a 3 decibel increase). Each time you double the power you'll increase the sound level by 3 decibels. So with 400 watts of power (assuming your speaker will handle it) you'll get a 116 decibel output. Not a big increase in sound level but it is a big increase in amplifier price and in current draw from your electrical system. The bottom line is you'll probably only use those 400 watts for a brief period of time because you'll risk hearing loss if you listen for extended periods at those levels.
Another factor to consider is the cost of electrical system upgrades you'll need if you do have a mega-watt system. The current draw can easily reach in the 150+ amp range which will almost certainly require an alternator upgrade. These are expensive both in equipment and installation cost. Then you'll need to use the larger 0/1 gauge wiring which is about $6/ft as compared to 4 gauge wiring which is closer to $2/ft. With large current draws everything is going to need to be bigger, not just your amps. The law of diminishing returns starts to kick in and your mega-watt system may cost you four times as much while only delivering 6 decibels more output as compared to a more modest system.
So the question is how much power do you need. For most listeners that want a clear top end and a solid bottom end you'll do well with a 500 watt or less system. That should probably be split 40/60 with the lion's share of the power going to the subwoofer system (200 watts for the highs and 300 watts for the lows). This will keep the current draw from your alternator at a more reasonable level while still providing punch and clarity to your system. The cost will also be much less overall and your pocket book (and your ears) will thank you.
There are several ways to wire speakers to an amplifier and to each other. The two main types are parallel and series. Anything else is just a combination of those two wiring schemes. Parallel wiring involves wiring each speaker positive terminal to the positive speaker output of the amplifier and likewise the negative speaker terminals are wired to the negative speaker output of the amplifier. See the figure below.
Series wiring involves connecting the amplifier's positive terminal to the positive terminal of the first speaker and then connecting the negative terminal of the first speaker to the positive terminal of the second speaker and so on. The final speaker in the chain will have its negative terminal connected to the negative terminal of the amplifier. See the figure below. This wiring scheme is best reserved for multiple subwoofers because any audio artifacts (distortion, frequency attenuation, etc.) caused by the first speaker(s) in the chain will affect the speakers in the chain after it. This is a major problem when mixing multiple speaker types (i.e. woofers with tweeters).
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. Because of the high efficiency design of those amplifiers a heavy heatsink is not required.
Another good indication of an amplifier's true output is the size of the fuse used. Genuine two hundred watt amplifiers do not use a ten amp fuse. For a quick and dirty method of finding an amplifier's output based on fuse size you can use this calculation.
Power output = Fuse size x Voltage x Efficiency
Generally you can use a voltage of 12-14.4 volts and an efficiency of 0.6 for class A/B amplifiers and 0.8 for class D amplifiers. Here's an example of a class A/B amplifier with a 30 amp fuse.
Power output = 30 amps x 14.4 volts x 0.6 = 259.2 watts
That's a very general equation but will give you an idea of whether the amplifier's stated power output is even close to its true output. If the amplifier above was listed as 1,000 watts you'd know it wasn't true.
Use your head when buying and keep in mind the brand's reputation for quality. 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. By minimizing the number of components the chance of noise entering into the system is lessened.
You may also be interested in How to Install Your Own Car Stereo System . It covers many topics including in depth car audio amplifier installation. Click here.
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 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. 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 (aka Mixed Mono) 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. Otherwise you will need to use power robbing passive crossover components in the setup. With a four channel amplifier you can usually get built in crossovers that are a much better option.
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. You are unlikely to run across tube designs if ever.
You may also be interested in How to Install Your Own Car Stereo System . It covers many topics including in depth car audio amplifier installation. Click here.