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Choosing an Amplifier

One of the most important components of a car audio system is the amplifier. It is the amplifier that takes in the small electrical signal originating from the head unit and amplifies it into a larger electrical signal that can drive the speakers. In recent years the amplifier has also been packed with a whole lot of features that help to enhance the signals going to the speakers. For example, amplifiers are now coming with built in crossovers which assure that the proper frequencies are routed to the correct speakers as well as equalizers that allow you to adjust the sound. Amplifiers have also been specialized with specific features that help subwoofer drivers perform to their designed potential. These features do not only include lowpass crossovers, they also include bass boosts, equalizers, and subsonic filters which protect the subwoofer from being forced to play frequencies that are too low and that can damage it.

Amplifiers are available with a variety of channels. There are single channel or mono amplifiers that are used to work the subwoofer or subwoofers; there are two channel amplifiers which work the midrange and tweeter speakers on the left side and right side of the car or the front and rear of the car; there are four channel amplifiers that work the mids and highs speakers in the four corners of the car (front doors and rear doors or front doors and rear deck for example) and there are even five channel amplifiers which work the four corners of the car as well as a subwoofer or center channel (a speaker commonly placed in the center of the dash).

Amplifiers also come in a variety of power ratings. These ratings are measured by the manufacturer either at 14.4 or 12 volts and the measurement defines how much wattage is being supplied by each channel of the amplifier.

The amplifier needs current in order to amplify the signal it receives. So it must get proper, clean and a sufficient amount of power from the car’s battery and alternator.

You should be considering many factors including the above when selecting an amplifier or amplifiers for your system. But first, you need to understand the terms that are used by salespeople and installers when they talk about amplifiers.

Definition of Terms
Manufacturers measure the power wattage of the amplifier at peak and at RMS. Scott Neill, national trainer for Alpine Electronics USA, Torrance, California,  explained that peak power is what the amplifier supplies in a split second to two or three second burst. “They can throw out a massive amount of power,” he volunteered. An RMS power rating is what the amplifier delivers continuously -- what it puts out all day long without overheating. When you compare power ratings of amplifiers it is the RMS power rating that you should be using. If you are comparing two amplifiers and one of those amplifiers do not provide you with an RMS power rating but instead offers a peak power rating, you should convert the peak power rating to RMS to make a fair comparison with the other amplifier.

Clipping is another term you may hear. This is when the amplifier is forced to go beyond its capability and distorts. Clipping occurs when the user demands more power out of the amplifier than it is capable of delivering cleanly. Clipping creates distortion and can also overheat smaller speakers because it increases high frequency energy. Sam Taylor, vice president of mobile entertainment for Phoenix Gold, Portland, Oregon, explained that this can occur when you play music too loud. He noted that many amplifiers that are now being offered today include a light that turns on to show too much signal is going in.

Bridged is another term you will hear. This explains the process of combining two channels of an amplifier into one channel to increase power. This could be done to a two channel amplifier assigned to power the subwoofers. By combining two channels into one you will be supplying more power to the subs. In addition, in some audio systems you may see a four channel amplifier with its front two channels running the mids and high speakers in the front of the car and the rear two channels bridged into one channel to run the subwoofers with the amplifier built into the head unit running the rear mids and highs, suggested Neill.

When talking about an amplifier’s ability to stay cool you may hear the term heatsink. This is the metal top of the amplifier which commonly includes fins. The construction of the amplifier causes the heat to rise from within up the fins and evaporate into the air. This is called convection cooling. There are many amplifiers that also include built in fans to assure that they stay cool.

Ohms stable is a term you will hear when a salesperson or installer is talking about the impedance capability of an amplifier. Impedance is the resistance to AC current flow presented by a speaker or speaker system. This is measured in ohms. So you may hear a salesperson say that a particular amplifier is 4 ohm stable or stable down to 4 ohms. There are amplifiers that are stable down to 4 ohms, 2 ohms, 1 ohm, even 1/2 ohm. The lower the ohm rating the more power the amplifier delivers. The ohm rating is based on the number of speakers wired to the line and how that connection is made. Most single speakers are rated at 4 ohm. If only one speaker is connected to the line, then the amplifier load will be 4 ohms. If two speakers are connected to the line in parallel (that is positive to positive and negative to negative) then the ohms load drops to 2 ohms. If the connection is made in series (that is amplifier positive to speaker 1 positive, speaker 1 negative to speaker 2 positive) the ohms load rises to 8 ohms. So if you are connecting a midrange speaker and tweeter to one line in a series installation, the amplifier should be 2 ohm stable. In this application, however, the passive crossover network that comes with the speakers will actually keep the impedance at 4 ohms. If you are connecting one 4 ohm rated subwoofer to a line then the amplifier that is running the subwoofer can be 4 ohm stable. If you are putting two 4 ohms rated subwoofers to a line in a parallel installation the amplifier running this configuration should be 2 ohm stable. If you are putting three or four subwoofers to one line in a parallel installation, then the ohms load rating drops to 1 ohm and you will need an amplifier that is 1 ohm stable. Taylor pointed out that as impedance of a speaker drops -- every time it is cut in half (4 ohms to 2 ohms to 1 ohm) -- the power of the amplifier will double. However, as impedance rises above the amplifier’s optimum load, the power drops unless the amplifier makes adjustments to maintain optimum power, as in the JL Audio Slash amplifiers and the Phoenix Gold Xenon amplifiers. Below a certain “optimum load” the amplifier is unable to deliver enough current and at that point it becomes unstable.

Then there are specs that are referred to. For example, when going over the specs of an amplifier you will see such listings as THD, signal-to-noise ratio, frequency range. THD means Total Harmonic Distortion and measures the amount of distortion coming out of the amplifier. Taylor, Neill, and Manville Smith, vice president of marketing for JL Audio, miramar, Florida, all agree that the amplifier you select should have a THD of less than 1 percent. The lower the number the better. Signal-to-noise ratio is a measurement that indicates how much noise the amplifier is passing into the sound system. Neill indicated that a good rating for signal-to-noise ratio is 90dBA. “The higher the rating the better,” he said. The frequency range spec shows the range of frequencies played by the amplifier. Most amplifiers have a frequency range of 20 Hz. to 20,000 Hz. These are the amplifiers you should be looking at. This frequency range covers beyond the frequencies that can be heard.

You need to be warned that some manufacturers fudge their specs. For example, they measure the specs of their amplifiers at 14.4 volts while other manufacturers measure at 12 volts. So what you may be doing is comparing apples to oranges when comparing specs of amplifiers from different manufacturers.

Neill pointed out that to assure that the specs are standard and that you are measuring apples to apples the Consumer Electronics Association (CEA) has introduced a standard for the rating of car audio amplifiers. The standard guideline is referred to as CEA 2006. Neill explained, “car audio deals with a 12 volt environment. When a car is running it is supplying 13.8 volts up to 15.2 volts. What CEA 2006 does is standardize the voltage at which manufacturers have to do their measurements. It is standardized at 14.4 volts. In addition, the guideline says you must measure at the full range of the audio frequency spectrum -- 20 Hz. to 20,000 Hz. Then the guideline says that since individual speakers are rated at 4 ohm impedance, all measurements have to be at 4 ohm impedance. Lastly, CEA 2006 says that distortion levels (THD) should be equal to, or below 1%. All of this assures that you are comparing apples to apples.”

Sam Taylor of Phoenix Gold and Manville Smith of JL Audio cautioned that spec ratings should be based on 12 volts because the car’s electrical system works at 12 to 13 volts.

The point is that you need to be certain that the specs you are comparing are based on the same standards so that you are comparing apples to apples when you compare one manufacturer’s amplifier with another manufacturer’s amplifier. So consult with the salesperson and/or an installer at a reputable shop and consider amplifiers offered by reputable manufacturers.

Things to Consider
There are a number of things to consider when selecting an amplifier. Manville Smith of JL Audio counseled that you take a good look at your car’s electrical system to determine if it is capable of supporting the system you want. “Establish an upper limit to what you can do in your car before you would have to upgrade the electrical system,” said Smith. If you have a small four cylinder car like a Honda Civic with a 65 amp alternator you are not going to be able to put in a 2000 watt system in the car unless you upgrade the electrical system. Too large a system will seriously compromise the car’s alternator and battery and the system won’t operate properly because there won’t be enough current in the charging system. Larger vehicles like a Ford Expedition which has a much bigger alternator would have a significantly higher upper limit for power.” He recommended a 500 watt system or below for a four cylinder car;  about a 1500 watt system or below for a six cylinder car; and a 2000 watt system or below for an eight cylinder vehicle.

Smith also noted that the power rating of the system does not necessarily translate to a louder system. It is one component of an overall picture especially when you are talking about bass. “You can get a very high output level with a small amount of power if you use enough speakers or large enough speakers in a large enough box,” he said. “For example, if you have only 200 watts of power you are better off with a pair of 15-inch subwoofers in a big box then with a single 10-inch subwoofer. You will get a ton of output with a small amount of power because the speaker system would be very efficient. To get a lot of output out of a small enclosure with let’s say a single 10-inch subwoofer choose a subwoofer that is capable of handling a lot of power like JL Audio’s W6V2 or W7 because you will need more power to get the advantage of that premium woofer. You will possibly use 500 watts or 1000 watts because you are trying to overcome the fact that the subwoofer system is small and not very efficient. The bigger the subwoofer the less power you need. So if you have a smaller car with fewer cylinders, give up more space for a larger box so you can use a larger subwoofer without putting a burden on your car.”

Smith suggested that power on the mids and highs speakers should be as much as you can afford. “When an amplifier drives a subwoofer you probably won’t hear distortion. You will hear distortion on mids and highs. So having a lot of clean power for the mids and highs will give you better sound and better dynamics. But there is a limit. Speakers can only handle so much power. So don’t go bonkers. My basic guideline is to strive to drive the front components with a minimum of 75 watts per side. One hundred and fifty watts a side would be even better.” Smith continued that you don’t need as much power for the rear speakers because in a system with front stage, which is the high end style, the rear speakers are supplying rear fill. So you can use a little less power in the rear. This can be achieved with a staggered four channel amplifier or using two two channel amplifiers.”

Smith advised that the amount of power for the subwoofer amplifier should be carefully matched to the power handling capability of the subwoofer system. “If you have a subwoofer with a power handling of 500 watts you don’t run it with a 1000 watt amplifier. You are just asking for trouble. What kills speakers is too much power.” He also suggested that when it comes to providing power to the front speakers, look for an amplifier that has tuning features -- highpass crossover and possibly an equalizer. Also consider what kind of input and output flexibility the amplifier has and whether it allows you to fade without effecting the sub-bass.

Finally, Smith said that you should consider the physical size of the amplifier. If you intend to have the amplifier mounted under a seat, you need to be certain that it is compact enough to fit. If you will be putting the amplifier in the trunk, then you can get a larger amplifier.

Sam Taylor of Phoenix Gold advised that you consider what kind of music you listen to and whether you play that music loud. Second, decide on the money you have to spend and third, find a brand that has been around for awhile and is known for quality.

As far as power rating is concerned, Taylor said, “I can’t see a really good quality sound system with less than 50 watts a channel into the mids and highs and a minimum of 100 watts to the subwoofer.” He added that you select an amplifier that allows you to adjust the output level and includes a bass control and built in crossover.

Scott Neill of Alpine said that you should match the RMS power handling of the speaker with the RMS power rating of the amplifier. “A 6-inch speaker rated at 50 watts RMS should be powered by an amplifier with a power rating of 50 watts RMS,” he said. Same for the subwoofer.

Neill added that you need to consider whether you intend to keep the factory radio or not. If you decide to keep the factory radio, does it have an RCA output. He said that most do not. So you need to select an amplifier with a speaker level input. “If the amplifier has both speaker level and RCA inputs then it can work with a factory or aftermarket radio. You can keep the factory radio and then upgrade to an aftermarket radio later and still be able to keep the amplifier.”

Finally, Neill suggested that an amplifier’s flexibility should be considered. This will allow you to build a system over time without discarding the products you bought earlier. He suggested, for example, that you buy a four channel amplifier and run the front two channels to the front speakers and bridge the rear two channels mono to work the subwoofers and run the rear speakers off of the amplifier built into the radio. Later you can get an amplifier for the subwoofer and use the four channel amplifier on the front and rear speakers.

And all three -- Neill, Taylor and Smith -- agreed that you consider amplifiers only from reputable manufacturers who have been in the industry for awhile. Choose brands that you know and ask the salesperson about the history of the brand and the warranty. And Taylor concluded that you ask the salesperson what amplifier he has in his car or what amplifier is he saving up to buy for his car.  

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