Making Sense of Amp Specs
When shopping for amplifiers, power per channel is just one specification or spec you'll want to scope out, if not the most important one. There are more specs you'll need to inspect.
A spec of 50 watts doesn't tell you much unless it includes the impedance at which it was measured. Impedance in this case is that of a speaker, and speakers are rated with different impedances. In car audio, however, most speakers are rated at 4 ohms, although subwoofers can be rated at anywhere from 8 to 2 ohms. All things being equal, the lower the impedance, the higher the current flow, meaning the less the current is being pushed back towards the amp. The higher the impedance, the lower the current flow.
You may be wondering why everyone doesn't just use low-impedance speakers to get more power out of amplifiers. A lower impedance increases distortion, which isn't always a bad thing with subwoofers, but is always bad for midranges and tweeters. Subwoofers, which produce low bass, do a better job of masking or covering up distortion, and their larger voice coils - the mechanical engine of the speakers - can handle more power and distortion. But with mids and tweets, distortion not only degrades sound quality but can also harm the speakers. It's always important to match the impedance of the speaker with that of the amplifier. Otherwise, you could end up damaging the amplifier and the speaker.
Bridging to Mono
Another power spec you'll often see is bridged power. Bridging an amplifier means combining two stereo channels to create a more powerful single mono channel, usually for powering a subwoofer. Sometimes bridging is accomplished by flipping a switch on an amplifier or, more commonly, by the way the subwoofers are wired to the amp. (The owner's manual for most amplifiers will specify how to wire the amp in mono.) Regardless, an amplifier will typically more than double its power in bridged mono mode.
Some amplifiers are specially designed to run at low-impedance loads so that they can squeeze more power out of the limited amount of current that a car's electrical system provides. Called high current amplifiers, they produce their maximum output power into 2-, 1- or even 0.5-ohm loads.
About now you're probably wondering, "So why don't I just buy a high-current amplifier so that I can get as much power as possible?" Well, for one thing, high-current amps are usually more expensive than standard amps, and they draw much more current than most cars' alternators are designed to provide. Adding several to a car audio system could necessitate an upgrade to a car's electrical system, which is as dicey as it is pricey.
High-current amps are also not as musical as other amps. They may be fine for bass-heavy music, but if you use a high-current amplifier to, say, pump up the volume of your favorite chamber music, the sound quality will likely suffer. That's because high-current amplifiers are primarily intended for brute-force power, usually to drive subwoofers.
In fact, high-current amps were originally designed for crank-it-up competitors looking for an edge in the competition. Car audio competitions are divided up into power classes, such as 0-150 watts, 151-300 watts, and so forth. In the early days of car audio competition, it was all about who had the loudest system, and in SPL (sound-pressure level) competitions, it still is. So a high-current cheater amp would allow a competitor to gain an advantage - that is, until everyone else in the class also started using high-current amplifiers.
Look for the CEA Logo
The Consumer Electronics Association (CEA) is the preeminent trade association promoting growth within the $148 billion U.S. consumer electronics industry and boasting more than 2,200 member companies. CEA efforts include legislative advocacy for consumers and member companies, market research, technical training and education, industry promotion, and the fostering of business and strategic relationships. CEA also sponsors and manages the International Consumer Electronics Show (CES), where entertainment, technology, and business converge. Find the CEA online at www.CE.org.
If an amp doesn't have a total harmonic distortion (THD) spec - or even a power supply voltage - you should be suspicious. It's not uncommon for some manufacturers to greatly exaggerate the power output of their amplifiers. For example, if you're shopping and you see an amp with "1000 WATTS MAX POWER" printed on its case, you should be suspicious - as you should be with any marketing claims. As with many other product categories, marketing plays a crucial role in selling car audio products, and marketers have figured out that bigger always looks better to a consumer. If you find a so-called 1000 watt amp that costs $100, you can be pretty sure it's not the one you need to power your 15-inch subwoofers.
Such claims are the reason the Mobile Electronics division of the Consumer Electronics Association got together and came up with the CEA-2006-A standard. CEA-2006-A is a voluntary standard that provides a uniform method for determining an amplifier's power. The standard requires using a power-supply of 14.4 volts into a 4-ohm impedance and achieving THD of 1 percent or less. So far, 21 car audio manufacturers have signed on to abide by the standard and many use it in their marketing efforts.
Power output and THD are two of the most important specs to consider when shopping for an amplifier, but there are others:
- Damping factor refers to the ability of an amplifier to control unwanted movement of a speaker. A higher damping factor indicates greater control and less unwanted movement (that is, distortion from the speaker). An amplifier's damping factor will also decrease as the speaker's impedance decreases.
- Signal-to-noise ratio compares the strength of the signal to the level of potential background noise in the signal and is measured in decibels (dB). A higher number is better.
- Frequency response is the measure of an amplifier's ability to reproduce sound across the audible audio spectrum, and is often given with a tolerance. For example, a frequency response spec of 20 to 20kHz +/- 1 dB means that the amp can reproduce frequencies from 20 Hz to 20,000 Hz without deviating (lowering or increasing output) more than 1 dB within that range.
- Input sensitivity refers to the range of input-signal strength that an amplifier can accept. In order to get the most performance from an amp, it should be able to accept as high an input signal as possible, which is why a head unit with a high output voltage is desirable. The greater the range of input sensitivity to match the varying ranges of head units, the better. A spec of 0.5 to 4.0 volts for example means that an amp can accept from ½ volt up to 4 volts of input voltage.
No discussion of car audio amplifiers would be complete without covering amplifier classes. There's a lot of misconceptions over the various amplifier classes available in car audio and their advantages and disadvantages.
The differences in amplifier classes mainly has to do with their efficiency: how well they take input power (from the car's electrical system) and turn it into output power (for the car's speakers). But amplifier classes also relate to sound quality. Some amplifiers are more efficient at turning input power into output power without wasting some in the form of heat dissipation, but have lower perceived sound quality. Others are less efficient but offer better sound quality.
While your head is spinning from trying to absorb all of this information, keep in mind that most car audio amplifiers from reputable manufacturers will provide good power for years to come. So don't sweat the specs and techie stuff too much. Just go with an amplifier with a good rep, and you'll be as happy as a winning politician on election night.
From Car Audio for Dummies, copyright © 2008 by John Wiley & Sons, Inc., Hoboken, New Jersey. Used by arrangement with John Wiley & Sons, Inc.