You're faced with a lot of different choices when it comes to buying an amp, and it can seem overwhelming and confusing to a car audio newbie. The first thing you'll have to decide is what type of amplifier or amplifiers you will need for your system: stereo, multichannel, or subwoofer.
Two-channel amplifiers are also called stereo amps, and they are essentially left and right mono or single-channel amps on one chassis. In a typical car audio system, one channel powers a speaker on the right and the other a speaker on the left. A stereo amp's two channels can also be bridged or combined to form a single channel to power a subwoofer.
As the name suggests, multichannel amps have multiple channels - or at least more than two. A multichannel amplifier is essentially several different channels on a single chassis. They can have as few as three channels or as many as six.
The main advantage of multichannel amps is that the same amount of power produced by, say, three separate stereo amplifiers can come from a single six-channel amplifier that takes up much less space and sometimes costs less money.
Multichannel amps also offer a lot of flexibility. You can usually bridge two individual channels into one mono channel to power a subwoofer.
Subwoofers require the most power in a car audio system because it takes a lot to move their larger cones. That's why it can be a good idea to have a single large amplifier (or amplifiers) dedicated solely to the sub (or subs) in your system.
Subwoofer amplifiers are also called mono or sometimes mono block amplifiers because they usually consist of just a single monaural channel. Mono power is usually used with subwoofers because the human ear can't distinguish the difference between stereo and mono at low frequencies.
Considering the Different Features of an Amp
Home audiophiles consider any bells and whistles on a power amplifier extraneous. But car audio manufacturers add all sorts of features that give you more control over your system and help cut down on the number of components. Some also help the amplifier deal with the harsh environment of the car, where temperatures can range from more than 100 degrees to below zero, and the power supply for the amp can fluctuate.
In a car audio system, a crossover ensures that the appropriate speakers get the appropriate signals - that the tweeters aren't woofing and the woofers aren't tweeting. This also protects the speakers because a massive bass note could easily fry a small tweeter.
Crossovers built into amplifiers come in configurations such as low-pass, high-pass, and bandpass. As the name indicates, a low-pass crossover only allows low frequencies through for subwoofers, whereas a high-pass crossover allows high frequencies through and blocks all others flowing to tweeters. A bandpass crossover allows a predetermined band of frequencies to pass though and blocks frequencies on either side.
Remote Subwoofer Control
A remote subwoofer control is, in effect, an extension of the bass boost circuit on an amplifier and allows adjusting the level of bass on the fly for different kinds of music or to suit your tastes. Some amplifiers come with a remote subwoofer control, but more often it's an add-on and extra-cost accessory.
Gain control is as important on an amplifier as a volume control is on a head unit, and many people mistake it for the same thing. Gain control isn't used to increase the overall volume of a system, but to match the output from the head unit to the amplifier's input to get the highest signal level with the lowest amount of noise.
High Level Inputs
Most amplifiers need a low-level, preamp signal from a head unit because an amplified signal will seriously overdrive an amp's input section. But amplifiers also have high-level inputs that allow direct connection to a stock head or even aftermarket head unit's amplified outputs.
Balance-line technology is borrowed from the recording industry. Balanced inputs are used in place of standard RCA inputs to reduce noise that can enter a system through wiring. The term balanced here refers to a method of connecting each wire - at the signal source and the amplifier - to identical impedances at each end. The amplifier at one end compares the two signals, and any noise that has been added is rejected. Use of fully balanced inputs in car audio amplifiers is rare, but differential balanced inputs are more common. Differential balanced inputs are a variation on balanced inputs. Differential balanced inputs don't connect part of the audio signal to the chassis ground.
Getting Input on Inputs and Outputs
Every amplifier has inputs and outputs. Inputs are where the signal from a head unit goes into the amplifier, and the outputs are where it leaves the amplifier for the speakers.
Inputs are usually RCA jacks that are typically gold-plated to ensure a good connection and guard against corrosion. Inputs can also be of the balanced-line variety.Outputs can be one of two general types. The most common is a screw terminal or barrier strip. The screw for each terminal is loosened, the speaker wire or a connector attached to the wire is placed under the screw, and then the screw is tightened.
The other type of output is a capture-wire. Capture-wire outputs come in a variety of styles. Some have holes into which a stripped or bare speaker wire is inserted and then a hex-head screw or some other fastener is tightened. Another is a spring-loaded connector; a part of the connector is pushed to open the hole into which the bare wire is inserted. Still another uses a removable capture-wire application that can be detached so that connections can be made outside of the vehicle.
Protection from heat is one of the most essential forms of protection an amplifier can have. Because heat is a major source of failure in amps, manufacturers build in circuitry that senses when heat rises to a dangerous level and shuts down the amp. After the temperature returns to a safe level, the amplifier fires up again.
In addition to thermal-protection circuitry, some amplifiers also have cooling fans built in. The fans will either run continuously, or they'll kick on when the temperature inside the amp reaches a certain threshold.
Short Circuit Protection
When positive and negative power wires touch or a power wire is pinched and is grounded to a car's metal chassis, a short circuit can occur, which can damage an amplifier. To prevent this, many car audio manufacturers build in a circuit that detects a short-circuit situation and shuts the amplifier down before any damage can occur.
A drop in impedance can damage an amplifier by placing too heavy of a load on it and making it work too hard. Some amps have circuitry that senses an impedance drop and shuts down the amp or lowers its output to prevent damage.
Some amplifiers come with a shroud that covers the connections to the amps to protect them from, say, spare change that falls under the seat that could touch the controls and cause a short circuit.
If the supply voltage drops below a certain threshold, rather than the amp working harder to compensate and potentially damaging its circuitry, low-voltage protection kicks in and dials the amp back or shuts it down entirely.
Keeping it Pretty: Cosmetic Considerations
Part of having a car audio system is being able to show it off to your friends. If that wasn't the case, all car audio amplifiers would probably be plain, utilitarian black boxes. Instead, you'll find amplifiers in all manner of shapes, colors, and finishes. Some are large and flashy, whereas others are compact and subtle.
From Car Audio for Dummies, copyright © 2008 by John Wiley & Sons, Inc., Hoboken, New Jersey. Used by arrangement with John Wiley & Sons, Inc.