Built-in Crossovers

Often used to keep high-frequencies from reaching a subwoofer, a low-pass crossover allows only frequencies below the crossover point to be amplified. A high-pass crossover allows only frequencies above the crossover point to be amplified - useful for keeping low bass away from small speakers, so they can play more efficiently. 

Bridged Power 

When you bridge an amplifier, you combine the power output of two channels into one channel. Bridging allows you to drive one speaker with more power than the amplifier could produce for two speakers. Because of this high power output, bridging is an ideal way to drive a single subwoofer. 

If your amplifier is bridgeable, the owner's manual will have directions that tell you how. Usually, an amplifier is bridged by connecting the speaker leads to the positive (+) terminal from one channel and the negative (-) terminal from the other channel. However, you should be sure to consult your owner's manual before attempting to bridge your amplifier. 

Also, keep in mind that most amplifiers need to see a 4-ohm load when bridged to mono operation. If you want to bridge your amp, you should use one 4-ohm speaker or, if you prefer multiple woofers, wire two 8-ohm speakers in parallel. (Again, consult your manual before operating your amplifier in bridged mode.) 

Discrete Output Devices 

There are three basic types of output devices found on car audio amplifiers — integrated circuits, bipolar transistors, or MOSFETs. An integrated circuit (or IC) is found only on relatively low-wattage (20 watts RMS per channel or less) amplifiers called "bridged transformerless" amplifiers. An IC cannot pass enough current to work on a more powerful amplifier and is not considered a discrete output device. 

Bipolar transistors and MOSFETs are found on the output stages of high powered amplifiers. They are fast enough and can handle enough current to send power greater than 20 watts per channel to your speakers. Both of these types of transistors are considered discrete output devices. Usually there are two per channel, but some amplifiers feature as many as four per channel.

High pass filter (HPF)

This is a circuit that prevents any low frequencies from reaching mid and high frequency loudspeakers.  This is desirable because low frequency signals sent to a high or midrange speaker at high levels can damage these dive units. This filter can be fixed or variable - variable allows better integration with subwoofer systems.

Infrasonic Filter

This is an amplifier circuit that removes subsonic frequencies (tones too low to be heard) to prevent excessive and possibly damaging subwoofer cone movement. This also allows higher power levels to be applied to frequencies that are audible.

Low pass filter (LPF)

This is a circuit that prevents high frequencies from reaching low frequency (woofer) speakers. Although a woofer can produce some high frequencies, these would sound distorted. This filter can be fixed or variable; variable ones allow for better subwoofer speaker integration.

Mono Amplifier 

Mono (or monaural) amplifiers are single channel amps, well-suited for low-frequency applications since the human ear cannot distinguish stereo in the extreme bass range. Also, since mono amplifiers are stable to 2-ohms, you can connect them safely to two 4-ohm woofers (wired in parallel). 

MOSFET 

Metal Oxide Semiconductor Field Effect Transistors have a higher switching speed than bipolar transistors and generate very little heat. They offer fast response and are highly efficient. 

Ohm 

The unit of measurement for impedance. It tells you how much a device will resist the flow of current. If you take two signals of exactly the same strength and send one to a 4-ohm speaker and the other to an 8-ohm speaker, twice as much current will flow through the 4-ohm speaker. In other words, the 8-ohm speaker will require twice as much power (wattage) to play at the same volume. 

Parametric 

Parametric equalization or tone controls allow you to set not only the degree (in dBs)  by which a certain frequency band is boost or cut, but also the width and/or center frequency of this band. This gives you extremely precise control of the tonal balance in your vehicle. Parametric equalizers are more versatile than graphic equalizers, which have fixed center frequencies and bandwidths. 

Preamp output 

Number of RCA outputs. An RCA output lets you pass the preamp signal to additional amps, and in many cases the internal crossover from the first amp can send a filtered signal, eliminating the need for additional crossovers. 

RMS Power vs. Peak Power 

The amount of continuous power, measured in watts, that an amplifier produces is called RMS power. The higher the RMS figure, the louder and cleaner your music sounds. When choosing an amplifier, the RMS rating is the power rating you should pay most attention to. 

Also, keep in mind that some manufacturers calculate the RMS power ratings of their amplifiers at different input voltages. For example, an amplifier rated at 100 watts RMS at 12 volts can produce considerably more power than an amp rated at 100 watts RMS at the more typical 14.4 volts. 

Stereo manufacturers often display peak power ratings on the face of their products. The peak power rating tells you the maximum wattage an amplifier can deliver as a brief burst during a musical peak, like a dramatic drum accent. The RMS figure is more significant. 

RMS Power at 2 ohms 

This spec tells you how much more power your amplifier delivers when presented with a 2-ohm stereo load. You can achieve a 2-ohm load by using parallel wiring or by using 2-ohm subwoofers. 

Theoretically, amplifier output should exactly double as the impedance drops from the usual 4 ohms to 2 ohms. However, amplifier manufacturers use different degrees of regulation on power supplies, which can restrict the actual increase in output. 

Less regulated power supplies come closer to doubling their output into 2-ohm loads. An amplifier with little regulation can achieve higher wattage into lower impedances. An amplifier with stiffer regulation maintains rated output from your amp as other electrical accessories demand voltage from the battery. 

Sigma Servo

This is an amplifier feature designed to stop unwanted and potentially damaging, excessive speaker cone movement. This not only protects loudspeakers from damage to their cones, but also improved bass reproduction.

Signal-to-Noise Ratio 

Measured in decibels (dB), this measurement compares the strength of the desired signal (music) to the level of background noise. A higher value indicates less background noise.