Amplifiers

Amplifier Overview
Your amplifier takes the signal from your head unit and makes it big enough to be able to drive your speakers. Most people start their system with an amplifier for the low frequencies (bass) and use their head units built-in power to drive the higher frequency speakers. This is enough but the built-in power in a head unit is usually not strong enough for high volume listening and not clean enough for the discerning ear. There are lots of choices when choosing an amplifier.
Things to look for:
RMS Power: The power output of an amplifier should be roughly matched to what the amp will be used for and what speakers it will be driving. The most typical problem with matching speakers and amps is using an amp that is too weak to power the speaker. When an underpowered amp is used to power a speaker, the listener tends to turn the volume up higher in order to get more output of the amplifier. Eventually the amplifier runs into its limit and begins to distort. This distortion can cause the output from the amplifier to become DC for short periods of time and DC signals of even low power can destroy a speaker. Under powering a speaker can be more dangerous than overpowering it! Also more power is usually necessary when powering subwoofers because of their large size and excursion. Do not plan on using an amp of less than 75watts per channel to drive a subwoofer. The converse holds true for higher frequencies (midrange and treble) only 25-50watts per channel are needed to drive speakers in those frequency ranges, however more power will not hurt, it just probably will not be used. Another factor in power output is stability in low resistance loads. It is possible to wire mutiple subwoofers to a single channel on an amplifier but the amp will have to work harder to drive this kind of load. Many affordable priced amps can drive loads as low as 2 ohms or less, with 4 ohms being the typical load of a single speaker.

Power Supply Regulation: The power supply in an amplifier converts the 12volt DC that is available in your car's electrical system to something the amp can use to produce greater power. Several designs are employed by manufacturers today. Two categories are regulated and unregulated. A regulated supply produces the same power regardless of whether your car's electrical system voltage sags (which a capacitor will help prevent). An unregulated supply's power output depends directly on the input voltage. This causes changes in the maximum output power with changes in the car's electrical system. I recommend getting an amp with a regulated power supply so power output will be constant regardless of input voltage changes. This changes if you have a stiffening capacitor or another regulation device (Accumatch) to smooth out your car's electrical system. In this case, buy an amp with an unregulated supply. Take care not to buy cheap amps which use unregulated supplies buy don’t give the benefits of a usual unregulated supply. One way to determine whether an amp has a regulated supply or not is to view the power output specs for 12volt and 14.4volt inputs. If they are the same then the amp probably has a regulated supply otherwise it has an unregulated one.
Tri-Mode: Some amps can play in what is called "tri-mode." In this mode, 2 channels are used to drive a pair of high frequency speakers and one subwoofer. The subwoofer receives power from both channels. This is a very good way to use an amp for more than one purpose. A special crossover is required to separate the two ranges of frequencies and it should have a way of adjusting the output level between the high frequency speakers and the subwoofer. This is a good way to reduce your expenditure.
Other Specs: THD (Total Harmonic Distortion) is a spec that often shows up with the power output spec. An example would be "45wattsx2 @ 0.01% THD" This spec says that at an output level of 45watts into each channel the THD will be no more than 0.01%. Sometimes manufacturers will quote the power spec at a THD of 1%. Be wary of this, 1% THD is poor and either implies that the amp is not very high quality or that the manufacturer is artificially inflating the power output spec by running the amp into a higher distortion region where it does produce more power but more distortion as well. Either way it is a sign of a low amp or marketing that deceives. Anything lower than 0.1% is negligible.
Built-in Crossovers: These allow you to use the amp to only amplify certain frequencies and dedicate the amp to a subwoofer or some other specialized speaker. By using an amplifier's built-in crossover you eliminate the need for a separate one which can save you quite a bit of money. There are sophisticated amps on the market today that combine multiple channels and built-in crossovers so that you can use them in place of multiple amps and a separate crossover. They are costly but often cheaper than buying separate components.
Pre-amp Outputs: Some amps have pre-amp outputs which allow you to "daisy-chain" multiple amps together without splitting the pre-amp output from your head unit. Also, if the amp has a built-in crossover, you can use it to drive another amp.
Input Sensitivities: Here's what an amp does: it takes its input and makes it larger so it can drive speakers. How much larger it can build the input signal is set by the input sensitivity and the max power output of the amp. You can turn the input sensitivity all the way up but that does not cause

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