Your amplifier takes the signal from your head
unit and makes it large enough to be able to drive your speakers. It is
preferable to use separate amps for high and low frequencies but it is not
necessary. The problem with using one amp for all frequencies is that you cannot
adjust the levels among different frequency ranges as easily as you can adjust
outputs of separate amps. Many people start their system with an amplifier for
the low frequncies (bass) and use their head units built-in power to drive the
higher frequency speakers. This is adequate 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 many options when choosing an amplifier.
I've also created a page for understanding power
amplifier specifications here.
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. Oddly enough, the most common
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.
Underpowering a speaker in this way 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 necessary 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. Sometimes you can 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 moderately 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 more power.
Several designs are employed by manufacturers today. Two classifications 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 amp using a stiffly regulated power supply will be able to
supply full power even when the input voltage dips below 12volts. However, it
will not gain any power if the input voltage goes above 12volts. 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.
Some cheap amps use unregulated supplies to save money but provide none of the
benefits of a typical 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 efficient 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 can be a nice
way to save money on your system although it wastes a little bit of amplifier
power because of the crossover and it can be more difficult to adjust the
relative level between the high and low frequency outputs.
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 poor amp or
marketing that decieves. Anything less 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 considerable 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 expensive 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. For
example if you have an amp you are going to use to drive a subwoofer with a
built-in crossover at 90Hz, you can use its built-in crossover to set the amp to
only amplify signals below 90Hz for the subwoofer and then have a pre-amp output
that only has frequencies above 90Hz which you can connect to an amp that does
not have a built-in crossover. That amp can then be used to power the high
frequency drivers.
Input Sensitivities:
I have received a number of questions about input sensitivities and their
importance especially as to why 4 volt outputs on a head unit are better. Here's
what an amp does: it takes its input and makes it larger so it can drive
speakers. How much larger it can make the input signal is set by the input
sensitivity and the maximum power output of the amp. You can turn the input
sensitivity all the way up but that does not make the amp put out more power
than its max, it just gets to that max level with a smaller input voltage. To
see why 4 volt head units are better lets say we have 2 head units, model A puts
out a 1 volt signal and model B puts out a 4 volt signal max. We're connecting
these head units to a 25 watt amp. The amp puts out 10 volts.
Power = Voltage^2/Resistance = 10^2/4 = 25watts.
To get maximum output from head A, the gain needs
to be 10 (10volts out per 1volt in, 10/1 = 10). Now let's say there's 0.1 volt
of noise in the signal. With our gain set at 10 with our input sensitivity
control we have amplified the noise to 1 volt. Consider what happens with head
B. The gain needs to be only 2.5 to get full output. We still get 10 volts of
output but the noise is only 0.25 volts. This noise level is 4 times lower than
with head A. By using a higher voltage head unit you can set the gain on your
amp lower and thus amplify less noise. Also lets say you left the input
sensitivity set for a gain of 10 and you used 4 volt head unit at its max. If
this did not make the input stage distort it would try to make the amp put out
40 volts (10*4) which would be 400watts! Obviously the amp can't do that and
just hits its 25watt limit. To set your input sensitivity, turn you amp's input
sensitivity almost all the way down. Now start with your head unit at its lowest
volume and turn it up until you hear distortion and then back off some. Some
head units will let you go to full volume without distorting the pre-amp level
outputs. Now with your head unit putting out its max clean voltage, turn the
input sensitivity up until you get to the loudest your system will play without
distortion or the loudest you ever care to listen, whichever is lower. Now your
amp is set to amplify the least amount necessary to produce full volume making
it amplify noise the least.
