Power?
There are different ways in which power
is measured by amplifier manufacturers to make people think
that their amps have more power than others. Laws of physics
tell us that Power can be obtained by multiplying Current
and Voltage. For example, if your amplifier gets 12 volts,
and it draws 20 amps, then power would be 240 watts, right?
Not exactly. In the real world, amplifiers waste 50% or
more of the power in the form of heat. That leaves you
with only 120 watts.
 Things get more complicated
than that. There are different ways to measure power.
Power can be measured for top to bottom of the signal
(Peak, or Max, etc). Another way to measure power
is From the zero-level to the top half (usually called
music power). The most accurate way to measure power
is RMS (root mean square) watts. The RMS value is
obtained by squaring the value of the signal, taking
the average, then the square root. This is the equivalent
of the actual power delivered. Most reputable manufacturers
use the RMS rating.
To get RMS power from peak or
max power just divide by three. Music power is
just half of peak power. For example, an amplifier
is rated at 100w (peak) per channel. The so called
Music power would be only 50w per channel. The
RMS power would be 33w per channel. Big difference,
isn't it? Be careful when checking specifications
of amps before buying, to see what you are really
getting. Always ask for the RMS power of an amplifier.
Confused enough? There is more.
Some companies rate their amplifiers using unrealistic
conditions, for example calculating power at 15
volts, under 2 ohms, at 10% distortion, etc. Make
sure you see the actual test voltages and loads. |
How to tell if
I am getting a good amp?
Shop for reputable brands. Look at the
size, weight of the amp. The more power the amp puts out,
the more wasted heat, and the bigger area it will need
to dissipate that heat (bigger heatsinks). This alone can't
be enough to determine if the amp is good or not. Watch
out for companies that use bigger heat sink than needed,
giving the idea of a more powerful amp.
Look at the fuses that are either plugged
into the amp, or specified by the instruction book. If
you see a 400w amplifier with a 5-amp fuse, you should
be suspicious. Remember what was said above, multiply size
of the fuse by around 6 (12v at 50% efficiency), and that
will give you a rough idea of what you are dealing with
in terms of maximum possible RMS power.
How much power
do I need?
For mids and highs, anywhere from 30 to
50 watts (RMS) per channel would be a minimum. For subs
you would need at least 80 - 150 watts (or more) per subwoofer.
There should always be more total power going to the subwoofers
than the rest of the speakers, since human ears are
more sensitive to higher frequencies than lower. For example,
if you have 4 x 50 watts going to all your mids and tweeters
(total=200 W), then you should have at least 200 W or more
going to your subs.
A lot of people wonder if too much amplifier
power can burn up the speakers. What damages speakers most
of the time is distortion, not power. If the speakers have
the proper crossovers and are not distorting, then it is
really hard to blow them. A bigger amp just gives you the
opportunity to go to higher volumes without distortion. Get
the biggest amplifiers you can afford and your car's electrical
system can handle. More power means louder sound,
but most importantly, cleaner sound.
What Else to Look
For in an Amplifier
It is a good idea to get an
amp with a built-in crossovers, so that you don't have
to spend extra money later on crossovers. If you are going
to be using multiple speakers, make sure the amp is 2-ohm
stable (or less). A bridgeable amplifier could come in
handy in the future if you are planning to upgrade. Overheat,
short-circuit, overload protections are good features that
any good amplifier should have. Look for a low THD
(total harmonic distortion) rating.
Amplifier Classes
There are different amplifier designs:
Class A, A-B, B and D
Class A amplifiers are the most sonically
accurate. On the other hand, they have some drawbacks that
make them a rare breed. Class A amplifiers use only one
output transistor that is turned
"on" all the time, giving out tremendous amounts of heat.
Class A amplifiers are very inefficient (less than 25%).
More heat means more heatsink area, so even though most class
A amps have built-in cooling fans, they are big. Class
A amplifiers are usually and expensive choice.
Class B amplifiers are the most common
by far.
They use two output transistors. One for the positive and
one for the negative part of the cycle. Both signals are
then "combined". The problem with this design is that at
the point when one transistor stops amplifying and the other
one kicks in (zero volt line), there is always a small distortion
on the signal, called "crossover distortion". Good amplifier
designs make this crossover distortion very minimal. Since
each transistor is "on" only half of the time, then the amplifier
does not get as hot as a class A, yielding to a smaller size
and better efficiency (typically 50%).
Class A-B amplifiers are a combination
of the two types described above. At lower volumes, the
amplifier works in class A. At higher volumes, the amplifier
switches to class B operation.
An increasingly popular kind is the class
D amplifier (known as digital amplifier). These amplifiers
are not really digital (there is no such thing), but operate
similarly in the same manner as a digital-to-analog converter.
The signal that comes in is sampled a high rates, and then
reconstructed at higher power. This type of amplifiers
produce almost no heat and are very small in size, but
really expensive. Although there are full-range class
D amplifiers available, most high-end manufacturers are
designing amps for low frequency applications. These
amps are capable of over 1000 Watts. Efficiency is
much higher in class D amplifiers (~80%).
