An introduction To Amplifiers!
Amplifiers take the signal from the 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.
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%).
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