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Car Audio - Amplifiers
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.
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%).