Amplifiers 101: How to pick the right amplifier for your system
By Paul Sonoda
An amplifier’s function is to convert the audio
signal from your source unit to a higher output level
capable of driving speakers without changing the
sonic characteristics of the audio signal. Sounds
simple, doesn't it? Actually it is quite a complex
process. Fortunately for us, we do not need to know
how an amplifier works to pick one that is right
for our needs. This article will cover the features
of amplifiers and how to choose the right amplifier
to fit your needs.
Amplifier Class
Amplifier Class refers to the circuit topology of the amplifier.
The common classes of amplifiers are Class A, Class
B, Class AB, Class D and Class H. Class A, Class
B and Class AB are variations of analog amplifier
circuit design. Class D and Class H amplifiers are
different implementations of digital driven amplifiers
designs.
Class A amplifiers are the most inefficient but typically have the best sound quality. Their efficiency rate can be as low as 25%. That means the other 75% is wasted in heat! A typical Class A amplifier similar in size to a Class AB amplifier will only deliver about half the power. Only choose a Class A amplifier if your goal is ultimate sound quality without regard to cost and power output. Class B amplifiers are much more efficient but have the problem of notch distortion. Class AB amplifiers are a combination of Class A and Class B design yielding the best combination of amplifier efficiency and sound quality. Typical amplifier efficiency for a good Class AB design is about 60% efficiency. This is the most common choice for an amplifier that needs to reproduce the entire audible frequency range.
If you have checked out high-powered subwoofer amplifiers
chances are you have heard the term Class D amplifier design.
A Class D amplifiers best feature is its efficiency. Good
Class D designs can meet and even exceed 80% efficiency,
drawing less battery power when compared to a Class AB
design. More important, they routinely have over double
the efficiency of Class AB designs when running less than
full power. The problem with Class D amplifiers is that
they are not so good (even bad) at powering midrange and
tweeter speakers. As a result, most Class D amplifiers
are bandwidth limited to frequencies for subwoofers. Class
H amplifiers are a newer version of digital amplifiers
designs. They share the efficiency of the Class D amplifiers
plus they can be very competent at powering midrange and
tweeter speakers.
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topNumber of Channels
This is one of the most important aspects of your amplifier
purchase decision. There are many different configurations
of amplifiers. Amplifiers range from signal channel mono-blocks
to 8 channels total system amplifiers. The most common
configurations are 2-channel and 4-channel amplifiers.
The first step to choosing the right amplifier (or amplifiers)
for your system is to first determine the number of speakers
you plan to use in your vehicle. In an average system,
this would consist on speakers in the front of the vehicle,
amplifiers in the rear with one or two subwoofers. For
this example, let’s create a simple system that
uses nice set of front component speakers with a single
4-ohm subwoofer. We will look at several different options:
one amplifier for the entire system, or two separate
amplifiers.
Two amplifiers vs. one multi-channel amplifier
Deciding between the two possible amplifier choices will
depend on comparing the benefits and advantages of two
amplifiers versus one multi-channel amplifier. Multi-channel
amplifiers have several advantages. The most obvious
is ease of installation. Installing one box is always
easier that installing two. Multi-channel amplifiers
with a common power supply can also provide extra current
for subwoofers when the amplifier duty is split between
powering midrange and tweeters on two channels and subwoofers
on the other channels. The multi-channel amplifier can
make the system simpler by needed only one set of RCA
signal wires and one set of power cables. Of course the
most obvious benefit is cost. Multi-channel amplifiers
will most often cost less for the same power as separate
amplifiers of the same power and brand. If “bank
for the buck” is most important, multi-channel
amplifiers are the right choice.
On the other hand, separate amplifiers also have their benefits. Separate amplifiers have less cross-talk (signal bleed between channels). If the subwoofer amplifier distorts, you do not have to worry about that distortion adversely affecting the sound quality of the other amplifier. Separate amplifiers typically have more heat sink area for better heat dissipation. Another benefit is the ability to better select amplifiers for a given task. Class D amplifiers, known for their high power versus current draw from your electrical system, are perfect for subwoofers but not so good for midrange and tweeters. With separate amplifiers, each amplifier can be chosen for optimal performance in the system. If cost and space is no object, separate amplifiers are the way to go. Next, we need to match the performance of the amplifier to the impedance of the speakers and subwoofers.
Ohms not Gnomes
When choosing an amplifier, it is important to match
the nominal impedance of the speakers with the performance
capability of the amplifier. Nominal Impedance is the
average load a speaker or subwoofer presents to an amplifier.
This is measured in ohms. (Ohms are the measurement of
electrical resistance.)
Most amplifiers are typically capable of 2 ohms stereo (unabridged outputs) and 4 ohms mono (bridged output). Higher performance amplifiers, typically 2-channel and mono-block (single channel) dedicated subwoofer amplifiers, are capable of dealing with lower impedances up to and exceeding 0.5 ohms! Choose an amplifier that meets your minimum requirements for impedance load. Always refer to the manufacturers spec and not what an enthusiast will say it can do. This will ensure the longevity of your amplifier.
Power per channel
Power is the most important feature of an amplifier. It
determines how loud your stereo can play as well as how
much power it will draw from your electrical system.
There are many misconceptions about power. The most common
myth is that overpowered speakers will break more often
than underpowered speakers. This is false. One of the
most common causes of speaker failure is damage due to
amplifier distortion (amplifier clipping). An under powered
amplifier will most likely be driven into clipping hard
which will result in blown speakers or worse, a blown
amplifier. When deciding on an amplifier it is best to
match the power of the amplifier to the RMS power rating
of the speakers. This will give you the maximum safe
output of the speakers. Make sure the amplifier’s
power rating meats CEA 2006 standards. At a minimum,
choose an amplifier that can deliver one half the rated
RMS power handling of your midrange and tweeters. For
subwoofers, the goal is to balance the output of the
subwoofer with the rest of the system. Choose an amplifier
that has at least one-quarter the RMS (continuous) power
handling of the subwoofer. In most situations, you will
not need to exceed the RMS power handling of the subwoofer
to balance the system. If high bass out put is your goal,
pick an amplifier that can deliver the peak power handling
of the subwoofer. Peak power handling is the amount of
power a woofer can safely handle for brief moments. In
any situation, never exceed the Peak power rating of
the subwoofer.
Internal Crossovers and Signal Processing
Today, most every amplifier includes internal crossovers
or additional signal processing. Signal processing
varies between simple high-pass or low-pass crossovers
to full adjustable digital crossovers with time alignment
and 31 band equalizers. For amplifiers driving midrange
and tweeters, make sure the high-pass crossover has
the frequency adjustment range suitable for your
application. If you plan a three way system with
separate amplifiers for the tweeter, midrange and
woofer, make sure the crossover range fits the recommended
frequency range for the speakers. For subwoofer amplifiers,
amplifiers with a bass boost or better a subsonic
filter. (A subsonic filter is the a circuit the protects
subwoofers for excessive low-frequency excursion)
Make sure the amplifier you choose has at least the
electronic crossovers you need to properly control the system. Additional controls and adjustments
make for a more flexible and potentially better sounding
system.
Conclusion
There are other important factors such a good warranty
and build construction to take in to consideration as
well. And, of course, amplifier looks are always important.
If you need additional assistance, rely on your local
car audio stereo shops for expert advice. They have the
latest information of what is hot and not. Plus they
will be able to recommend amplifiers based on their observed
reliability. Choosing an amplifier that is perfect for
your needs can be a challenging task. But the results
will be worth the effort.
