Watts Up with
Amplifier Power?
How many watts do I need? Is the power rating the only
amplifier spec I should concern myself with? Is bigger always better?
Amplifier power behaves logarithmically. Each time an
amplifier drives a loudspeaker to an increase in sound pressure level of 100% (or adds 3dB
of gain to the signal), it must double its power output. This means that the loudness
ratio between 2W and 4W is exactly the same (3dB) as that between 100W and 200W,
even though the difference in wattage is far more extreme.
Put differently, at full output, a 200W amplifier
can only play 3dB louder than a 100W unit. However, it's just as rare to drive an
amplifier to full output as it is to push a car to full redline. Besides, if you did
drive your amplifier to full output, you'd know it -– it would audibly clip and you'd
hear distortion. So the 3dB advantage of the larger amp would probably end up just
sitting there. So why buy more amplifier power than you might need on average?
Because music frequently has wide extremes in dynamics (which is another way of saying
that it ranges greatly from the quietest passage to the loudest), and while an amplifier
might cruise along happily outputting 10W, some passages might require ten times that
amount of power for an instant.
Put in the most basic terms, four different factors
determine how much amplifier power you need.
- Speaker sensitivity. This was covered in detail in our last
installment. Briefly, with the same power, a more sensitive speaker will play louder than
one with less sensitivity.
- Room size. Everything else being equal, a larger room
absorbs more power than a small room. That's because the speaker sound we hear is the sum
total of direct and reflected sounds, and sound pressure diminishes with distance. Hence,
to overcome the increased volume of a large room, an amp needs to put out more power.
- Listener distance. This can be independent of room size. If
you sit close to the speakers, even in a large room, you will be using less power than if
you sit farther away.
- Your average playback level. Are you a background music type
or party animal? Average listening levels (neither background nor party but, well, normal)
require less power than you may think -- the average listening level requires somewhere
around 10 watts in a standard 14'W x 20'D x 9'H room.
In addition, most popular music recordings compress their
dynamic range electronically to keep the differences between the quietest and loudest
passages to a minimum. The average power requirements barely fluctuate. Classical music
can feature extremes that range from pianissimo to triple forte. Without altering the average
power requirement, this type of material might demand spontaneous peak power bursts that
could be 100W or more.
This brings us to the way power ratings must be read.
Continuous or RMS power simply tells us how much power a given amp can deliver
day-in/day-out on a steady basis. (RMS stands for root means square, an equation
that specifies average power.) An amplifier's peak power might be three times its
continuous rating, or even greater, but that rating is only available for
milliseconds. There aren't many manufacturers left who'll try to fool you by passing off
peak power ratings as average, but there are still quite a few who will specify an
amplifier as having an output rating at a specific frequency rather than over an range.
When you see a spec like 100W @ 1kHz, you should be suspicious -- this is not the
same thing as an amplifier that delivers 100W from 20Hz-20kHz. The first is power output
at only one frequency. The second is power output at the full bandwidth of what’s
considered important for human hearing. Also look for the words both channels driven,
since any amplifier can deliver more than its true peak output when only one channel is
driving a loudspeaker (and for home-theater enthusiasts, look for something like all
channels driven since there may be five or six channels of power).
Make sure you're not comparing apples with oranges when
studying power specs. This means you must consider the impedance rating to which the power
rating is attached. Since most speakers (especially in the affordable arena) are
rated at 8 ohms, amplifiers and receivers most frequently list their power output into 8
ohms. However, speaker impedance generally varies with frequency, so you'll see some
speakers specified as having a "nominal" 8-ohm impedance. And sometimes the
specification will simply be an average taken over a wide frequency range, so be careful
and ask your stereo salesman if a given speaker's impedance is a real figure, a nominal
reading or an averaged reading. If the speaker you like has an impedance rating that drops
below 4 ohms, you'll want to look at an amp with a robust power supply.
Amps with robust power supplies will sometimes feature a
4-ohm rating. Ideally, this rating should be double that of the 8-ohm figure, which
indicates that such designs are stable and compatible with low-impedance loads. Certain
amps will also specify their power ratings into 2 ohms and even 1 ohm. The true brutes
among these will double power into each lower impedance, all the way into 1 ohm, giving
rise to astonishing figures of 1000W or higher. But, since affordable speakers tend to be
either 8 ohms or 4 ohms, you won't need to worry too much about power output into lower
impedance loads.
For completeness' sake, we now need to cover amplifier current.
A reasonably close car analogy is the horsepower/torque equation. A high torque rating is
useful when negotiating steep inclines and towing trailers. Depending on how you use your
car or truck, high torque could be more important than ultimate horsepower specs.
Similarly, decent current delivery in amplifiers is necessary to control speakers with
large and/or multiple woofers. Such speakers make greater current demands and (within the
context of our analogy) could be thought of as steeper hills or heavier trailers.
The subject of current delivery gave rise to the popular
notion that not all watts are created equal. In a limited sense, this is true, as
long as it is not taken literally to mean that Denon watts are different from Kenwood
watts. It simply means that an amp's current capability and power rating need to be looked
at in conjunction with one another.
Our earlier explanations indicated how much (or, more
accurately, how little) average power we tend to use. You can now appreciate that a
50W/high-current amp may outperform a 200W/low-current design when mated to demanding
high-end audio-type speakers.
While massive amplifier current ratings are one way to
evaluate a power amplifier's ability to drive all loudspeakers, they are not
essential if the loudspeakers you have chosen are reasonably sensitive, your room is of
moderate size and your listening habits are relatively sane.
The truth is that a simple listening session will tell you
whether a particular amp/speaker combination is copacetic or not. Try to listen to an
amplifier at home, so your room and other equipment can be part of the audition. Play some
of your favorite records -- the ones you are familiar with, not special "test"
discs (and not an "approved" type of music -- if you normally listen to rock,
audition with rock, not classical). First listen at your regular loudness level and see if
the amp/speaker combination can play at the level you prefer. Listen for the things you
like about your musical selections. Is the bass punchy and tight and deep? Are the vocals
clear? Is there a big difference between the loudest and softest passages (keeping in mind
that not all recordings have such a distinction)? And if it all sounds good at your
regular playback loudness, turn it up to about the loudest you'll ever listen at and check
for the same properties. You'll be surprised at how much your ears can tell you in your
own room.
Besides, that's where you'll be listening to it once you've
bought your ideal amplifier, so listen carefully and put it through its paces before you
buy it.
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