Things to Consider When Buying a Stereo Amplifier
Most audio components deal with very low-level electrical
signals. The voltages at a tuner's antenna terminals, for instance, are minuscule, and
must be boosted to what is vaguely called "line level" for their trip through
the rest of the system. But even line level is a tiny amount of electricity.
In the end, however, the signal will be called upon to do
some real work. A loudspeaker is a kind of electric motor, which pushes air about in a
controlled fashion that results in sound -- musical sound, one hopes. That takes a
considerable amount of effort, and to accomplish it, the line-level signal must be boosted
to many times its original strength, while still retaining the shape of its waveform. That
task falls to an audio amplifier.
In most systems, the amplifiers are either separate
components that do nothing but amplify, or are combined with control functions as integrated
amplifiers. By far the most popular format, however, is the receiver that includes
radio capability as well -- nowadays along with sophisticated surround signal processing.
Whatever form it may take, the amplifier is the heart of
any audio system, and any flaws or shortcomings here will affect all your listening.
Fortunately, there isn't a great deal of variation between the performance of one
amplifier and the next, so the number of factors you have to consider is relatively small.
Amplifiers are about power, so the main concern in choosing
one is how much power you get, and at what cost. How much you need varies considerably,
depending on your listening habits, your room, and your speakers. But it's output power,
measured in watts, that determines an amplifier's suitability for a particular situation;
virtually everything else is window-dressing. Power ratings refer to an amplifier's
capacity over a period of time; virtually all can produce higher levels for the short
durations of musical peaks.
There is a huge range of options on the market, from a few
watts for use in small spaces to many hundreds; in most cases a modest amount of power --
from 50 to 100W, say -- is all that's needed. Remember, however, that doubling the power
output results in an acoustic increase of only 3 decibels, which is only barely audible.
In theory, you can't have too much power. Bear in mind that
if an audio signal is loafing along at an average level that translates into amplifier
output of a couple of watts (not at all unusual), and a modest 20dB peak comes along, it
will require an increase in power of 100 times, or 200W.
Most of us can't afford infinite amplifier power, and
fortunately most amps are able to put out levels above their rated output for very short
durations, so most applications don't really require hundreds of watts. If you do find you
require a large power reserve, one way to achieve that is to use two amplifiers each
"bridged" or "strapped" to mono. Only a few models permit this,
however, so be sure that the one you select does. The owner's manual will tell you how to
go about bridging the amp; if it doesn't say anything about it, you can't do it and will
damage the amplifier if you try.
High outputs and difficult speaker loads can strain
amplifiers in different ways. If you foresee a problem, or if your dealer suggests you may
have unusual requirements, check for such things as thermal protection circuits (or, in
extreme cases, cooling fans) and high current capacity. The last is particularly important
if the speakers you choose have low load impedances (4 ohms or less), or if you
contemplate running more than one pair of speakers from a single stereo amplifier.
A pure power amplifier is likely to have only a single set
of inputs and, perhaps, one pair of level controls, which can be useful for matching the
amplifier to the rest of the system. If you choose an integrated amplifier or receiver,
however, you will have to take the same sort of care to make sure it has enough
flexibility as you would in choosing a separate preamplifier. Such things as video
switching, adequate inputs, tape loops, and digital inputs are as important in combined
components as they are in separates.
Amplifier manufacturers have always been hampered somewhat
by the similarity of their products: performance levels are so high that they are now
meaningless as a basis for choosing one amp over another. Numerous features and design
wrinkles have been used over the years to urge you to buy this model instead of that. You
can forget all of them.
There's no real mystery to amplifier design, and the
simplest, most traditional techniques often yield the best results (or results as good as
anybody else's but at a lower price). A good rule of thumb is that if an audio company
gives its amplifier design a name, there's a good chance you will pay a premium for
it, but hear no improvement. If an amplifier produces maximum watts for minimum bucks, go
for it and ignore how the thing is wired together.
Ditto for the components inside the box. Expensive
amplifiers often boast unusual devices within and these rarely have a negative effect on
performance. But any advantages they may provide are usually so subtle that they hardly
justify their price. And the ultimate "in" component -- the vacuum tube -- can
be a positive disadvantage. Even tube fans will admit that the sonic improvement gained by
tubes is slight, but it's undeniable that they are hot, expensive, fragile, and wildly
unreliable. Most of the electronics biz abandoned tubes 50 years ago for very good reason.
Finally, don't worry too much about specifications.
Obviously if there is some massive flaw in an amplifier you have to take that into
account, but these days there are almost never such flaws. The differences between one
amplifier's performance and another's, although easily measurable, are almost always
inaudible. Even the minority of audiophiles who claim to hear variations from one amp to
the next have to work very hard to do so.
...Ian G. Masters