Matching Speakers and
Amplifiers
In a stereo system, every part reacts with and influences
every other part. Although they perform very different functions, power amplifiers and
speakers form a single component, whose task is to take the tiny source signals and turn
them into pressure variations that we hear as sound. To get things right, it's vital to
get all the pieces working together optimally.
The area of greatest concern by far is the matching of
speakers, not only to the amplifiers but to the listener and the room. If, for example,
you live in a small apartment, speakers that need a lot of space are obviously
inappropriate. In choosing models more suited to your situation, you may find that this
places certain demands on the amplifier. Often, for instance, the compromise a speaker
designer must make in producing a small unit is to make it less sensitive than a larger
one, so the amplifier power must be that much greater to provide the same acoustic output.
If you do need high power, you could just buy a big
amplifier. The same thing can be achieved, however, by choosing a more sensitive speaker,
one that will produce more acoustic output for a given signal from the amplifier. One
caution: while speaker sensitivity is important, it has to do only with quantity of sound,
not quality.
Some speakers need a lot of power -- they are quite
insensitive -- but can't handle a lot. Overpowering such speakers may cause damage
if you push them too hard. A delicate balance between power, power handling, and
sensitivity is thus essential if the most crucial part of your system is to perform to its
utmost.
Mirage's new OM-5 loudspeaker has a built-in 250W
amplifier specifically design to power the woofer section. The listener then uses their
own amplifier for the midrange and high frequencies.
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A power amplifier has no other function than
to take a small signal and turn it into a large one. The traditional audio virtues -- flat
frequency response, low distortion, good noise performance, and so forth -- are as
important for amplifiers as for other components, but it is rare to encounter one that
doesn't excel in these respects. Where amplifiers do differ is in the one critical
attribute of output power. The basic figure normally given is the amount of power, in
watts, the amplifier can put out continuously over a period of time. Most amplifiers can
deliver considerably more power during brief musical peaks.
In theory, you can't have too much power; you are far more
likely to damage your speakers with too little power than with too much. Most of us can't
afford infinite amplifier power, however, 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.
There are other electrical considerations to be taken into
account as well when matching amplifiers and speakers. In effect, an amplifier's output
stage is missing a resistor, whose function is taken by the speaker and its associated
cable. Some amplifiers are designed to accept a fairly wide range of values for the
completion of the circuit, but most are quite restricted as to what will operate safely --
too little resistance and the increased current drain may cause amplifier damage.
If a speaker were a purely resistive device, the choice
would be simple: just pick one with the appropriate value. But speakers are dynamic
devices, and their effect on the output of an amplifier is not resistive but
"reactive": it varies with frequency. The load a speaker presents to an
amplifier, therefore, is termed "impedance" rather than resistance, although
both are expressed in ohms. Too low a load will increase the current flow in the
amplifier's output stages, possibly to levels that could damage them.
Most home speakers have a nominal (or rated) impedance of 8
ohms, and virtually all amplifiers are happy with that, but a few are specified at 4 ohms;
amplifiers vary widely in their ability to handle low impedances, and this must be taken
into consideration when choosing both amplifier and speakers. The problem gets much worse
if more than one pair of speakers is used with the same amplifier, as this causes the
total impedance to drop dramatically.
A speaker is also only one half of an acoustical system;
the other is the room itself, and it can have a profound effect on the sound you hear. In
any room, sound reflects off the walls, ceiling, and floor. If the distance between two
opposite parallel surfaces is a simple fraction of the wavelength of a particular
frequency, notes of that frequency will bounce back and forth in perfect phase -- an
effect called a "standing wave". At some point in the room, this note will be
reinforced substantially; at others it will cancel out almost entirely.
Almost all rooms are susceptible to some standing waves at
low frequencies, but their effects can be minimized by careful positioning of both the
speakers and the listening seat. The only way to find out what works best is by
experimentation.
In particularly difficult cases, the use of a separate
speaker that handles only very low frequencies -- a subwoofer -- may be necessary. The
range of places you can put the main speakers and still get proper imaging may be fairly
limited, and some of these positions may result in standing waves that can't be tamed.
Positioning of the bass speakers is much less critical from an imaging point of view, so a
subwoofer can be located with only standing waves in mind.
...Ian G. Masters
ian@mastersonaudio.com
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