Properly matching loudspeakers
and power amplifiers
By Keith Clark
PSW Editorial Director
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One of the questions most commonly asked of loudspeaker manufacturers
concerns the matching of power amplifiers to loudspeakers.
Manufacturers will tell you that the majority of loudspeaker failures
encountered by their service department are the result of improper
power matching. It might seem logical to use a smaller amplifier as a
safety measure; after all, it stands to reason that there won't be
enough power to damage the loudspeaker.
Yet they see many more loudspeaker failures due to under powering than
any other cause. When the amplifier exceeds its maximum voltage
capability, it can "clip" the peaks off the electrical signal it is
supplying.
Once the amplifier starts clipping, the average (or RMS) level rises,
which can be extremely damaging to the loudspeaker. In layman's terms,
for example, your 200-watt amplifier can approach delivery of 800 watts
of power, but it will be volatile and heavily distorted, and thus
potentially quite harmful to the loudspeaker.
However, using an amplifier that is much more powerful than the
loudspeaker's rating can do just as much damage. Supplying the speaker
with more power than it can handle will often result in thermal (heat)
and/or mechanical damage.
Basically, any amplifier with a 20Hz - 20kHz power rating that matches
other specifications of a given loudspeaker will safely drive that
speaker at full output under most conditions. Therefore it's vital to
understand the meaning and impact of the most important loudspeaker
specifications as they relate to amplifiers.
First, there's power handling capacity, usually defined by terms such
as "Continuous," "Program" and "Peak". Continuous is the level of long
-term average power accepted by the loudspeaker; Program is based upon
a test signal that simulates a "real-world" program signal; and Peak
refers to the maximum amount of short-term power the loudspeaker will
handle.
Based on these terms, then, what is the proper amplifier choice to
capably and safely drive a loudspeaker rated at 100 watts continuous,
220 watts program, and 400 watts peak? The best choice would be an
amplifier delivering about 200 (continuous) watts per channel. Plenty
of power for normal full-range operation while also capable of meeting
short-term peak demands without going into clip.
It should be noted that although an amplifier delivering 200 watts
per channel is appropriate for use with a full-range loudspeaker rated
at 220 watts program, subwoofer enclosures tend to require a bit more
power to generate their lower frequencies. In other words, a single
mono subwoofer rated at 200 watts should be paired with an amp offering
absolutely no less than 200 watts.
In fact, a good rule of thumb when dealing with subwoofers is to employ
an amp rated somewhere in the vicinity of 20 percent higher than the
subwoofer. With that in mind, a 250-watt amplifier would be ideal in
this application.
Another factor to be aware of is the impedance load that the amplifier
is being required to drive. Impedance is the resistance to the flow of
the audio signal by the loudspeaker. Measured in ohms, it is usually
called "nominal impedance" by most loudspeaker manufacturers.
A mismatch in impedance between an amplifier and loudspeaker can result
in poor performance, in the form of improper frequency response and
excessive distortion. Serious damage to one or both components is also
a very real possibility.
Most professional loudspeakers are rated at either 8 ohms or 4 ohms,
although a number of power amplifiers are specified to drive loads
ranging from 16 ohms to 2 ohms. You'll notice that the lower the
impedance, the greater the amplifier power rating. More power is
required to overcome the increased resistance.
For example, a loudspeaker delivering 110 watts at 8 ohms will also
likely be specified to deliver about 150 watts at 4 ohms. Consider our
prior discussion of power handling, and you quickly see why its
important to note a loudspeaker's impedance specification in the
matching process.
It's usually best to avoid 2-ohm operation, which can unnecessarily
stress an amplifier, and also can require much larger (heavier gauge)
cable to effectively and safely transport the load.
And speaking of cable, why go to all of the trouble of properly
matching loudspeakers and amplifiers if the link between them is
subpar? Most manufacturers recommend using heavy gauge (#14-#10),
professionally manufactured cable in all applications. A good rule of
thumb is the heavier the cable, the more efficient it will be as a
conduit between the devices.
(Thanks to the gang at EAW for their input on this topic.)
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Post By：2008-7-10 7:53:36 [只看该作者]
