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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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