What is an Audio Watt?
of us that have any reason to visit RF Cafe know what a watt is both
from a DC, Ohm's Law perspective and from an RF power perspective. Some,
though, are not quite sure what an audio watt is, as far as whether
it is the same a watt of DC or a watt of AC (sinewave or not?). This
short article from the August 1955 Popular Electronics answers the question.
August 1955 Popular Electronics
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Popular Electronics
was published from October 1954 through April 1985. All copyrights are hereby
acknowledged. See all articles from
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What is an Audio Watt?
By Eugene F. Coriell, Major, USAF
A question that puzzles
many hi-fi enthusiasts is answered
To accomplish useful
work, whether it be moving a car along the highway or moving the cone
of a loudspeaker to produce sound, power must be expended. The more
power expended, the more work done. Mechanical power is generally expressed
in horsepower and electrical power in watts or kilowatts.
The concept of electrical power is important to the audio hobbyist because
almost every element of a tape recorder or hi-fi rig generates, transmits,
or absorbs power-electrical watts. The end result is the driving of
the loudspeaker, and since the air opposes movement of its cone or diaphragm,
power must be expended to overcome that opposition. In the case of the
loudspeaker, the driving power is furnished by the amplifier which is
rated as having an output of 10, 20, or more audio watts. It follows
that the speaker must also be rated as to the electrical power or wattage
it can absorb. This value should be at least equal to the amplifier
rating, and in the case of several speakers, the combined power rating
should equal the amplifier rating.
Within the amplifier, the
most obvious element important from an audio wattage standpoint is the
output transformer. This is the last component in the final stage of
the amplifier, and is the component that the loudspeaker "sees." The
output transformer is a device which matches output tubes to the speaker
voice-coil. If the amplifier is rated at 10-watts output, the transformer
must be able to handle this power. If it is too small, distortion or
burn-out may occur.
Another element within an amplifier which
is important, wattage-wise, is the lowly resistor. There are many of
these in the average amplifier, although not all of them handle appreciable
power. Resistors are available in many different resistance ratings
in ohms, and in various power ratings. In home amplifiers, the resistor
power rating may range from one-quarter watt to several watts. The resistor
rating is important at some positions in the amplifier circuit since
the current forced through the resistor by the applied voltage results
in absorption of power by the resistor. This power is dissipated as
heat, and a resistor of correct resistance value but with too low a
power rating may fail completely or cause noise in the amplifier output.
These two instruments are typical of the commercially
available units for measuring audio watts. At the right is the Heathkit
Model AW-1, which utilizes the v.t.v.m. principle to measure a voltage
across internal load resistors. On the left is the General Radio Model
583A, a professional laboratory unit. Both of these instruments have
provisions for matching the impedance of the amplifier under test.
It might be well here to distinguish between the
audio power input and the 115- volt house a.c. input to the amplifier.
For 10 watts of audio output, the house system must furnish several
times that much power to the amplifier 115-volt a.c. input. On the other
hand, the microphone or record player furnishes only an extremely minute
amount of program power to the amplifier audio input. This tiny audio
input power, ranging from a few microwatts to milliwatts, in effect
controls the 115-volt power that the amplifier draws from the house
a.c. system, which is the real source of the power fed to the loudspeaker.
Fig. 1. This is the test arrangement for measuring
the power output of a hi-fi amplifier. The vacuum-tube voltmeter must
be sensitive enough to read the extremely small voltage input to the
amplifier as specified by the manufacturer, which may be only a few
millivolts. Output wattage is equal to the square of the voltage as
measured by the v.t.v.m. divided by the value of the resistance of the
load resistor; this resistor is a substitution for the speaker. The
oscilloscope is not required for actual wattage measurements, but enables
the sine-wave pattern to be checked for distortion. Power output is
measured at various frequencies over the response range of the amplifier.
See text for details.
Continuous power of one watt
delivered to the speaker will result in an uncomfortably loud program
in the average living room. The question then arises: why use amplifiers
rated at 10, 20 or more watts? The cone-type speaker has a very low
efficiency. This means that only a small fraction of the electrical
or audio power fed to the speaker creates useful sound (horntype speakers
are more efficient but are less common in home systems).
reason for building considerable power output into amplifiers is the
fact that there are many instantaneous peaks of sound in music which
are far above the average level; lack of power capacity in the amplifier
will result in distortion on these peaks. Still another reason is the
need for reserve power when a room full of guests creates a high background
noise level - or when the hi-fi fan insists on turning up the volume
for better reproduction of the extreme lows and highs. The need for
reserve power becomes more apparent when it is realized that doubling
the audio output from the amplifier will achieve only a moderate increase
in loudness. While most amplifiers will deliver more than their rated
output, the distortion percentage will generally rise above the manufacturer's
stated value, which is the percent of harmonic distortion at rated output.
A really high-fidelity amplifier will deliver its rated output with
1 % distortion or less.
Figure 1 shows a simple setup for measuring
amplifier output power. A sine-wave audio oscillator furnishes a steady
signal at a particular frequency, perhaps 1000 cycles, to the amplifier
input. Amplifier output is loaded by an appropriate resistor in place
of the speaker (see speaker impedance rating). A vacuum-tube voltmeter
- and an oscilloscope if available - is in parallel with the resistor.
Amplifier volume control is set at the point at which the output is
to be measured and the oscillator control is adjusted to obtain the
amplifier input voltage recommended by the manufacturer. Output of the
amplifier in watts at this 1000-cycle frequency will be the square of
the voltmeter reading divided by the resistance of the resistor. The
purpose of the oscilloscope is to observe the output waveform for distortion.
When the waveform begins to depart from a smooth sine wave, the amplifier
output contains distortion which, as mentioned earlier, must be considered
along with the power output.
The method of measuring audio power
just described is well within the capability of the audio hobbyist who
has an audio oscillator and a vacuum-tube voltmeter sensitive enough
to read the minute output of the oscillator as well as the amplifier
output voltage. For the hobbyist who desires to make his own wattmeter,
a simplified design using a crystal element is shown in the Sylvania
Electric Products Inc. booklet called "Forty Uses for Germanium Diodes."
The Heath unit shown in the photograph is sold as a kit for home construction.