August 1955 Popular Electronics
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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.
What is an Audio Watt?
By Eugene F. Coriell, Major, USAFA question that
puzzles many hi-fi enthusiasts is answered
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
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.
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.
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).
Another 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.
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