WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual, Page: 155 / 194

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20 W Stereo Audio Amplifier
7 Performance Measurement
For performance measurements, the builder has three basic options:
• Run no further tests on the amplifier under the assumption that if built correctly it will
perform correctly. This may be a valid approach for an assembly line product where the
assembly process is well refined, but in the case of a custom-built audio device it is not
recommended.
• Perform tests at the maximum rated output of each channel (10 W). This is a reasonable
approach and should yield valid results under the assumption that if the amplifier can meet
specifications at high power, then it can very likely meet specifications at other operating
points.
• Perform comprehensive tests across a range of operating levels. This is the approach
detailed in the following section.
It is tempting, and quite acceptable, to put the 20 W Stereo Amplifier into service upon
completion of the previous assembly steps. However, if the test equipment is available, it is
recommended that measurements be taken at various power output levels (e.g., 1 W, 5 W, and 10
W, the rated output power of each channel).
It is important to note that each channel of the amplifier is capable of producing more than 10
W output into an 8 Ω load. Because the performance of any amplifier, when measured at or near
its operating limit, is not always repeatable over time, this amplifier is described as being capable
of producing “10 W per channel.” In point of fact, it is capable of producing more power than that
at low distortion levels, but it is difficult to guarantee such performance over time as the tubes
age.
Users may hear shortly after the amplifier is powered on a slight buzzing noise from the
chassis (not from the speakers) for a few seconds. This is normal. As the amplifier warms up, the
surge-limiting power thermistor bypass circuit of VR52 and C64 ramps up the voltage to relay
RYL51. During a brief portion of this ramp, the relay may vibrate slightly; however, within a few
seconds the relay will close. The speed of this action is a function of the ambient temperature.
Check the Speaker On/Off function as follows. Connect speakers to the amplifier. Feed a tone
or program material into both channels. Operate the Speaker On/Off switch and confirm proper
operation.