WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

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7 Performance Measurement
It is tempting, and quite acceptable, to put the 20 W Stereo Integrated amplifier into service upon
completion of the previous 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) and with various input sources.
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.
7.1 Transistorized Preamplifier Performance Measurement
The performance of any integrated amplifier is a combination of the capabilities of the
preamplifier and power amplifier stages. It is certainly possible to measure the preamplifier and
power amplifier stages separately; however, this configuration would never be used in actual
operation of the system. Therefore, in measurements of the preamplifier we make the assumption
that the power amplifier, when operating a 1 W output, is transparent with regard to performance.
The typical measured parameters of the 10 W amplifier block at 1 W are given below:
• Frequency response = ±1 dB, 15 Hz to 65 kHz
• THD = 0.35% (30 Hz to 20 kHz)
• IMD = 0.4% (SMPTE)
• Noise unweighted, (shorted input terminals) = –75 dB