WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

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times 0.815). The speaker terminal voltages for the IMD measurements at various power levels
are shown in Table 7.1.
7.2 Hum Balance Adjustment
Setting of the Hum Balance control can be accomplished as follows:
• Install the chassis bottom plate using the #6 self-tapping screws provided.
• Connect an 8 Ω 30 W load to the output terminals of each channel. Connect an audio
frequency voltmeter to the output of the right channel.
• Set the right channel Volume control fully clockwise.
• Apply a 1 kHz signal at 0.9 V rms to the right channel input.
• Set the audio voltmeter to the dB range and adjust the meter for 0 dB.
• Remove the RCA input cable from the amplifier and insert a shorting plug.
• Increase the audio voltmeter sensitivity to the point that observable deflection is displayed
at the mid point of travel. (While this adjustment can be done with a digital display, an
analog meter typically provides for easier reading.)
• Carefully adjust the Hum Balance control for minimum signal level. A typical value is
about – 90 dB.
• Repeat these steps for the left channel.
Adjustment of the Hum Balance control may or may not have a significant impact on the
noise floor. Also, the optimal adjustment point may be different from one channel to the next. In
this event, split the difference. The most likely cause of an amplifier not being able to meet the
noise target is a noisy 6U8A input tube. If one channel can meet the target but the other cannot,
try swapping the input tubes. If the problem moves with the tube, then the device is defective and
should be replaced.
7.3 Clipping Point
The clipping point of the amplifier may be checked as follows:
• Connect an 8 Ω 35 W load to the output terminals. Connect an oscilloscope and audio
frequency voltmeter to the output.
• Set the right channel Volume control fully clockwise.