WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual, Page: 158 / 194

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• Apply a 1 kHz signal at 1.0 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 – 85 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.
It is possible that lowest noise is found at opposite ends of the Hum Balance control for the
right and left channels. If this is the case, remove power and switch the filament connections to
the right channel PWB (switch the wires to 6VH1 and 6VH2).
7.1.3 Clipping Point
The clipping point of the amplifier may be checked as follows:
• Connect an 8 Ω 15 W load to the output terminals. Connect an oscilloscope and audio
frequency voltmeter to the output.
• Set the Volume control fully clockwise.
• Apply a 1 kHz signal to the Line input while observing the oscilloscope. Find the point at
which the top of the trace begins to flatten. This is the clipping point. Check the
oscilloscope trace to confirm that clipping is symmetrical; i.e., the same amount of
clipping is observed on the positive and negative peaks. For this amplifier, the clipping
point should be reached at an input level of approximately 1.2 V rms.
• Back off the audio input level slightly so that clipping just disappears. For this amplifier,
this point is typically 1.1 V rms.