WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

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reason, distortion measurement is specified at 30 Hz on the low end. This issue is more critical at
high power levels than at low levels.
With this circuit as configured, the difference in distortion performance with bias set at the
optimal level and distortion with bias set to a higher (more negative) level is relatively small, as
documented in the example given in Table 7.3. It can be seen that the increase in distortion is
typically modest at normal listening levels, and generally an acceptable trade-off for reduced
power consumption and longer tube life. Excessive idling current can result in short operational
life of the output tubes and/or tube failure.
The difference in idling current with a various bias settings can be substantial, as documented
below:
• At a bias of –30 V, the no-signal B+ idling current = 60 mA per channel
• At a bias of –25 V, the no-signal B+ idling current = 105 mA per channel
Important note : The amplifier should never be operated with a bias setting of less than (more
positive than) –20 V. Component damage may result.
It is important to remember that high distortion readings on a given channel can be caused by
a variety of issues in addition to the operating bias point. If high distortion figures are
encountered, first try replacing the output tubes and see if that solves the problem. Next, try
replacing the input pentode. Check also for any signs of abnormal component conditions on the
circuit board, or overloading at the speaker terminals. Adjust the bias point as a last resort.
Table 7.3 Example of Measured Distortion Performance of the Power Amplifier
Block With Different Bias Points
Conditions Parameter Measurement with Optimal
Bias (–25 V)
Measurement with Higher
Bias (–30 V)
Measurements at 1 W power
output
THD
(note 1)
0.35% 0.5%
IMD 0.23% 0.35%
Measurements at 5 W power
output
THD 0.4% 0.5%
IMD 0.28% 0.9%
Measurements at 10 W
power output
THD 0.7% 0.9%
IMD 1.0% 1.75%
Notes:
1 THD measured at 30 Hz, 100 Hz, 1 kHz, 10 kHz, and 20 kHz. The worst (highest) reading is the one
reported.