WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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• Connect an audio signal generator to the right channel input. Adjust the Volume control
fully clockwise. Set the signal generator to 1 kHz and adjust the input signal to provide
peak output from the amplifier (just below observable clipping); this is approximately 1.1
V rms.
• With an input signal of 20 kHz, measure THD and adjust the bias control for minimum
distortion reading. Power output should not change significantly within the bias range of –
25 V to –35 V dc.
• Change the input signal to 30 Hz and measure THD. Adjust the bias control again for
minimum distortion. The optimal setting should roughly coincide for each frequency
extreme. In the event that the optimal settings for 30 Hz and 20 kHz do not coincide, split
the difference between the two settings.
• Repeat the process for the left channel. The optimal points between channels should
roughly coincide. Note that some compromises may be necessary to provide for the best
relative performance from the right and left channels of the amplifier.
For a typical implementation, the lowest distortion point is usually found to be at a bias of
about –25 V dc. While varying the bias, it may be observed that distortion on either side of this
point increases, albeit only slightly. Be careful when adjusting bias since lower settings (less
negative) will increase the idling current. For test purposes, setting the bias for best performance
is reasonable. For long-term operation, however, it is advisable to find the ideal operating point
and then back off somewhat; e.g., if –25 V is found to be the optimal setting, increase bias to –30
V for long-term operation. Experience and performance should be the guide here. Keep in mind
that the bias may need to be adjusted over time as the tubes age. When replacing tubes, it may be
worthwhile to repeat the bias setting adjustment.
Note that the output transformer used in this circuit (Hammond 1620) is rated for operation
down to 30 Hz. The characteristics at 20 Hz are not documented and as such it is not advisable to
force the amplifier (through low bias) to make the target distortion numbers at 20 Hz. For this
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 7.2. It can be seen that the increase in distortion is