WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

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60 W Stereo Power Amplifier
• With no input applied, adjust the bias supply for approximately –40 V (if not already set at
this value).
• Connect an audio signal generator to the right channel input. Set the right channel 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). An input voltage
of 0.90 V rms is typical for this amplifier. This assumes a line input voltage of 120 V ac.
For lower line voltages, the clipping point will occur at a lower output voltage.
• With an input signal of 1 kHz, measure THD and adjust the bias control for minimum
distortion reading. Power output should not change significantly within the bias range of –
35V to –45 V dc. Do not adjust the bias too far from the default –40 V value. Monitor the
bias voltage at the test points adjacent to the bias set control.
• Change the input signal to 30 Hz (0.9 V rms) and measure THD. Adjust the bias control
again for minimum distortion. The optimal setting should roughly coincide for each
frequency. In the event that the optimal settings for 30 Hz and 1 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 at 1 kHz is usually found to be at a
bias of about –40 V. At 30 Hz the lowest distortion point is usually found with a slightly lower
(less negative) setting; e.g., –38 V. While varying the bias, it may be observed that distortion on
either side of the optimal low frequency 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 –37 V is found to be the optimal setting, increase bias to –40 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 1650HA) is rated for
operation down to 30 Hz. The characteristics at 20 Hz are not documented and as such it is not