WhitakerAudio 60 W Stereo Power Amplifier, Руководство пользователя и по сборке

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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 slightly higher (more negative) level is relatively
small. The increase in distortion is typically modest, 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.
7.7 Drive Balance Adjustment
Potentiometer R20 is provided in order to make it possible to precisely balance the drive voltage
to the two output tubes. If the currents through the push-pull output tubes are not well matched,
increased distortion will result. The default setting for R20 is described in Section 6. Builders may
wish to optimize the setting of R20 for each channel in order to gain the best performance
possible from the amplifier.
There are two ways of adjusting drive voltage balance—one requires an oscilloscope and the
other requires a distortion analyzer. Use the following procedure:
• Connect an 8
Ω
30 W non-inductive load to each channel. Connect appropriate test
equipment to the right channel speaker output (at minimum, an audio voltmeter and a
distortion analyzer or oscilloscope).
• 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.
• On an oscilloscope, observe the maximum and minimum points of the waveshape.
Increase the input signal until clipping is visible on either the positive or negative peak.
Ideally, clipping should occur at the same point on both peaks. If not, adjust the Right
Channel Balance control so that clipping is symmetrical.
• When using a distortion analyzer, apply an input signal of 1 kHz at 0.9 V rms. This will
produce about 30 W output. Measure THD and adjust the Right Channel Balance control