WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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20 W Stereo Audio Amplifier
The performance targets listed in Table 7.1 are just that—targets. Any given amplifier may
perform better than the target or worse than the target. It is not uncommon for one channel to
perform slightly better than the other with regard to THD and/or IMD. The most likely
explanation is differences in tube performance, although other items may factor in as well. In any
event, the amplifier should be able to come close to the numbers listed in most cases without the
need to tweak the bias operating point beyond the default value of –30 V.
With no audio input applied, the amplifier draws about 800 mA from the ac line at 120 V ac.
With typical program material applied to both channels, peak current of about 1 A is typical.
7.1.1 IMD Measurement
When measuring intermodulation distortion (IMD) it is important to remember that the two-tone
signal used alters the generally accepted relationship between the rms, average, and peak values,
which only hold true for single sine waves. In the case of a single-frequency signal, the meter will
indicate the rms value, while responding to the average value of the signal. In the case of the
complex waveform of a two-tone signal, the meter will respond to the average value of that
signal, but the altered relationship between the average and rms values will cause a discrepancy in
the indication. Such discrepancies are generally not large; however, the difference in peak value
of single and two-tone signals that give equal meter indications may be substantial. A correction
factor, therefore, is often applied; e.g., 81.5%.
For example, since (where P = power in Watts and R = load resistance in Ohms),
for an IMD measurement at 10 W power output, the input signal would be adjusted to yield a
measured value at the speaker terminals on an rms-reading audio voltmeter of 7.2 V rms (8.9 V
times 0.815). The speaker terminal voltages for the IMD measurements at various power levels
are shown in Table 7.1.
7.1.2 Hum Balance Adjustment
Setting of the Hum Balance control can be accomplished as follows:
• Install the chassis bottom plate using the #6 self-tapping screws provided.
• Connect an 8 Ω 10 W load to the output terminals of each channel. Connect an audio
frequency voltmeter to the output of the right channel.
• Set the right channel Volume control fully clockwise.
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