WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

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60 W Stereo Power Amplifier
• Apply a 1 kHz signal to the right channel input while observing the oscilloscope. Find the
point at which the top of the trace begins to flatten. This is the clipping point. Check the
oscilloscope trace to confirm that clipping is symmetrical; i.e., the same amount of
clipping is observed on the positive and negative peaks. For this amplifier, the clipping
point should be reached at an input level of approximately 0.95 V rms.
• Read the value displayed on the ac voltmeter. Determine the power as follows: P = E
2
/ R ,
where E = the voltage measured by the audio voltmeter and R = 8 (the load resistance).
For this amplifier, with 120 V ac line voltage input, the voltage measured at the clipping
point is typically about 16 V, which translates to 32 W. This is the maximum operating
point of the amplifier.
• Back off the audio input level slightly so that clipping just disappears. For this amplifier,
this point is typically 0.92 V rms. With 0.92 V rms input and an ac line voltage of 120 V,
an output of approximately 15.5 V rms should be observed at the output speaker terminals,
which translates to 30.0 W output. In order to keep a sufficient margin below clipping, the
measurements specified in Table 7.1 call for an input voltage of 0.9 V rms.
Repeat the steps above for the left channel. The results should closely match the performance
of the right channel.
7.4 Crosstalk
Crosstalk is a parameter that identifies the amount of signal applied to one channel that may
appear in the other channel. This parameter is not specified in Table 7.1 since nearly all stereo
material has a high correlation between channels. In any event, it may be measured as follows:
• Connect a suitable resistive load to each channel. Set both Volume controls fully
clockwise.
• Apply a 1 kHz signal at 0.9 V rms to the right channel input. (This will result in
approximately 30 W output.) Connect an audio voltmeter to the right channel output.
Adjust the meter to read 0 dB on a suitable scale.
• Move the 1 kHz signal to the left channel input. Place a shorting plug in the right channel
input jack.