WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

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20 W Stereo Integrated Audio Amplifier
While not fully “transparent” this level of performance is adequate for measurement of the
preamplifier circuits. Measurement of the preamplifier and power amplifier circuits separately
would normally only be done as a troubleshooting step.
Measurements are detailed in the following sections for two input positions:
• Phonograph
• Line Input
Note that the performance of the Line input and Front Aux input should be essentially
identical, with the exception of sensitivity. The buffer stage consisting of Q111 and Q112 provide
additional gain for the variety of personal entertainment devices available. Builders may wish to
run spot checks on the Front Aux input. A full set of tests, however, is typically unnecessary.
7.1.1 Tone Control Measurements
Set the tone-control potentiometers for their center (flat) response. The flat setting points for the
Bass and Treble controls are best found by using a sweep generator while monitoring the output
of the preamplifier with an ac voltmeter. If a sweep generator is not available, use an audio
generator set to the frequencies specified below as start and stop points; then vary the operating
frequency between those points.
Small variations from the optimal flat response are not unusual between channels. Any
deviation of more than about 1 dB may be cause for further investigation. The following
procedure is recommended.
1) Connect a sweep generator to the right channel Line input. Switch the Input to Line. Connect
10 W or greater 8
Ω
loads to the right and left channel speaker outputs. Connect an audio
voltmeter to the right channel speaker terminals.
2) Set the sweep generator to produce a 1 kHz signal at 1 V rms. Adjust the front panel Volume
control to produce 2.8 V rms at the right channel speaker terminals. This is the 1 W output
reference level. Adjust the audio voltmeter to read in decibels and adjust so that 0 dB is
displayed on the meter.
3) Set the generator to begin the sweep at about 20 Hz and end the sweep at about 1 kHz. Begin
a repetitive linear sweep of about 1 second duration.
4) Slowly adjust the Bass control for the flattest response, which should be in the range of +/–1
dB.