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19
Low-Pass Filter
Matching
The low-pass filter for the left program channel is not adjustable;
rather, it employs close-tolerance components to meet design
parameters. Since close frequency and phase response matching
between stereo channels is essential to acceptable Stereo Generator
crosstalk performance, trim controls have been included in the right
channel filter so that it may be tuned to match left channel response.
1. Set the
AUDIO PROCESSING
switch to the
OUT
position.
Frequency and phase response matching is optimized with all
processing bypassed.
2. Clip the two ‘scope probes to the output of the filter buffer
amplifier, IC20. The most convenient monitor points are the right-
hand ends of R22 and R27, for the left and right channels,
respectively. The right-hand ends of the resistors are the ends
closest to the
COMPOSITE OUTPUT
connector.
3. Apply a 10kHz sinewave test signal to both the
LEFT
and the
RIGHT
PROGRAM LINE INPUT
. This is most simply done with clip-leads if
the barrier screw-strip is first unplugged from the chassis. Connect
the “hot” side of the oscillator output to
both
+
terminals, the
grounded side of the Audio Generator output to either
G
. The
–
input terminals may be left unconnected for test purposes.
4. Set both ‘scope attenuators at 2V/div. Position the left channel
signal (from R22) above the right.
5. Adjust the Audio Generator amplitude for a 6-volt (3 divisions)
peak-to-peak signal on both ‘scope traces. If necessary, trim either
the
L
or
R
front-panel
INPUT GAIN
control so that both traces are
equal in amplitude.
6. Increase the Audio Generator frequency to the first filter null,
approximately 19.1kHz. Observing the upper (left channel) trace,
down-range both oscilloscope vertical attenuators and fine-tune the
Audio Generator frequency for the deepest null in the left channel
signal.
HINT:
When properly nulled, the signal waveform will be very close to
the noise floor. If you find the nulled signal difficult to resolve, try
triggering the ‘scope timebase directly from the Audio Generator. This
will make the waveform “hold still” and stand out from the (random)
noise.
7. When the Audio Generator has been fine-tuned to null the left
channel, adjust R112 (between IC26 and IC27) for a null in the
right channel filter. Both channels should null to the same relative
amplitude.
8. Increase the Audio Generator frequency to the second filter null,
approximately 21.8kHz. Again, fine-tune the Audio Generator
frequency for the deepest null in the left channel signal.
