SIGNAL GAIN CONTROL CIRCUIT
Signal Gain Control circuit consists of T R109, 111, 113 and 115 (L ch) and TR 110, 112, 114 and
116(Rch).
This circuit configuration is identical to the Frequency Equalization circuit. For operating, see
"Frequency Equalization description" section.
The input signal is fed into TR 109 base.
TR111, TR113 and TRl15 are connected so that the voltage at TR105 base always follows the
voltage at T R111 base.
Total gain can vary from negative feedback value which consists of R135, VR101 and R143 (L ch)
[R136, VR102 and R144 (R ch)].
When base voltage of T R111 is equal to base voltage of TR 113, the gain is unity.
COMPARATOR CIRCUIT FOR SIGNAL GAIN CONTROL'INDICATOR(LED}
Comparator circuit consists of three IC's NJM 4558DM.
Comparator operates by comparing the INPUT signal to output of TR 115. These signals are
supplied to Pin 6 and Pin 2 of IC201 (Lch) [or IC202 (Rch)] . Both input levels are doubled to
output terminals (Pin 6 to Pin 7 and Pin 2 to Pin 1).
Output of Pin 7 is positive, half-wave rectified by D203 and filtered into an average plus DC voltage
by C207 and R221, and this voltage is applied to
(+)
input at pin 3 of IC203.
In the same manner, the output of pin 1 is positive, half-wave rectified by D201 and filtered into
an average plus DC voltage by C205 and R233, and this voltage is applied to the (-) input at pin 2
of IC203.
Thus, the difference level between Pin 2 to Pin 3 appears at Pin 1 of IC203. This voltage at Pin 1
serves as input to the Signal Gain Control indicator LED.
For example, when the input level at Pin 2 is small compared with Pin 3, the output at Pin 1
appears as positive voltage (approx. 13.6 V DC).
Thus, LED D1003 will light but D1002 will not.
When the input level at Pin 2 is greater compared with Pin 3, the output at Pin 1 appears as a
negative voltage (approx. -12.3 V DC).
Thus, LED D1002 will light but D1003 will not.
When the input level at Pin 2 is equal to Pin 3 input level, the output will appear as zero, and thus
both LED's D1002 and D1003 will light. In this case, the gain is unity. (Figure D)
OSCILLOSCOPE
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INPUT OUTPUT
AUDIO OSC. FREQUENCY AC V.TV.M.
(20. 20KHz) EQUALIZER
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01002 01003
Figure D
-8-
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