Precision Half-Wave Rectifier And Amplifier Circuit; Comparator Circuit For Led Level Meter Description - ADC Sound Shaper Three Service Manual

PRECISION HALF-WAVE RECTIFIER AND AMPLIFIER CIRCUIT (Refer to Figure C)
The basic fau It with diode rectifier circu its is that the diodes do not conduct until a specific voltage
is reached. The above circuit eliminates this problem and also amplifies the output.
Referring to the basic circuit, note that: (1) With no input signal 0801 is conducting slightly to
establish 0 V DC at
A (-)
input and 0 V DC at
A
out; (2) When 0801 is barely conducting a small
amount of feedback exists. Therefore, the gain of the amplifier is very high. Less than one millivolt
will cause the output to change by a volt or more. Operation is as follows:
A
negative input causes
a positive-going change in the amplifier output. Since the gain is high until 0801 conducts fully, the
amplifier output jumps to 0.2 Volts long before the input reaches a millivolt (in a very short time).
At this point 0801 is fully conducting, has a low impedance compared to RF (n), and exhibits a
0.2 V drop across it. The rectifier output is now -85 mV DC. Since the feedback loop (RF) has
been completed by 0801 any further decrease in input voltage is amplified by the ratio of RR'.(n)
which is approximately equal to 5 for this circuit.
In
A positive input causes the output to decrease in a manner similar to just described except the
feedback is through 0803 and RF (p). The amplifier output is blocked from appearing at the
rectifier output by 0801 (0801 is reverse biased with respect to the following stages which are
returned to the 0 V DC line).
In this manner, appearing at 0801 cathode are negative half-wave pulses whose amplitude is directly
proportional to the input signal level.
The following refers to the complete schematic:
The negative pulses at 0801 cathode are filtered into an average DC voltage by R809 and C807, and
this voltage serves as input to the LED meter comparator.
R803 and 0803 serve to maintain positive feedback around RC4709 during the positive half-wave
excursions of the input signal. As the amplifier's output is positive during this time, 0803 is forward
biased (0201 is off) and the feedback path now consists of R803 and 0803. The amplifier is thus
kept out of saturation and free from oscillations throughout the full input cycle.
Bias circuit uses split power supply which provides +15.4 V DC at Pin No. 14 and -15 V DC at
Pin NO.7 on IC801.
r-_R_F-"/V( N"y-e_g_.)
----.--0
RECT I FIE R
OUTPUT
0801
R F (Pos.)
0803
(AMP OUTPUT)
BASIC OP. AMP. RECTIFIER-AMPLIFIER CIRCUIT
Figure C
COMPARATOR CIRCUIT FOR LED LEVEL METER DESCRIPTION
Comparator circuit operation
Comparator circuit consts of 13 IC's MJM-4588DM.
Comparator operates by comparing the (-) input level to (+) input level, in which output voltage is
changed from minus to plus Voltage. Thus DC currents flows to each LED.
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