- 6 -
- 21 -
4.3.2 Frequency Selected Circuit
The signal amplified by the N417B is coupled by C508 and then sent to the 7-band
filter composed of the operational amplifiers. Setting its feedback capacitor's capacity
can define the corresponding frequency amplitude. The frequency value marked on
their output points is the frequency band's central frequency. There's a half-wave
rectifying circuit connected to the every band filter's output end. The circuit will rectify
the amplified AC signals into the DC voltage. The circuit is mainly performing the
frequency sampling function which can show the amplitude of every frequency of the
whole audio signals through the DC voltage. If the low frequency of the current audio
signals is strong, the DC voltage of the 35Hz and 100Hz band filter is higher. If the
high frequency is strong, the DC voltage of the 10K and 16K band filter is higher. The
output end of these seven band filters is connected to seven input terminals of the
electronic switch N419 (CD4051). CPU will send commands to make this electronic
switch select the frequencies circularly (Refer to the above real value table). The
output terminal of the pin 3 of N419 will send out a series of voltage representing the
corresponding frequency's amplitude.(See Figure 3)
35HZ
100HZ
300HZ
1KHZ
3KHZ
10KHZ
16KHZ
35HZ
T
V
Switch interval
Frequency selected circularly period
Note:the diagram's
voltage amplitude
is not definite
(Figure 3)
SCHEMATIC DIAGRAM OF THE MAIN AMP BOARD
O
U
T
4
1
O
U
T
3
2
O
U
T
2
3
O
U
T
1
4
A
V
D
D
5
S
W
IN
6
G
N
D
S
7
S
R
IN
8
S
L
IN
9
G
N
D
C
1
0
C
IN
1
1
G
N
D
R
1
2
R
IN
1
3
G
N
D
L
1
4
L
IN
1
5
B
Y
P
A
S
R
1
6
B
Y
P
A
S
L
1
7
L
T
R
E
1
8
L
B
A
S
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3
1
9
L
B
A
S
S
2
2
0
L
B
A
S
S
1
2
1
R
B
A
S
S
1
2
2
R
B
A
S
S
2
2
3
R
B
A
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3
2
4
R
T
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E
2
5
C
R
2
2
6
C
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1
2
7
C
L
2
2
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C
L
1
2
9
A
V
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3
0
L
O
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T
3
1
R
O
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3
2
C
O
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3
3
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L
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3
4
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R
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3
5
S
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3
6
A
G
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3
7
D
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3
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3
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4
0
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4
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6
2
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94
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7
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7
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7
4
4
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4
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1
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4
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9
3
3
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4
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+
6
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3
6
1
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4
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C
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5
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4
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3
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4
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4
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V
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To
VF
DB
oa
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5
5
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k
