Read Sharp 28JW-74H (serv.man2) Service Manual online
50Hz Widescreen Colour Television – GA20 Chassis
Sharp Electronics (UK) Ltd
Technical Support
Issued June 02
Page 12
Audio Output
TDA7480 is a 10W class ‘D’ audio amplifier; it operates on a dual supply or +/- 16v d.c. which is
derived from the switch mode power supply
Pin 12 has three levels, <1.8v d.c. The device is in standby mode (No output)
>1.8v d.c. and <3.8v .d.c The output is attenuated by 80dbs (This feature is
not used)
>3.8v d.c. The device is operating normally
not used)
>3.8v d.c. The device is operating normally
At switch on pin 12 is held low by the mute circuit, preventing any audio output until supply has been
established. Once supply has been established and the operating system is ready for audio to be
produced pin 12 will rise to < 3.8v d.c. The internal oscillator (external timing components at pin 9) will
start, producing a PWM signal at pin 4. This PWM signal has a frequency which is equal to the
internal oscillator plus the frequency of the input signal i.e. if the oscillator frequency = 200,000 kHz
and the input signal = 5,000 kHz, the PWM frequency = 205,000 kHz. The mark space ratio of this
PWM represents the amplitude of the signal at pin 11
R1302
12k
C1301
100pF
100pF
C1306
4700pF
IC1301
TDA7480
+v
cc
cc
-v
cc
-v
cc
-v
cc
-v
cc
-v
cc
-v
cc
ou
t
bo
ot
Os
c
c
Fr
eq
in
St
an
db
y/M
ute
-v
cc
11
20
12
13
14
16 15
17
18
19
1
10
9
8
7
6
5
4
3
2
C1304
0.1uF
+16v
-16v
Power On Mute
+v
cc
cc
Audio Input
C1313
0.47uF
Gn
d
d
C1305
560pF
560pF
C1307
0.47uF
R1306
150R
R1302
100k
L1301
Protection
Loudspeaker
C1313
0.47uF
R1307
27k
R1316
27K
Q1305
R1317
22K
Left Channel
IC1302 pin12
LED 5v
Right Channel
LOW
PASS
FILTER
L1301 and C1307 (Right Channel) L1306 and C1326 (Left Channel) forms a low pass filter for
conversion of the PWM signal into an analogue signal to drive the loudspeaker. A proportion of this
drive signal is fed into an averaging circuit via R1302 (Right Channel) and R1313 (Left Channel) as a
protection signal
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