Read Sharp 66DW-18H (serv.man3) Service Manual online
Sharp Electronics (UK) Ltd
Technical Support
February 99
Page 30
The principles of all the audio output stages, including the sub-woofer are the same. And the principle
of operation is the same, as the vertical output stage except the frequency of the PWM single will vary
depending on the audio being inputted to the circuit.
of operation is the same, as the vertical output stage except the frequency of the PWM single will vary
depending on the audio being inputted to the circuit.
The base frequency of the PWM is much higher than that which is used for the field output stage due
to harmonics that could be heard by pets. And it will increase by as much as 20kHz (audio bandwidth)
to harmonics that could be heard by pets. And it will increase by as much as 20kHz (audio bandwidth)
Figure 1 Centre Channel Switching Amplifier
The oscillator used to produce this changing frequency is a 'triangle wave oscillator', and the key
components are R352 & C330.
components are R352 & C330.
The time that it takes C330 to charge will determine the frequency of the oscillator. To enable this
oscillator to change frequency; the charging voltage is derived from the input to the low pass filter
(L302) shown in Figure 2, fed back via R329. R352 will determine the time the C330 takes to
discharge.
oscillator to change frequency; the charging voltage is derived from the input to the low pass filter
(L302) shown in Figure 2, fed back via R329. R352 will determine the time the C330 takes to
discharge.
+
-
Audio I/P
Q302
Q301
R320
R354
C342
R350
R321
R325
R322
R326
R334
R324
D333
C4V7
C4V7
C335
D322
C4V7
C4V7
D323
C4V7
C4V7
C358
C347
R349
D335
D334
R328
R329
C330
R352
C383
IC303
+19V
-19V
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