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Appendix
Service manual WP-795
2-6 Sound amplification
The TDA8944J (TDA8946J) is a stereo BTL audio amplifier capable of delivering 2 x 7 W (2 x 15 W)
output power to an 8
load at THD = 10%, using a 12 V power supply and an external heatsink. The
voltage gain is fixed at 32dB.
With the three-level MODE input the device can be switched from 'standby' to 'mute' and to 'operating'
mode.
The TDA 8944J outputs are protected by an internal thermal shutdown protection mechanism and short-
circuit protection.
Power amplifier
The power amplifier is a Bridge Tied Load (BTL) amplifier with an all-NPN output stage, capable of
delivering a peak output current of 1.5 A.
The BTL principle offers the following advantages :
- Lower peak value of the supply current.
- The ripple frequency on the supply voltage is twice the signal frequency.
- No DC-blocking capacitor
- Good low frequency performance
Mode selection
The TDA894xJ has several functional modes, which can be selected by applying the proper DC voltage to
pin MODE.
Mute : In this mode the amplifier is DC biased but not operational (no audio output). This allows the input
coupling capacitors to be charged to avoid pop-noise. The devices is in mute mode when 2.5 V < V
<
MODE
(Vcc-1.5 V).
Operating : In this mode the amplifier is operating normally. The operating mode is activated at V
<
MODE
0.5 V.
2-7 Vertical deflection
The vertical driver circuit is a bridge configuration. The deflection coil is connected between the output
amplifiers, which are driven in phase opposition. The differential input circuit is voltage driven. The input
circuit is especially intended for direct connection to driver circuits which deliver symmetrical current
signals, but is also suitable for asymmetrical currents. The output current of these devices is converted to
voltages at the input pins via resistors R350 and R351. The differential input voltage is compared with the
output current through the deflection coils measured as voltage across R302, which provides internal
feedback information. The voltage across R302 is proportional to the output current.
Flyback voltage
The flyback voltage is determined by an additional supply voltage V
. The principle of operation with
flb
two supply voltages (class G) makes it possible to fix the supply voltage Vp optimum for the scan voltage
and the second supply voltage V
optimum for the flyback voltage. Using this method, very high
flb
efficiency is achieved. The supply voltage V
is almost totally available as flyback voltage across the coil,
flb
this being possible due to the absence of a coupling capacitor.
Protection
The output circuit has protection circuits for :
- Too high die temperature
- overvoltage of output stage A
Guard circuit
- 31 -
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