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Circuit Descriptions, List of Abbreviations, and IC Data Sheets
The Audio Amplifier is located on the SSM. The output power
for the Audio Amplifier is 10 Watts per channel for the Basic
and Basic Plus versions. The Core Version has an output of 15
Watts per channel. IC 7700 is the Audio output IC. This is a
class D amplifier. Left and Right Audio from the SSB is fed to
Pins 10 and 18 of the IC. The output stages are basically switch
mode circuits, driven by the audio input signal. The frequency
of the output is keep at 200 kHz by an internal 200 kHz
oscillator. The pulse width of the output is determined by the
signal level of the input signal. The Right Channel pulse width
signal is output on pin 3. The output is filtered by 5701, 2777,
2776, 2717, 5716, and 2737. The amplified audio is output on
connection 1349. In the same manner, Left audio is output on
pin 23. The IC is powered by +19 and -19 Volt supplies. The
supplies are protected by fuses 1700 and 1701. The Supply
voltage for the Core version is 4 volts higher than the Basic and
Basic plus version. The extra supply voltage is needed to
produce the additional 5 Watts per channel output.
Muting and volume control for the audio takes place in the
Audio Signal processor on the SSB. The STBY MUTE line on
pin 6 goes to 2.5 Volts to mute the audio when the set is turned
"on" or "off". This line goes "low" to place the amplifier in a
Standby mode if a DC voltage is detected on the Audio Output
lines.
9.4.4
Shutdown Mute
Figure 9-42 Shutdown Mute
The Audio Amplifier will be placed in a Standby mode and the
set will shut down if a DC voltage is detected on the Speaker
Output lines. The Left and Right audio output lines are
connected to the base of 7704 and the emitter of 7705 via
resistors 3718 and 3717. Filter capacitor 2778 filters out the AC
component of the waveform. If the DC voltage goes positive,
7704 will turn "on". If the voltage goes negative, 7705 will turn
"on". This turns 7706 "on" which turns 7707 "on". This causes
the Front Detect line to go Low, preventing the microprocessor
from receiving any commands. It also turns SCR 6701 "on",
turning transistor 7708 "on", causing the Standby line to go
"high". If this SCR is turned "on", it will be necessary to remove
power from the set to reset the circuit. This will turn the set "off".
Transistor 7707 also turns "on", causing the Standby-Mute line
to go Low. This places the Audio Output IC in a Standby mode.
If the Power Fail line should go "high", 7710 will turn "on",
causing the Standby-Mute line to go Low.
9.4.5
Centre Channel Amp Switch
The Core versions have a Centre Channel amp switch, located
on the rear of the set. The Output of the Audio Amplifier on the
Basic and Basic Plus versions are fed directly to the speakers.
In the Core sets, the Output is fed to the Centre Channel amp
switch before being fed to the speakers. When the Centre
Channel amp switch is placed in the External position, the Left
and Right speakers in the set are placed in series with the
external speaker input. In cases where the Customer has a
separate sound system, this switch allows the speakers in the
set to be used as the centre output.
9.4.6
Headphone Amplifier
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IC 7403 is the Amplifier for the Headphone output. The volume
control for the Headphone output is on the Audio Signal
processor located on the SSB. The Headphone Amplifier is
located on the SSM. The output of the amplifier on Pins 1 and
7 is sent to the Headphone Jack, located on the Side Jack
panel.
DPTV465
Figure 9-43 Centre Channel Amp Switch
Figure 9-44 Headphone Amplifier
9.
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