BBK MA-950S, MA-850S Service Manual
SERVICE MANUAL
MA-950S/850S
2004-10-03
MA-950S/850S Principles and Maintenance
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1. Product profile
MA950S is a medium-grade active sound box consisting of five
satellite sound boxes, i.e. one subwoofer sound box, two pre-positioned
sound boxes, two surround sound boxes and one centrally-positioned
sound box. The product features good sound effect in appropriate space,
and it has the following features:
1) 2.1/5.1 sound channel output;
2) Tone and volume adjustment
3) Built-in five sound channel power amplification, and adaptable to
AC-3/DTS and stereo music replay;
4) Six sound channel volume control and independent level control, with
tone adjustment function.
2. Principle block diagram
signal
input
Electronic
volume an
tone
adjustment
Power
amplific
ation
Output
CPU system control
Remote Receiving an
display part
3. System composition
The device comprises input board, output board, power amplification
board and control panel. The input board and output board only consist of
a few terminals. The power amplification board is the most important part,
fulfilling control, amplification and power supply for the whole system.
In the following is the brief description of the principles for the power
amplification board.
1) Power supply part
In consideration of the big output power of the subwoofer, the device
employs a ring transformer for power supply. A group of 18V(AC)
voltages is output from the transformer, which will be subject to
rectification and filtering and will output +22 V voltage to power IC
TDA8947 and IC TFA9843 for power supply.
One line of output 22V voltage goes through the current limiting resistor
R101, voltage regulating diode VD104 and capacitor C108 and then
output +9V voltage to IC 75347; the other line goes through R100 and
voltage regulating diode VD105 and then output 5V voltage power
supply for the CPU and the control panel.
2. Signal input, volume and tone adjustment
The device has six channels for signal input. The signals input through
each channel will go through the filtering capacitor before being directly
delivered to the IC LC75347 for volume and tone adjustment. In addition,
one line of signals will be derived from the L and R main sound channels
and overlapped on the SW input. As a result, 2.1 output result will be
achieved even when there are only L and R sound channel inputs.
IC LC75347 is an IC with six-channel independent volume adjustment.
With its gains adjustable from 0 to 95 single DB, it is applicable to home
theater products. With benefits such as big range of power supply, low
power consumption and wide temperature scope, it is a quite competent
IC, and the functions of its main pins are listed in the following table:
Pin name Pin Numbering Function description
FLIN 7 Signal input
FRIN 24 Signal input
RL 20 Signal input
RR 18 Signal input
C 22 Signal input
SBW 16 Signal input
FLOUT 9 Signal input
FROUT 10 Signal input
COUT 11 Signal input
RLOUT 12 Signal input
RROUT 13 Signal input
SBWOUT 14 Signal input
VREF 23 Reference voltage
VDD 37 Power voltage
CE 40 Part signals selection control
line
DI 39 Data control line
CL 38 Clock control line
MUTE 35 External mute control
The IC internal unit equivalent circuit:
3. Amplifying circuit part
Principle: as far as the signals output from the IC 75347 are concerned,
the signals of the main sound channel will be delivered to the N101 IC
TFA9843 for power amplification; the central surround signals will be
filtered and delivered to the N106 IC TDA8947 for amplification; the
subwoofer signals will first pass experience amplitude limit (the
amplitude limit circuit comprises VD110 and VD113 diodes) and
pretreatment before being delivered to the N102 IC TFA9843. Amplitude
limit is aimed at preventing excessive size of signals, which may, after
amplification, damage the loudspeaker due to excessive power. Because
the power supply of the amplification IC is fulfilled by a single power
source, all amplified and output signals will have DC, therefore, the
signals will still experience filtering before being output to the
loudspeaker.
IC TFA9843J is used for dual sound channel power amplification.
With standby and mute modes, free from switch impact, and with
functions such as short circuit protection and overheat protection, it can
effectively be prevented from being damaged in case of IC abnormality.
In ordinary output mode, the IC can supply for each channel a maximum
of 20W power output. When BTL output is employed, the power can be
as high as 40W. In addition, the IC features powerful anti-ripple
performance, with little power consumption in the standby state.
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