SERVICE MANUAL
MA-950S/850S
2004-10-03
MA-950S/850S Principles and Maintenance
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 |
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Electronic |
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Power |
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input |
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volume |
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amplific |
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tone |
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ation |
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Output |
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adjustment |
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CPU system control
Remote Receiving and 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 |
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FLIN |
7 |
Signal input |
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FRIN |
24 |
Signal input |
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RL |
20 |
Signal input |
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RR |
18 |
Signal input |
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C |
22 |
Signal input |
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SBW |
16 |
Signal input |
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FLOUT |
9 |
Signal input |
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FROUT |
10 |
Signal input |
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COUT |
11 |
Signal input |
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RLOUT |
12 |
Signal input |
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RROUT |
13 |
Signal input |
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SBWOUT |
14 |
Signal input |
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VREF |
23 |
Reference voltage |
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VDD |
37 |
Power voltage |
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CE |
40 |
Part signals selection control |
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line |
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DI |
39 |
Data control line |
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CL |
38 |
Clock control line |
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MUTE |
35 |
External mute control |
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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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