Analysis of the principle of Qisheng AV-747DB power amplifier

Qisheng AV-747DB is the latest home theater AV center launched by Qisheng, adopting front and rear split structure. Its predecessor uses the HD6473714P microprocessor as the control center, and the super large scale integrated circuit M62460FP as the core of Dolby Pro Logic Surround Sound Decoding (PRO.LOGIC) and Digital Surround Sound Field Effect (DSP) processing. L, R), center channel (C), surround channel (S) volume, tone adjustment. All functions of this machine except the karaoke system have realized full digital control and remote control. Thanks to the use of a fluorescent display for operation instructions, the man-machine dialogue function is greatly enhanced, making the operation very simple and clear. The rear stage performs independent power amplification on the L, R, C, and S channels, and the output power is strong. The output protection circuit has functions such as power-on noise suppression, over-current protection, over-voltage protection, and output short-circuit protection. The karaoke circuit of this machine adopts the PT2399 reverberation integrated circuit, which has a better performance on human voice. The microphone volume, tone and reverb depth, delay time and other control circuits are designed separately, and users can adjust it according to their own preferences. Almost every winding in the front / back stage power circuit is equipped with a fuse to ensure the safety of the whole machine. The machine is also equipped with a subwoofer line output and two-way video selection switching function, which is very convenient to use. The principle block diagram is shown in Figure 1.

After processing various instructions and status information, the microprocessor IC801 drives the fluorescent display screen to display related information while outputting to various controlled circuits to realize various functions. Because the I / O interface of IC801 is not enough, IC802 is added as an interface extension to meet various requirements.
After the signals of the four channels L, R, C, and S enter the post stage, each channel is amplified independently. Among them, the S channel enters the rear stage and is divided into two, which are amplified and output directly; the L, R, and C channels are output through L201, L101, and L301, and after being controlled by the relay J, the speaker is pushed to sound.
According to the technical requirements of Dolby, when working in the wideband mode surrounded by Dolby directional logic, the center channel must have the same bass volume as the left and right channels. Therefore, the three channels of R, L, and C all use super-high-power amplifier circuits composed of discrete components. A total of 5 pairs of 2SC5198 / 2SA1941 high-power tubes are used for power amplification. With the cooperation of a professional large radiator and a high-power power supply system, the rated output maximum undistorted power reaches 85W / 8Ω × 3 (in the straight-through state, the maximum undistorted power can reach 120W / 8Ω × 2), making these three sounds Dao has a wide dynamic range.
Because when watching movies, the sound source in the program is mainly in the front, and the surround sound in the back generally only plays the role of sound field positioning and background sound playback, so its power does not need to be too large, but it cannot be too small. This circuit uses the power integrated circuit LM1876 for power amplification of surround channels. The rated power of LM1876 is 30W / 8Ω × 2. It has over-current, over-voltage, over-heat protection and mute functions. It can perform self-protection and speaker protection, with stable performance, safety and reliability. The S channel signal enters the amplifier from the {13} pin and ⑧ pin of the LM1876, and is output from the ① pin and the ③ pin as left and right surrounds after power amplification.
In order to eliminate the start-up impact sound and avoid damage to the user's expensive speaker when the power amplifier stage fails, or the user accidentally shorts the speaker cable and damages the power amplifier, AV-747DB has designed a perfect protection circuit. At power-on, the delay circuit of the protection circuit controls the relay to pull in and switch on, while muting the LM1876 for a period of time, so that the impact signal generated by the power supply circuit at power-on cannot be output to the speaker. R125, R225, R323 in Figure 1 are the output resistance of the overcurrent (or speaker line short circuit) detection circuit, and R501, R502, R503 are the midpoint potential monitoring resistors. When one of the R, L, and C channels has overcurrent or the midpoint deviates from the ground potential, the protection circuit quickly releases the relay and cuts off the power output line, thereby protecting the speaker and the power amplifier.
The AV-747DB rear stage uses two large pointer tables for dynamic indication of the R and L channels to increase the aesthetic appearance. The driving signal is taken from the R and L channels near the output terminal, and enters the driving circuit through R418 and R419. After attenuating, clamping, smoothing and other processing in the drive circuit, the signal becomes a signal representing the dynamic change of R and L, which is driven by the meter head.
The power circuit of AV-747DB provides more voltage categories, but because the front stage uses 7812, 7805, 7809, 7912 and other three-terminal voltage regulator integrated blocks, the circuit is simple; the rear stage uses two 300W toroidal transformers for power supply, and its rectification The filter circuit is similar to the power supply circuit of other power amplifiers, and will not be repeated here.