GELIN9658H Green Audio has functions such as radio (FM, SW and MW band), tape recording and playback, record player, karaoke, tone control, subwoofer, one-way stereo sound source input and light display. The speaker is a two-unit combination: the woofer is a Green brand YD166-2A6 rubber-edged speaker, and the tweeter is a far-sound YDG50-1A2 paper cone speaker. The size of the speaker is 540 (height) Ã— 250 (width) Ã— 210 (depth) mm. The speaker cabinet is made of medium density board, the front board is 13mm thick, and the rest are only 7mm thick. The frequency division function is only completed by a 4.7Î¼F common electrolytic capacitor. The FM band has an automatic squelch function. The author made the following changes to this combination of sound: (1) keep all the sound source parts, keep the pre-stage parts such as tone control. In order to improve the receiving sensitivity of the FM band, increase the indoor horn antenna (the TV is idle after changing the cable reception), and an outdoor antenna can also be erected to increase the number of receiving FM stations; (2) The subwoofer is not used after the transformation due to distortion; ) The original power amplifier IC is TA7270, the power is relatively small, and it is decided not to use it, adding a new power amplifier circuit; (4) Speaker modification.
New power amplifier circuit The new power amplifier circuit is shown in the figure. The circuit of only one channel is shown in the figure, and the circuit of the other channel is the same. LM1875 is a monolithic 30W power integrated circuit. It is a product of National Semiconductor Corporation (NS). It is famous for its sweet sound and full of guts. It is often used in well-known AV power amplifiers abroad and is the preferred device for amateur power amplifiers. When using the LM1875 as a power amplifier, the author made reference to the relevant data to create a "DC servo current negative feedback power amplifier." The sound quality of this power amplifier does indeed sound full and thick. The strength of the bass and the resolution of the sound quality have been improved to a certain extent. However, the component data given in the relevant data should be adjusted greatly and auxiliary circuits must be added to eliminate high-frequency self-excitation, which is very difficult for amateur production. Therefore, according to the characteristic that the LM1875 output offset voltage is only 2Î¼V, I decided to cancel its feedback capacitor (22Î¼F ground capacitor), which theoretically can extend the frequency response to DC, and eliminate the signal phase shift and non-signal caused by this capacitor. Linear distortion, DC power amplifier. The power amplifier circuit is stable and reliable, the sound quality is greatly improved, and it is also suitable for amateur production. Of course, the circuit is stable and reliable on the premise of high-quality power supply and peripheral components.
The power amplifier newly added one 100W transformer, and the original transformer is exclusively used for the pre-stage. All resistors use NEC's five-color ring resistors, small capacity capacitors use CBB models, and input capacitors use tantalum capacitors.
The newly added power amplifier circuit board and transformer are placed in the bottom space of the box, and maintain an appropriate distance from each other.
A pair of double-pole and double-throw switches are newly added, which are installed in the box and used for sound source switching. Solder the input cable of the original audio source from the front-end circuit, and connect it to one end of the double-throw switch, so that the external high-quality audio source can be directly amplified by this power amplifier without artificial modification to meet the Hi-Fi requirements: connect the original tone circuit to the potential The signal line of the potentiometer is welded away from the potentiometer and connected to the other end of the double-throw switch, and then the output line of the potentiometer is connected to the power amplifier from the original power amplifier. Connect the ground point of the power amplifier to the ground point of the original power supply. The original volume potentiometer has noise that is difficult to eliminate, and use the APLS50 Ã— 2 potentiometer. Disconnect all connections from the original power amplifier, and the original power amplifier is "retired".
The speaker is modified to make two 6dB frequency dividers (frequency division frequency 4000Hz) to replace two 4.7Î¼F capacitors. The original woofer and tweeter are retained because their actual performance is comparable to commercially available general speaker speakers. The wall of the box is too thin and needs to be modified. The steps are as follows: open the back cover, reinforce the inner wall with this strip first, then deposit small nails, and evenly coat with 5mm thick cement concrete. This is to strengthen the wall thickness and increase the weight. Reduce box vibration and improve sound quality. Then lined with 15mm thick sponge, and finally installed the back cover, and sealed with glass glue. The original connected speaker cable is an ordinary thin cable, and use a commercially available speaker cable.
After completing the above steps, power on the test machine, pull down the double-throw switch, input as the source of the machine, and listen to FM high-quality music broadcasts; pull up the switch, connect to the VCD, and play the "Titanic" sung by Celine Dion Theme song "MY HEART WILL GO ON". The beautiful and beautiful melody is fascinating, and the rich tone can be reproduced very well.
Finally, in order to prevent the VCD signal voltage from being too large, two 150kÎ© resistors are connected in series at the input end of the double-throw switch sound source.
The above reconstruction cost is only more than two hundred yuan, but the sound performance after reconstruction can be compared with a commercially available multi-amplifier and speaker, which can fully meet the requirements of ordinary families to form a home theater, which is actually a better way to upgrade and upgrade common combination speakers .
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