[Photo] LM1875 Multimedia HI-FI Amplifier

Multimedia Hi-Fi amplifier DIY
2003, Issue 5, Category: Professional Audio

In recent years, with the popularity of PCs and the price of DVD-ROMs, it has become a pleasure to listen to music and watch DVDs with PCs. Secondly, a large number of "gold ears" and "silver ears" are emerging, and the sound quality requirements of PCs are becoming higher and higher. But for thousands of high-end active systems, we can only stay away from the "shyness" in our pockets. So the author DIY set up a set of multimedia active power amplifier to listen to Hi-Fi music on PC.
This circuit is quite simple, and the sound processing uses the Canadian QSOUND company's single-chip QS7779 virtual surround circuit. Because the circuit has built-in Dolby Pro Logic and Dolby Digital Mixed Signal Decoder. Not only can the effect of ordinary stereo signals be enhanced, but when playing a Dolby digital signal of DVD, it can also be decoded first, and then matrix operation is performed, which is virtualized into a two-channel output. In addition, you can switch different effect. The use effect and sound quality of the circuit have been finalized in the audio industry, and it is more realistic than the "3D surround" which is virtually useless. The buffer stage of this circuit uses only a piece of OP275 to improve the input and output impedance after passing through the volume potentiometer. This op amp is used here to enhance the driving force for the final stage and reduce the interaction between the front and rear stages under large dynamics. The last-stage power amplifier uses LM1876TF, which is well-known in the audiophile world. The IC comes from the US company NS, with a conversion rate of 18μV / ms, a signal-to-noise ratio of 92dB, and a quiescent current of only 4mA. Due to the integration of two completely independent channels, sharing a heat sink, to ensure that the circuit has a higher degree of separation and the same temperature characteristics. The IC's output power is 25W, which can't be better used in a small multimedia active system with a small listening environment. The power amplifier has a small number of peripheral components, only 8 per channel, so it is easy to make boards and modify existing multimedia speakers.
Production points: The resistor should be a metal film resistor with a small temperature drift, and the coupling, negative feedback and decoupling capacitors everywhere should use ELNA supplement electrolysis. The transformer uses 200W high-quality ring cow, the secondary is two groups of dual 18V output, one group is small current, and the other group is large current. The filter capacitor at the power supply should use Rubycon's 2200μF / 50V in parallel. Only in this way can we ensure strong power under great dynamics. For capacitors C14, C21, C16, C23, C18, C42, C37, C38, C40, C27, C31, C44, C45 use WIM0.1μF / 63V. Three-terminal regulators use general LM7815, LM7915 and LM7805. The sound processing part can be selected from commercially available finished boards or self-made. The radiator of the LM 1876 power amplifier block should be large enough. After all components are installed and wired correctly and without error, the power will ring immediately.
Actual listening: Connect the innovative SB-LIVE sound card on the PC. The speaker is a self-made two-way small bookshelf box with Hivi 6.5-inch bass and SS1 + treble. First set the sound effect switch to straight-through, and put a familiar "In That Far Away", the wind blowing effect sound produced by the electronic synthesizer in the intro part, the violin in the string music is crystal clear and rich in overtones. The high-frequency “squeak” sound emitted by the triangle iron is snug, clear and pleasing, and the entire sound field is wide. Then put the sound effect switch in the enhanced gear, put a "Future Warrior Sequel", play it with the POWERDVD playback software, and set the sound effect to "stereo". Many explosion scenes on this disc were interpreted as turbulent and magnificent. The powerful airflow emitted by the low frequency hits the chest directly, but the power amplifier is not "feet soft". The sense of movement and positioning of the bullets fired by various guns is extremely strong. There are also good performances for game sound effects. When I play "Resident Evil", the background effect sound effects, especially the sounds of various "ghosts" moving in different directions, accompanied by desolate music, create a good A creepy atmosphere. In exchange for such a high performance at a price of more than two hundred yuan, it is indeed worth the money.

? Guangdong Shi Zhong

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