[Image]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 drop of DVD-ROMs, listening to music and watching videos on PCs has become a kind of enjoyment. Secondly, a large number of "golden ears" and "silver ears" have emerged, and the requirements for the sound quality of PCs are getting higher and higher. However, for high-end active systems that cost thousands of dollars, we who are "shy" in money can only look at them. So the author DIYed a multimedia active amplifier for listening to Hi-Fi music on PC.
　　This circuit is quite simple, and the sound effect processing adopts the single-chip QS7779 virtual surround circuit of Canada QSOUND company. Because the circuit has built-in Dolby directional logic and Dolby digital mixed signal decoder. Not only can it enhance the effect of ordinary stereo signals, but when playing the Dolby digital signal of DVD, it can also decode it first, and then perform matrix operation to virtualize it into two-channel output. In addition, different effects can be switched through switches. The effect and sound quality of this circuit have been determined in the audio industry, which is more real than those "3D surround" that are in name only. The buffer stage of this circuit uses only one OP275 to improve the input and output impedance after the volume potentiometer. This op amp can be used here to enhance the driving force of the final stage and reduce the mutual influence between the front and rear stages under large dynamics. The final amplifier uses the well-known LM1876TF in the audiophile industry. This IC comes from NS Company in the United States. The conversion rate reaches 18μV/ms, the signal-to-noise ratio reaches 92dB, and its static current is only 4mA. Because two completely independent channels are integrated and share a heat sink, the circuit has a high degree of separation and the same temperature characteristics. The output power of this IC is 25W, which is perfect for small multimedia active systems with small listening environments. The number of peripheral components of this amplifier is small, with only 8 per channel, so it is easy to make boards and modify existing multimedia speakers.
　　Key points: The resistors should be metal film resistors with small temperature drift. The coupling, negative feedback and decoupling capacitors should be ELNA electrolytic. The transformer uses a 200W high-quality ring transformer. The secondary has two sets of dual 18V outputs, one for small current and the other for large current. The filter capacitors at the power supply should be Rubycon 2200μF/50V and connected in parallel. This is the only way to ensure strong power under large dynamics. Capacitors C14, C21, C16, C23, C18, C42, C37, C38, C40, C27, C31, C44, C45 use WIM A0.1μF/63V. The three-terminal voltage regulator uses general LM7815, LM7915 and LM7805. The sound effect processing part can choose commercial finished boards or homemade. The heat sink of the LM1876 amplifier block should be large enough. After all components are installed and wired correctly, the sound will come out immediately after power-on.
　　Actual listening: Connect to the Creative SB-LIVE sound card on the PC, and use a self-made two-way small bookshelf box with a 6.5-inch bass and SS1+ treble. First, set the sound effect switch to direct pass, and play a well-known song "In That Faraway Place". The wind effect sound played by the electronic synthesizer in the prelude is pervasive, and the violin in the string music is crystal clear and rich in overtones. The high-frequency "cha-cha" sound of the triangle is crisp and clear, and the whole sound field is wide. Then set the sound effect switch to enhanced gear, play a "Terminator 2", play it with the POWER DVD player software, and set the sound effect to "stereo". The numerous explosion scenes in the disc are performed turbulently and majestically. The strong airflow from the low frequency hits the chest directly, but the power amplifier is not "soft", and the direction and positioning of the bullets fired by various guns are extremely strong. The game sound effects are also quite good. When I was playing Resident Evil, the background sound effects, especially the sounds of various "ghosts" moving in different directions accompanied by sad music, created a creepy atmosphere. The price of only more than 200 yuan for such high performance is really worth it.