参考设计:唱机前置放大器

LSK489非常适合于高性能的移动磁铁留声机前置放大器.他可以达到非常低的噪声,同事显示出非常高的阻抗给动磁式唱头. JFET设计中输入偏置电流的缺席排除了输入噪声电流,BJT(双极结型晶体管)设计所受的痛苦.到EMI的阻抗和软负债特性使得JFET选项更加有优势.

The absence of input bias current in the JFET design eliminates input noise current from which BJT designs suffer. Resistance to EMI and a soft overload characteristic make the JFET choice even more attractive.

由于唱头感抗(大约300-600mH)产生的共振,使得动磁式唱头源阻抗在更高的频率处上升,负载电容.这个共振通常位于18-22Khz,能够产生大量的Q.在共振频率处,阻抗可以上升到47K的名义负载阻抗.这个增加了唱头和非线性输入电容的可能性.低输入阻抗的LSK489 JFET对管减少了唱头干扰，高频互调失真.动磁式唱头通常是和一个特定负载电容一起工作的大于200pF,
The MM cartridge source impedance rises at higher frequencies due to the resonance formed by the cartridge inductance (on the order of 300-600 mH) and the load capacitance. This resonance usually lies around 18-22kHz and can have substantial Q. At the resonance frequency, the impedance can rise to almost the nominal load resistance of 47k. This raises the possibility of cartridge interaction with the nonlinear input capacitance of the amplifier. The low input capacitance of the LSK489 JFET pair reduces cartridge interaction and high-frequency intermodulation distortion. Moving magnet cartridges are usually designed to work with a specific loading capacitance on the order of 200pF, so an input stage with capacitance on the order of 20pF, like that of an LSK389, may create a significant disturbance, especially given that the JFET input capacitance can be nonlinear.
Figure

上图展示了一个留声机前置放大器设计,采用了LSK489来达到一个低噪声和高输入阻抗.此电路包括了一个混合的JFET-bipolar运放,使用LSK489作为其低噪声输入级. 低失真,低噪声LM4562完成了混合的运算放大器, which is configured for a flat gain of 21dB. The 75us high-frequency corner of the RIAA equalization characteristic is implemented by R12 and C4. The remainder of the RIAA equalization implements time constants at 3180 and 318μs. It is implemented with the second half of the LM4562 op amp and the surrounding feedback network. Total gain of the preamp is 36dB at 1kHz. LSK489 JFET输入级提供了出色的EMI抗干扰能力.

The author’s VinylTrak phono preamp [3] uses a similar arrangement, but the front-end stage is a discrete 20-dB amplifier without negative feedback. That design provides a true high-impedance differential input and very soft overload characteristics. The small signal levels from a moving magnet phono cartridge allow distortion in the no-feedback arrangement to be very low.

BOM: 低噪放LSK489

用途: 麦克风前置放大器

优势: 低噪声,高保真


南频推荐的料号给麦克风前置放大器应用:

1, LSK489


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