兩級RIAA唱機前置放大器

　　Two Stage RIAA Phono Preamplifier

　　Designers of quality audio equipment have long recognized the value of a low noise gain block with“audiophile performance”。 The LM833 is such a device： a dual operational amplifier with excellent audio　specifications. The LM833 features low input noise voltage typical）， large gainbandwidth　product （15 MHz）， high slew rate （7V/μSec）， low THD （0.002% 20 Hz-20 kHz）， and unity gain　stability.

　　A phono preamplifier‘s primary task is to provide gain （usually 30 to 40 dB at 1 kHz） and accurate　amplitude and phase equalization to the signal from a moving magnet or a moving coil cartridge. （A　moving coil device’s output voltage is typically around 20 dB lower than that of a moving magnet pickup，　so this signal is usually amplified by a step-up device—either an active circuit or a transformer—before　being applied to the input of the phono preamplifier）。 In addition to the amplification and equalization　functions， the phono preamp must not add significant noise or distortion to the signal from the cartridge.

　　Figure 1 shows the standard RIAA phono preamplifier amplitude response. Numerical values relative to　the 1 kHz gain are given in Table 1. Note that the gain rolls off at a 6 dB/octave rate above 2122 Hz. Most　phono preamplifier circuits in commercially available audio products， as well as most published circuits，　are based on the topology shown in Figure 2（a）。 The network consisting of R1， R2， C1， and C2 is not　unique， and can be replaced by any of several other networks that give equivalent results. R0 is generally　well under 1k to keep its contribution to the input noise voltage below that of the cartridge itself. The 47k　resistor shunting the input provides damping for moving-magnet phono cartridges. The input is also　shunted by a capacitance equal to the sum of the input cable capacitance and Cp. This capacitance　resonates with the inductance of the moving magnet cartridge around 15 kHz to 20 kHz to determine the　frequency response of the transducer， so when a moving magnet pickup is used， Cp should be carefully　chosen so that the total capacitance is equal to the recommended load capitance for that particular　cartridge..