As shown in FIG constituted by INA217 professional miniature microphone (microphone) typical preamp circuit. FIG set a switch for selecting whether the choice of phantom power
function, when the switch connected to + 48V when the choice of phantom power function, when the switch to the resistor R3 is prohibited phantom power. Elected to use phantom power feature, + 48V power supply through R1, R2 was added 3,2 socket contacts, providing a current path. The socket is left and right channel mix outlet, therefore + 48V phantom power through the power socket to the distal miniature microphone (microphone) power supply. After the distal end of the miniature microphone (microphone) voice signal input through the socket after the INA217 amplifier output. C1, C2 isolates phantom power power supply DC voltage into the INA217, while coupling the AC signal. If instead of phantom power, C1, C2 available nonpolar capacitors. R4, R5 provide access INA217 input bias current, and input offset current (typical 100nA) produces a DC differential voltage, the DC differential voltage at the output terminal will generate output offset voltage, output offset at maximum gain G 1000 (60dB) when voltage may be a few volts. This is generally acceptable, because the output using the AC coupling due to the blocking effect of the coupling capacitor, the output offset voltage does not affect subsequent circuit stages. Cheap OPA137 operational amplifier feedback loop for driving the DC output voltage to 0V, since the audio channel OPA137 not, it will not affect the signal quality. R6 is used to set the maximum gain, the gain can be varied to adjust R7, R6 and R7 of the total resistance determines the minimum gain. The effect of changes in linear dB usually R7 selected antilog potentiometer can generate a rotation.