Phantom-Powered Professional Microphone Pre-Amplifier Circuit Using INA217


The Circuit is based on INA217 from Texas Instruments and the IC used in professional audio equipment such as professional microphone preamps, moving-coil transducer amplifiers, differential receivers, and bridge amplifiers.



  • 48V Phantom Powered, Remotely Located Microphone
  • Circuit Operates from +/-15V Power Supply
  • Low Distortion and Noise Over The Audio Frequency Band
  • Gain Range from To 20db To 60 db


R1 and R2 provide a current path for conventional 48V phantom power source for a remotely located microphone. An optional switch allows phantom power to be disabled. C1 and C2 block the phantom power voltage from the INA217 input circuitry. Non-Polarized capacitors should be used for C1 and C2 if phantom power is to be disabled. For Additional input protection against ESD and hot-plugging, four IN4148 diodes may be connected from input to supply lines. R4 and R5 provide a path for input bias current of the INA217. Input offset current (typical 100nA) creates a DC Differential input voltage that will produce an output offset voltage. The generally the domination source of output offset voltage in this application. With a maximum gain of 1000 (60db) , the output offset voltage can be several volts. This may be entirely acceptable if the output is AC-coupled into the subsequent stage. Gain is set with a variable resistor, R7, in series with R6. R6 determine the maximum gain. The total resistance, R6+r& , determines the lowest gain. A special reverse-log taper potentiometer for R7 can be used to create a linear change (IN db) with rotation. This Circuit is from Texas Instruments Application.

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