Simple-mixer

  
This mixer is built around a TL072 dual BiFET op amp with a JFET input stage, and can be powered from a single-ended 9-to 18-V power supply. The microphone input is capacitively coupled to the noninverting input of U1a. Resistors R1 and R3 set the voltage gain at about 26 dB and serve as a negative feedback network for U1a. Capacitors C1 through C3 are de-blocking capacitors. Most high-impedance microphones have outputs of a few mV.
Simple-mixer - schematic

Often, a preamp stage just isn"t enough, so the microphone signal· is given a boost of about 20 db in the mixer. The noninverting input of U1b is biased to half the supply voltage by R6, R7, and C6. Resistors R5 and R8 make up the negative-feedback network and set the voltage gain of U1b at unity. Capacitor C5 is for de blocking at this input.




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