Low-Impedance Microphone Input_Preamplifier

Figure 1.  Low-Impedance Microphone Input Circuit Diagram.

This is a low-impedance (Z = 50 to 200 ohms) microphone input circuit or pre-amplifier that employs low-cost, low-noise precision operational amplifiers such as the OP27 and the OP37. 

The simple circuit above amplifies differential signals from low-impedance microphones by 50 dB.  Because of the high working gain of the circuit, use of the OP37 (which is a high-speed op amp) is recommended if bandwidth is important to the application.  To ensure stability, a dummy resistor Rp must be placed between the OP37 inputs. This will prevent amplifier oscillation due to 100% feedback from the open input in case the microphone is unplugged.     

Primary Reference:  Analog Devices Datasheets for OP27/OP37