This is a circuit for 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. This is the figure of the circuit;
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.
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