Another useful audio circuit using an operational amplifier is the one shown in the figure. It is a pre-amplifier for dynamic low impedance microphones. This circuit is indicated for operation with microphones from 50 to 200 ohms and is of the type with balanced input to avoid capturing hum. For the balanced input to have maximum efficiency in the rejection of noise and snoring it is important that high precision resistors are used. 0.1% resistors are recommended in this case. The circuit gain is 50 dB and the input impedance is 2 k ohms. As the gain is very high, the pass-through range only reaches 110 kHz, for audio applications this is more than enough.
