This effective, fixed-gain transformer less microphone preamp schematic amplifies differential signals from low impedance microphones by 50 dB, and has an input impedance of 2 k. This microphone preamp schematic electronic circuit project is based on the OP37 operational amplifier manufactured by Analog Devices. Because of the high working gain of the circuit, an OP37 helps to preserve bandwidth, which will be 110 kHz. As the OP37 is a decompensate device (minimum stable gain of 5), a dummy resistor, RP, may be necessary, if the microphone is to be unplugged. Otherwise the 100% feedback from the open input may cause the amplifier to oscillate.
Microphone Preamp Schematic Circuit Diagram:
Noise performance of this circuit is limited more by the input resistors R1 and R2 than by the op amp, as R1 and R2 each generate a 4 nV noise, while the op amp generates a 3.2 nV noise. The rms sum of these predominant noise sources will be about 6 nV, equivalent to 0.9 uV in a 20 kHz noise bandwidth, or nearly 61 dB below a l mV input signal.
As you can see in the schematic circuit diagram, this low noise amplifier circuit require few external electronic parts and it must be powered from a dual +/- 15 volts DC power supply.
