The signal from a microphone is too weak for a standard line input. This low-noise DC-coupled microphone amplifier offers an effective solution for anyone looking to connect a microphone to their hi-fi system. As illustrated in the schematic diagram, a well-designed circuit doesn’t have to be complicated. The differential amplifier is built around T1 (MAT-03E), a low-noise dual transistor. The combination of T2 and LED D1 creates a constant-current source for the input stage. A low-noise opamp (OP-270E) amplifies the differential signal at the collectors of the dual transistor, resulting in an analog signal at line level. The amplifier’s bandwidth ranges from 1 Hz to 20 kHz, and within the audio range (20 Hz to 20 kHz), distortion is less than 0.005 percent. Since only half of the OP-270E is utilized, the remaining opamp can be used in the output stage of a stereo version. The amplifier can be powered by a stabilized, symmetrical supply with a voltage between ±12 V and ±15 V, which is commonly found in many amplifiers.
