The signal from a microphone is two weak for a standard line input. This low-noise DC-coupled microphone amplifier pro¬ vides a solution for anyone who wants to connect a micro¬ phone to his or her hi-fi installation. As can be seen from the schematic diagram, a good circuit does not have to be com¬ plex. A differential amplifier is built around T1 (MAT-03E), which is a low-noise dual transistor. The combination of T2 and LED D1 forms a constant-current source for the input stage. A low-noise opamp (OP-270E) amplifies the difference signal that appears at the collectors of the dual transistor. The result is an analogue signal at line level. The bandwidth of the amplifier ranges from 1 Hz to 20 kHz. Within the audio range (20 Hz to 20 kHz), the distortion is less than 0.005 percent. Since only half of the OP-270E is used, the remaining opamp could be used in the output stage of a stereo version.
The amplifier can be powered from a stabilised, symmetrical supply with a voltage between ±12 V and ±15 V. Such sup¬ ply voltages are already present in many amplifiers.