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The signal from a microphone is too low for standard line inputs. Yet, most audio setups don't have a dedicated microphone input. This low-noise, DC-coupled microphone amplifier will be a godsend for any audiophile wanting to connect a microphone to their audio system. As the schematic demonstrates, a high-quality circuit doesn't have to be complex. The heart of the circuit is T1 (designated here by T1a and T1b), a low-noise dual transistor of the MAT-03E type, configured as a differential amplifier.
The combination of T2 and LED D1 forms a constant current source used to power the input stage. A low-noise operational amplifier, also of the OP-270E type (Burr-Brown), amplifies the differential signal present at the collectors of the dual transistor. The result of this processing is an analog signal at line level.
The amplifier's bandwidth ranges from 1 Hz to 20 kHz. Distortion within the audio range, between 20 Hz and 20 kHz, never exceeds 0.005%. Since only half of the OP-270E is used, in the case of a stereo version, the other half, i.e., the second operational amplifier, can be used for the second stage.
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