Here is audiopreamp mic(dinamic). 200-600-ohm output impedance and low noise pre-amplifier for dynamic microphone is ideal for strengthening. This is a three-stage discrete amplifier with gain control. Alternatives such as the BC109C transistors, BC548 BC549, BC549C may be used with little change in performance. Thepreamp mic?circuit first stage is built around Q1 operates in common base configuration. It can not be used in the audio stage, but in this case allow Q1 to improve low-noise operation and the overall signal to noise ratio. Q2 and Q3 form a direct coupled amplifier, similar as with my previouspreamp mic. When the signal from a dynamic microphone is low, usually much less than 10mV, then there is too little to be gained by the collector voltage of Q1 voltage to half the supply voltage. Here is a schematic PreAmp MIC:
In power amplifiers, bias, half the supply voltage allows maximum voltage and maximum overload margin, but where the input is low, any value in the linear part of the operating system is quite nature. Here Q1 operating with the collector voltage of 2.4 V and a low collector current of about 200uA. This ensures a low collector current low noise and also increase the input impedance of the stage to about 400 ohms. This is a great addition to preamp mic any dynamic microphone with an impedance between 200 and 600 ohms.
source: http://www.hqew.net/circuit-diagram/Audio-PreAmp-MIC-%28dynamic%29-Circuit-Diagram_3346.html
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