Sensitive microphone with low noise transistor amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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The design of sensitive amplifiers for listening to speech has its own characteristics. This device contains a two-stage low-frequency amplifier based on low-noise transistors VT1 and VT2, a corrective filter on a VT3 transistor, and a final amplifier assembled according to a push-pull transformerless circuit based on transistors VT4-VT6.
The acoustic amplification of the audio frequency signal by the given device is 85 dB, the initial current consumption is 1,8 mA, the amplified frequency band is from 0,3 to 3 kHz, the maximum output signal level is 124 dB.
(click to enlarge)
The signal from the microphone M1 type "Pine" through the capacitor C1 is fed to the base of the transistor VT1. Since the sensitivity of the audio frequency amplifier is limited by the internal noise of the transistors, low-noise transistors of the KT3102 type are used to reduce noise in the first stages of the amplifier.
Amplifying cascades on transistors VT1 and VT2 are covered by deep negative feedback, which allows for stable operation of the cascades and a more linear frequency response. The load of the second stage of the amplifier is the variable resistor R3, which is also the volume control. A complex RC filter consisting of elements R3, G5, R6, C6, R7, C7 cuts off the "noise" RF components received by the microphone, and leaves only signals in the frequency band up to 4 kHz. This range provides the greatest intelligibility of speech information.
From the filter output, the signal is fed to the final audio frequency amplifier, made on transistors VT4, VT5 of the KT315 type and transistor VT6 of the KT361 type. The amplifier load is a TM-2A or TEM headphone. Resistors in the circuit are used type MLT-0,125. Resistor R3 - SP 3-41 or other small size.
Setting up the device comes down to selecting the resistances of resistors R1 and R16 to set the voltage at points A and B equal to half the supply voltage.
Authors: Andrianov V.I., Borodin V.A., Sokolov A.V.
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