Microphone preamp. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Preamplifiers
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Cables that connect a microphone to a complex of sound reproducing devices very often become sources of additional noise. The reduction in useful signal level that occurs on a long connecting cable can be compensated for with a simple single-stage input amplifier, but noise will also be amplified at the same time.
The best results are obtained by turning on the amplifier directly at the microphone and transmitting the signal through a balanced line. In this case, the noise at the level of the amplified signal is masked to a greater extent.
An interesting way to turn on the microphone amplifier is suggested in the diagram below. The amplifier itself is divided into two parts: the left part according to the scheme is connected directly to the microphone, the right part - to the sound reproducing complex, the symmetrical connecting line - between the stages of the microphone amplifier.
The input stage gain of the left side is determined (approximately) by the ratio of the resistances of resistors R5 and R6. The signal from the microphone, amplified 10 times, is fed to the base of the transistor VT2. Connecting lines 1 and 2 are connected to different points of the transistor VT2, the signal in the lines is antiphase.
The input stage (transistors VT3, VT4) on the right side is an adder with a phase shift of 180°. Thus, the out-of-phase signal is added and a useful signal at the output with double amplitude is formed. And the same noises and interferences that arise in each of the lines cancel each other out. The total signal is fed to the base of the transistor VT5. This stage has a gain of 4. Through the C4R15 filter, the amplified signal is fed to the output of the microphone amplifier.
The convenience of the proposed method also lies in the fact that the left side of the amplifier does not require an independent power source. The supply voltage of the sound reproducing complex is supplied through the switch SA1 to the right side of the microphone amplifier mounted in it, and through the resistor R8, the connecting line 1 and the resistor R7 to the left side.
Resistor R2, shown by a dashed line on the diagram of the left side of the amplifier, serves to optimally match the output
microphone impedance with amplifier input impedance. With the elements R3 and R4 indicated in the diagram, the input impedance of the amplifier is about 57 kOhm. Connecting a resistor R2 with a resistance of 100 kOhm reduces the input resistance to 36 kOhm. It can be selected according to the microphone used.
When mounting the device, the left side of the microphone amplifier should be placed as close as possible to the microphone head and placed in a metal shield. The right part of the amplifier is located near the mixing console of the sound amplifying complex.
In the proposed devices, 2T3169 transistors can be replaced by domestic KT342B transistors; 2T3109, VS549S - KT3102E; 2T3309, VS559S - KT3107E.
Publication: cxem.net
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