Microphone amplifier with noise reduction
The scheme is divided into two parts. The left one is in the immediate vicinity of the microphone, for example in the housing. The right - hand-anywhere - in amplifier, mixer. Connect these two pieces of two-wire, shielded cable.
How does it work? Very simple - the signal from the microphone, amplified left part is fed to the right side, the input stage of which (VT3, VT4) is an adder with a phase shift of 180 degrees (the phase of the signal, and not what you think). Thus, the useful signal in opposite phase on the two wires is formed and its value is doubled. The vile and common mode noise are mutually self-destructs.
The input impedance of the amplifier is about 57 ohms. During Assembly, the left part is preferably placed in a metal screen.