In this circuit, D.A. which is actually O. A. of an operational amplifier, which can be any type with symmetrical source, the gain is fixed by the feedback resistor. With 1 M we have a voltage gain of 1000 times. The microphone can be of any type, the gain being fixed according to its characteristics. The circuit is from Newton C. Braga's Mechatronics Sourcebook, published in the United States.
