This circuit was obtained in a 1980 edition of Elementary Electronics a magazine that no longer exists, but the basic idea can be utilized with the use of a BC548 transistor or equivalent. The circuit consists of an amplifier microphone. Placed between the microphone and the input of the audio equipment it increases the signal level, hence the original name of booster (booster). In fact, we can say that it is a preamplifier, a most common term between us. The circuit can either be powered with 9 V battery as a 12 V car battery, if this use. As consumption is very low, the life of a battery will be very large. You can add a switch that switches the input, leaving the signal passes directly unreinforced if this is necessary. Input is average impedance and output provides a signal of a few hundred millivolts, enough to excite common audio amplifiers. The signal cables (input and output) must be shielded. If power supply is used, it must have excellent filtration.
