This excellent 3-transistor audio preamplifier is suitable for use with high-impedance microphones. The transistors may be the BC548 and power comes from a 9 V battery. If used power supply, filtering should be excellent. Consumption is very low. The original circuit was found in an 1979 English publication. Changes have been made to make it feasible today. Input and output cables must be shielded. The level of the output signal is sufficient to excite the input of most amplifiers. The input impedance is high and output impedance is low. The potentiometer controls the loop gain and must be adjusted to obtain full output power without distortion.