Electret microphones are used to an ever-increasing extent in modern telephone apparatus, since they give good sound quality, have low power consumption and are inexpensive in manufacture. As with all electrostatic microphones, the electret microphone has a very high impedance and low output power, so an amplifier must therefore be connected between the microphone and the line. Due to the electret microphone's high impedance, this amplifier should be immediately adjacent the microphone to avoid circuit noise. Suitably, the microphone and amplifier are built together to a capsule which is placed in the telephone handset. Current supply then suitably takes place using the same conductor as for the signals from the handset to the telephone set. A suitable circuit for connecting a source having very high impedance to a load with low impedance suitably contains an input stage with a field-effect transistor directly connected to one or more emitter follower stages. Such circuits are known in the literature e.g. from "Electronics" Feb. 28, 1972, page 80, which illustrates an amplifier for a capacitive transducer.