Electret microphone for telephone mouthpiece

Although many older telephone sets are electrically and mechanically perfectly sound units, their speech quality is poor compared with that of modern, all-electronic, sets (except the cheap types used in domestic intercoms). The reason for this deficiency is the carbon microphone in the mouthpiece. Here, an up-to-date replacement is discussed for the carbon microphone. It takes the form of an electric microphone and an amplifier with a special band-pass characteristic.