a hearing aid has a behind - the - ear housing containing a microphone , an amplifier , a current source and an ear phone , and a group of regulators for controlling various operating characteristics and an acoustic coil . the regulators and the acoustic coil are insertable through an opening in the rear of the housing and are combined in a single unitary unit which can be attached to the amplifier . the precise orientation of the regulators and the coil with respect to each other is thus always the same and can be easily achieved during assembly . the necessary electrical separation between the parts is also guaranteed .