Headset with limiter circuit

Claim 1

A battery-free protective hearing aid headset for protecting the ears of the wearer from acoustical sound originating externally of the headset, and for receiving and converting into sound audible to the ears of the wearer, electronic signals from a source, such as a microphone, positioned remote from the headset, comprising two ear-pad casings, an electrically operated earphone mounted in each casing for converting electronic signals to sound, and means for operating said earphones including a flexible cord operatively connected at one end to said earphones and adapted to be connected at its opposite end to an external electronic signal source for supplying audio electrical signals to said earphones, each of said earphones being operable by said electrical signals without use of any separate voltage supply, said means further including a limiter circuit in one of said casings and connected between said signal source and said earphones to limit the sound level from the earphones to a value harmless to the wearer's hearing, and to limit the magnitudes of the signals applied to said earphones to values harmless to said earphones, and said limiter circuit being directly responsive to the signals from said source and comprising a first stage connected in parallel across the earphones and operative to limit the voltage applied to said earphones, and a second stage connected in series with said earphones and operative to limit the current flow therethrough, wherein said first stage comprises a pair of Zener diodes mounted in parallel and in opposed directions to each other, and connected in parallel with said earphones, and said second stage comprises a field-effect transistor having its collector-emitter circuit in series with said earphones.