An interesting patented hearing aid amplifier

Patent:


US3729588


Inventor Berland O


Applicant: American Danish Oticon


https://worldwide.espacenet.com/publicationDetails/biblio?DB=EPODOC&II=2&ND=3&adjacent=true&locale=en_EP&FT=D&date=19730424&CC=US&NR=3729588A&KC=A