...back in 1952 :icon_eek:
US-pat.#2762875 fig.2 (http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2762875A&KC=A&FT=D&ND=3&date=19560911&DB=EPODOC&locale=en_EP)
even comes with transformer-coupled speaker-output :icon_eek: :icon_eek:
US-pat.#2761916 (http://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2761916A&KC=A&FT=D&ND=3&date=19560904&DB=EPODOC&locale=en_EP)
^ I've tried that!