His pseudonyms for piano rolls included Henri Bergman, Edwin Gabriel, Robert Romayne, Robert Summers. After studying with Constantin von Sternberg he became a concert pianist, then branched out into conducting and a composing for radio, then television and film. He debuted as a pianist with the Philadelphia Orchestra at the age of eight.
In his teenage years, he started recording piano rolls for the Aeolian Companies Duo-Art reproducing pianos and turned out hundreds of classical and salon-type performances for them.
He attracted particular notice as orchestra leader of The Voice of Firestone, the songs "Cuddle Up" and "High Barbaree" and his compositions "Western Ballet" and "Variations in Miniature on Chopsticks". During the 1940s, Armbruster was the conductor of the National Broadcasting Company Hollywood Orchestra for many charitable specials.
In 1948, Armbruster was musical director, conductor and solo pianist for the Kraft Music Hall program starring Nelson Eddy. A fine pianist, he was featured each week in classical piano solos.
The program had an excellent orchestra of about 35.
Regular arrangers included Billy May and Nelson Riddle. In the sixties, Armbruster was head of the music department at Metro-Goldwyn-Mayer Studios.