Johnny-one-note

Etymology
"Johnny One Note" is a show tune from the 1937 Rodgers and Hart musical . The lyrics tell a story of a male vocalist named Johnny who sang with a band and could sing only one note, but sang that note extremely well.

Noun

 * 1)  A person (or organization) who often expresses a strong opinion or viewpoint on a single subject or a few particular subjects.
 * 2)  An opinion or viewpoint on a single subject or a few particular subjects which is frequently expressed by a person or organization.