“The guitar is a miniature orchestra in itself.”
― Ludwig van Beethoven
Does anyone know the origin of this?
My Google-fu is weak. I only find circular references to various listings.
Not going to argue the point, though! 
From my searching, the origin of this seems to be an interview by Segovia printed the the now defunct magazine The Etude in May 1930. I found a site with the text of the article:
May need a Beethoven scholar to track that actual source down, or it may just be that Segovia was passing on something he heard.
As a vaguely related fun fact, Johannes Brahms was a fan of the banjo, having encountered it late in life. There are various claims that he intended to write something for banjo orchestra but these appear to be largely spurious.