Carl Ignaz Anton Demmer, also known as Karl, (baptised 11 February 1766 – after 1824) was an operatic tenor at the Vienna court opera, and possibly Florestan in the first version of Beethoven's opera Fidelio on 20 November 1805, then titled Leonore, oder Der Triumph der ehelichen Liebe.
that's a wonderful man - he played Tamino with excellence - and our operas have won a lot through him - his wife only performed as Claudia once - you can't say much yet.
[10] On 27 February 1804, Demmer gave his farewell performance in Frankfurt in the title role of Mozart's La clemenza di Tito.
[11] Demmer made his debut there on 20 June 1804 at the court opera as Edwinsky in the Vienna premiere of François-Adrien Boieldieu's Die Verwiesenen auf Kamtschatka.
[13]The Vienna correspondent of the Berlinische musikalische Zeitung edited by Johann Friedrich Reichardt wrote somewhat more sceptically: For a first lover he is no longer young enough, his voice has too little sound and flexibility.
The performance was a great failure for Beethoven, as even the protagonists were not given a good testimony: "Milder, despite her beautiful voice, had far too little affect and life for the role of Fidelio, and Demmer almost always intoned flat."
[15] Among Demmer's great successes was the role of Ober-Seneschall in the Boildieu's singspiel Jean de Paris, which was first performed at the Theater an der Wien on 29 August 1812.
Ignaz Franz Castelli writes in his memoirs: Two Demmer brothers were employed as singers, one at the court opera, the other at the Theater an der Wien; they played all the chevaliers, they looked so alike, and had so much the same manners and so much the same language that it was almost impossible to tell them apart.