When Field Marshall Mannerheim celebrated his 75 birthday on
4th June 1942, in a suite of first class railway carriages taken to
a secret location near Imatra in Finland, he had an unexpected guest -
Adolf Hitler.

YLE (Finnish Radio) had planned to broadcast the proceedings
and speeches of the birthday celebrations, and despite strict
instructions to the contrary, YLE sound recordist Thor Damen
recorded the discussions with Hitler by concealing a microphone
in the overhead hat-rack.

This is said to be the only recording of Hitler where he speaks like
a normal human being! Link below. Hitler begins to speak at: 03:20
He and Mannerheim discuss in German, and there is a Finnish voice
over.

To keep this thread on topic, at least as much as the discussions
on taxes and expensive watches, I would add that the recording
was made on a vintage 1941 AEG K4 tape recorder at 76cms.
(30 ips) This recorder was far ahead its time and included AC
bias, which only appeared some 6 yeas later on the American
Ampex 200 as the result technology acquired in the victory spoils.

Risto Syrjä also of YLE, had studied in Germany and
had been one of the team working on AC bias for tape
recording, which produced an improvement of 25dB
in signal-to-noise ratio compared with DC bias.

Iain

http://www.yle.fi/elavaarkisto/?s=s&g=1&ag=1&t=&a=376