The film is almost the first demonstration of stereophonic sound to accompany moving pictures, an invention of Alan Blumlein.
An alternative construction uses two capacitors connected as a Blumlein generator.
the Ultra-Linear output stage (a tapped push-pull output transformer providing power intermediate between triodes and pentodes, at lower distortion than either) was originated by Alan Blumlein in 1937 in the UK, but popularised following publication of a paper by David Hafler and Keroes in the USA in 1951, and became the dominant topology during the post war recovery of consumer products
In 1937, Alan Blumlein originated a configuration between a "triode strapped" tetrode and normal tetrode, that connects the extra grid of a tetrode to a tap from the OPT part way between the anode voltage and the supply voltage.
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Blumlein Pair, a stereo recording technique invented by Alan Blumlein