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brownhunter wrote:I do know the local classical station will be airing the concert on Saturday. Perhaps this was some sort of indication for the control booth?

This makes sense. I don't know about the instrumentation of "El Niño", but Adams has been a big fan of synthesizers and samplers in the symphony orchestra for decades, and balance and amplification are always issues because he always requests that the electronics not be amp'd through the house PA system.

Aha! The answer has been found on the composer's blog!

http://www.earbox.com/posts/100" target="_blank (Last paragraph)

The conductor had a small monitor behind him and that control was to shut it off.