Switch that turns on when detected audio

I'm looking for switch that turns on my amp after detected an audio
signal.I recall seeing such a device in Silicon Chip or Electronics Australia
a long time ago. I can't for the life of me recall when it was or what mag
it was in.
Can anyone help me with this. Or even better, does anyone know of a
commercial product and whether it is more cost effective to build or buy.

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Wing Wong.