If you are talking about turning the power on and off automatically when an audio signal is present then yes there are DIY versions
of these switches around. Some SUB's have this built in. Anyways, look for automatic stereo switches etc. The switch is a cct which
is in parallel to the signal watching it and if there is a signal present it energies a relay which in turn powers on the sub amp or whatever
is plugged in.
These methods of hands free control of power have gone by the way side as modern equipment such as sub bass amps etc now employ class D operation and use so little power when no music is present that the extra cct to do what you ask is not needed. Also I think
it is bad practice to have audio and 120vAC in the same box as a consumer item so they died out.
Have a look at this page
http://sound.whsites.net/project38.htm
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