Line level out without disturbing impedance

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I want to put a device on my amplifier to turn it off if no music plays for 10 minutes. Seems to me I can do this on the signal coming into my amp or the speakers going out. But how to do it so it is not really in the signal path? I was thinking an opamp on the input because opamps do not flow current thru the inputs they shouldn't affect the source impedance. But I really dont like the idea of doing it pre power amp as any artifact will be amplified. Open to suggestions.
 
Overcome your fear, so long as the opamp is powered up its inputs are not going to disturb a signal, they are multi-megaohm and a few pF only. opamp followers into an envelop detector would be reasonable. An opamp-follower actually bootstraps its non-inverting input anyway so it looks even less of a load. Could add 100k input resistors if you want the device not to clip the audio if its powered-down - use JFET input opamp so input offset isn't large.
 
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