Tweakers' Asylum

Probably would regret it

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In Reply to: Need a circuit that turns on amp when an audio signal is fed to it posted by Heresy on December 28, 2004 at 00:14:40:

Such circuits do exist for powered subwoofers. The early ones were subject to unintended turn on from induced voltages such as garage door openers, appliances on the same circuit, etc, etc. I think that is avoided with current units by designing them not to respond to signals at higer frequencies than normally go to the sub.
For a reciever, it would have to react to anything between 20Hz and 20kHz to funcyion as desired. However, false turn-ons would soon drive the owner up the wall. Also for obvious reasons it would not accomplish anything if all one wanted to do was listen to the FM or AM section.

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20Hz to 20Khz ??? - clifff 03:35:31 12/29/04 (0)
Why? Sound is a composite waveform, not a bunch of descrete sine waves!
All you need is a rectifier charge pump on the input, and set the detecting time constant to, say, 200mS.
That will filter out glitches and noise but catch anything like a real sound wave.
But actually, I agree with a previous comment - just leave it on all the time!

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