I have a pair of ATC SCA50 active speakers. They make a great sound but the
amplifiers run all the time, even when there's no signal to amplify. Since
they run in class A there is a significant current draw 24/7. As they are
at opposite sides of the room, turning them off involves two switches and
doesn't get done unless we're going away.

My previous Bowers Active 1s had an audio signal sensor in each speaker so
they went onto standby mode if there was no audio and "woke up" when audio
was present. Given the cost of running the ATCs all the time I'd like to do
the same with them.

There obviously needs to be a "switch off" delay so that the speakers don't
power down in quiet passages.

Do any of you experts know if something like the Velleman 4700 would do the
job? Or of any other circuit? I'm perfectly happy assembling something but
I'm no circuit designer.

Thanks in advance.