Norman Billingham wrote:

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.

Do you turn the rest of your audio system off, or does it go
into standby? Jim or Chris have suggested clear solutions if
you habitually switch another part of the system off.

If the rest of your system is left on standby, and you are
into a bit of assembly, it should be possible to tie a mains
switch for the speakers to the standby function somehow.
Possibly easiest would be to use the standby indicator light
to trigger a simple logic circuit, and take a strong
low-voltage signal from there to a 5V mains relay or SCR in
each speaker. If this results in power-up or -down thumps or
whatever, things could end up a bit more complicated.

A switch that would sense an audio signal and make a
decision would require a "watchdog" timer. Every time it
senses a signal it would reset itself, otherwise it would
time out and shut down the system. Not particularly hard to
do, although preventing it from interacting with the audio
signal in a bad way may be a bit tricky, considering you
would need it to interact...

If it were dead easy, the speaker makers would have done it
I suppose.

Ian

not an expert