Automatic switch for output amplifiers - controlled from the speaker cables

People with a passion for hifi equipment and active speaker units are bound to have sought ways in which to switch on the output units via the pre amp. Funnily enough, many hifi manufacturers seem to regard automatic switch mechanisms as an unnecessary luxury. Automatic switches are, however, extremely useful and avoid having to lay yards and yards of leads throughout the house. Instead, a single or several "remote" active units may be switched on by way of the original AF lead. As the switch mechanism is always "listening in" anyway, it is also able to detect the prolonged absence of a signal, in which case it will simply switch off the output unit. Relatively few components are required for the circuit. Basically, it involves a double opamp, a timer IC and a relay to switch the mains voltage. Opamp Al is connected as a non-inverting AC amplifier. Note that its negative input is connected to the positive supply voltage by way of R3/C2. This prevents the relay from operating as so...