Subwoofer without a Remote Input

[fingers]

I have a factory fitted stereo which doesn't have a remote out to switch my Subwoofer Amp on and off. I have got a power outlet in the boot, so I can use the +V from that to switch the Amp on/ unplug it to switch it off.
I would like to have a way to switch the remote line On and Off automatically when the stereo is turned on. Is it possible to do this from the signal from the speaker inputs? How many volts/ amps(milli) would there normally be going through a speaker wire.

 

[Nigel Goodwin]

How many volts/ amps(milli) would there normally be going through a speaker wire.

None, it's an audio signal, when it goes quiet there's nothing there at all. Sub-woofer amplifiers often have the capability of auto-switching, switching ON when it receives a music signal, and switching back OFF when it's not received any signal for a number of seconds. If your amplifier doesn't include this you could always build one, there are probably various designs about, I seem to recall seeing them in magazines?.

Are you sure your radio doesn't include a switching output?, it's pretty standard these days, intended originally for use with electric aerials.

 

[jrz126]

Why not just tap into the radio's power wire? It should be switched with the ignition already.

 

[Peter82]

I think the remote just needs upto 12v so any live that comes on with the ignition could be used.

 

[fingers]

The power outlet in the boot is permanently on, and I don't know of any wires in the boot which become live when the ignition is switched. I've already put the radio back in the dash, and don't fancy the idea of taking it all out again cos it involves taking the fans out and most of the dash apart - its a ridiculous design.
Just wondered if I could have a comparator op-amp circuit to switch a transistor when the speaker sends an audio signal or something along those lines.
What do you think?

 

[zevon8]

If you only want the amp on when the car is running, then you could sense when the voltage rises to the alternators running voltage. This would keep all the circuitry in the trunk ( boot ). What you would do is have a comparator sense when the voltage rises above say 13 volts, and operate a small transistor to control the remote-on line of the power amp.

If you want to do it without the car running, you will need to sense an incoming audio signal or run a new wire from an ignition line.

Have a poke around the trunk, see if there is a line for a power antenna, or electric trunk release. Either will often only have power when the ignition is on. Even if your car does not have these accessories, often the wires are there.