Line powered audio switch, is this feasible ?

Hi

I've got a mechanical audio switch that does this:

o- Spk L Out L -o--__ o- TV L

o- Spk R Out R -o--__ o- TV R

It's really a simple thing, and for my own knowledge, I was wondering if it would be possible to replace this with an electronic switch. I can always use analog switches (FSA4157A, ADG884 for instance) but these require an external power source. So I was just wondering if one could draw enough current from the line to actually power the switches. If this is not feasible, that's no big deal, I'd just like to hear about possibilities in this area.

Cheers Olivier

By 'line powered' do you mean powered by the audio signal ? Is it speaker level ?

Graham

Ok - if it's 'line level' there's no power in the signal to talk of. It might just be possible to do something with a speaker level connection.

Other than that it's down to 'phantom power' being used. Are you familiar with this ? The centre tapped transformers suggested by Martin could be replaced by active circuitry btw. Then again you could replace the switch with a relay and simply run a DC control / power signal to it.

Another option is to find a sound card with 2 sets of line outs, wire them independently to their destinations and select the one you want from the sound card's control panel.

Graham

try center tapped tranformers and phantom power to a relay at the distant end. QED

martin

After the first death, there is no other. (Dylan Thomas)

It comes out of my PC, so I guess it is Line level. Basically, for the "fun" of it, I just want to select between two destinations for the signal, depending on a touch button. Press, goes to A, Press again, goes to B.

bet you cant do that with "one touch of a button "

martin

After the first death, there is no other. (Dylan Thomas)

Who said GUIs make sense ? :-)

Graham

Why not just a DPDT, push-on, push-off switch? I guess it'd be more accurate, with a DPDT, to call it "push-A, push-B". :-)

Good Luck! Rich

Yeah, that's definitely possible, but it has some mechanical parts, and I'm in my "all electronics" period. So i'm just wondering if it's possible to draw enough power from a regular line out of a PC to drive very few components that would act just like a DPDT switch. But it seems I'm dreaming awake, so i'll let that great idea go away...

Well, the output works well with headphones, but I agree these do not require much power anyway.

No, I'm not familiar with phantom power, and I would greatly appreciate any pointers. I will do a search on google, but pointers from those with experience is always better.

That is not an option, it's the laptop sound card I'm talking about, and I can't replace it.

Thanks Olivier

Others have already discussed options and feasibility. To fill in the background:

"Line level" from a non-pro-audio sound card (that is, the kind whose output is on a stereo minijack that you can plug headphones into) is typically a nominal voltage (at full output) of a bit less than 1V, with a source impedance of perhaps 100 ohms. So, with maximum signal, the power you can get is 1V at 10mA, that is, 10mW.

But that's assuming you're sending a lot of signal out the output. You also need to consider what happens in between songs, when there is no signal at all. At that point you're getting zero power.

And, you need to take into account that if you do derive power from the audio signal, you're going to be presenting the sound card with an additional load. It is challenging to turn AC (your audio signal) into DC (what you probably need to power your switch circuitry) without creating a load that is different at different voltages; that's what people are talking about when they talk about "power factor correction" in power supplies. The consequence of an uneven load in this case is significant audio distortion.

So, plan on being able to derive around a microwatt of useful, reliable, continuous power from your consumer line-level audio output. And even getting that will be a little tricky.

But why bother? If your circuitry can run on a microwatt - which is not impossible these days - then it can run on a lithium battery for 10 years. Is that really so unacceptable? It's probably longer than the life of the computer.

Still, sometimes mechanical is the right way to go. This is one of those times.

crood circuit on

martin

After the first death, there is no other. (Dylan Thomas)

Thanks,

That requires a battery, which is not really good.

How do headphones work? Where do they get the power from? Or are they small enough that the very little current on the line is enough?

just reposted it with a battery free remote end. just watch out for putting too much current through the transformer

martin

After the first death, there is no other. (Dylan Thomas)

Is it me or there are two batteries in that new diagram? Sorry to bother you with that.

nope ,it's you!

the top cct, just has a power source at the sending end, and the center tap on the distant end drives the relay coil directly.

The lower circuit has a second power source, so that you dont send a lot of DC through the tranny, just enough to turn on the transistor, which controls the relay. This is the prefered method, by us, who know these things!

the screen of the cable it how the circuit is completed.

This may not be suitable for you if you are running single screened cables. Normally most pro audio stuff is run balanced, But doing what you want without external power is basically "difficult".

martin

After the first death, there is no other. (Dylan Thomas)

Oh okay. Didn't see it like that.

That, I understood already.

Ok, I'll forget it then and keep my mechanical inverter. Thanks for the help.

You want your "Line out" to go to either the "Line in" of equipment A or the "Line in" of equipment B, right?

Why not just "Y" it and sent it to both? You're not going to be using them both at the same time, are you? Just let the one you're not using ignore the input. If you're ookie about isolation, and don't trust your grounds, then put a 1K ~ 10K resistor in series with each arm of the "Y".

+----/\/\/\/----- to A | From Source ------------+ | +----/\/\/\/----- to B

Cheers! Rich

If you want to switch it remotely via a single two conductor wire, you could use the phantom power of the sound cards microphone input or power from the sound cards MIDI\\game port to power an analog switch.

Alternatively you could derive power from a serial or parallel port connector or a USB port.