Wiring a TV to & stereo to same pair of speakers!!!

[Bucks]

Someone has wired a tv & small stereo unit to a set of sterio speakers for my parents, by just connecting all the positive wires together for say the left speaker, TVs, stereo all together with a chocolate box, then the same for the negative & so on! Is this ok & safe as there is no switch or anything to switch between the two different devices, I assume only one can be switched on at a time???? What would happen if both tv& sterio was on at same time competing to send audio to the same speakers?

Hope I've made this clear enough & someone can help? I recon it needs some sort of switch?

 

[oldbutnotdead]

That's a rubbish setup if they've linked the loudspeaker wires together- no predicting what would happen if both units were on at the same time but good odds they wouldn't enjoy the experience.

But on most TVs you won't get a direct out from the loudspeakers, it'll be line level out via SCART or phono sockets. If they've linked these with some other thing going into the stereo (due to no spare inputs) then it is still rubbish but won't go bang. A photo or 2 will help determine what has actually been done, what brand chocolate box has been used for the joint?

You can get speaker switches- better method is to get line level from the TV into the stereo (TV audio output stage is going to be fairly pathetic) and thence to the speakers.

 

[Bucks]

Thanks for the reply, I'm not there now so can't get a picture at the moment!

 

[Sam Gangee]

Connect the TV audio out (yellow + white phonos or Scart pins & 3) to the line input on the amplifier. Select line in when you want to hear TV audio. Simple.

 

[Lucid]

Someone has wired a tv & small stereo unit to a set of sterio speakers for my parents, by just connecting all the positive wires together for say the left speaker, TVs, stereo all together with a chocolate box, then the same for the negative & so on! Is this ok & safe as there is no switch or anything to switch between the two different devices, I assume only one can be switched on at a time???? What would happen if both tv& sterio was on at same time competing to send audio to the same speakers?

Hope I've made this clear enough & someone can help? I recon it needs some sort of switch?

Wow.... where to start with this...

First off, wiring two amps (the TV and the stereo) in to a single set of speakers just smacks of bodgery. The current from one amp will flow out to both the speakers and to the other amp, and that's where the problem is. Current flows to the path of least resistance. Depending on the quality of the stereo then the speaker impedance could be anything from 3 to 8 Ohms, and that's massively more than the output impedance of speaker outputs on the TV or stereo system. At a rough guess they'll be 0.1 Ohm at most and possibly less than that. 3/0.1 gives a 30:1 difference, so for every 1W making it to the speakers then 30W is going somewhere it shouldn't.

The quickest way to kill an amp is to short out the speaker terminals with a very low resistance (or zero resistance) load. The amplifier wired in parallel with the speakers, which is what you have now, does exactly that. The 0.1 Ohm load can draw a lot of current to the point where it kills both the live and the switched-off amp. That's not good. The saving grace is that this won't happen until the volume on the TV or Hi-Fi amp reaches a certain level, but it's really only a matter of time.

As long as the TV really does have speaker outputs then a speaker switch is the answer as you correctly surmised. Something like this BTech switch for around £14 will do the job.

 

[winston1]

Someone has wired a tv & small stereo unit to a set of sterio speakers for my parents, by just connecting all the positive wires together for say the left speaker, TVs, stereo all together with a chocolate box, then the same for the negative & so on! Is this ok & safe as there is no switch or anything to switch between the two different devices, I assume only one can be switched on at a time???? What would happen if both tv& sterio was on at same time competing to send audio to the same speakers?

Hope I've made this clear enough & someone can help? I recon it needs some sort of switch?

Something like this BTech switch for around £14 will do the job.

Well that certainly WON'T do the job. That is for switching two sets of speakers to one amplifier not the other way round. If used it would still be possible to switch both amplifiers together.

 

[Lucid]

Well that certainly WON'T do the job. That is for switching two sets of speakers to one amplifier not the other way round. If used it would still be possible to switch both amplifiers together.

You're correct in that it's not designed for the task, and also that it does present a possibility of having both amps on live and connected to each other. However, finding a speaker level switch so that two amps can feed a single pair of speakers is a hell of a challenge. That's not the same as saying it's impossible, but there just doesn't seem to be a large selection of such switches.

So as long as the switch isn't designed with transformers for impedance matching and it doesn't include anything with a diode-type restriction on the signal direction then it should be possible to use the switch in reverse. Therefore it would actually work, albeit with some caveats, and it would be infinitely preferable to having to two amps and single set of speakers permanently wired together as they are at the moment.

 

[Bucks]

Thanks Guys for the replies, it's been sorted now

 

[Sam Gangee]

Would you care to divulge how, in case someone else wants to do it?

 

[Bucks]

Pulled it all out LOL
The TV just plays through its own speakers now & the sterio is wired through the ceiling speakers!

 

[Ricardus]

I have a QED MA19 switching unit for this task - switch one pair of speakers between the AV Amp (Marantz) and the HiFi Amp (Rotel). They're out of production but they can be found on EBay occasionally.