installing an analog audio output jack on a television?

[peace brainerd]

I didn't realize this could be so problematic. I thought this was one of the simplest electronic feats possible (My knowledge is pretty elementary)

I would like to get a signal level (appropriate for aux inputs on a standard integrated amplifier) tap/pigtail/pickoff from the stereo speakers inside of my plasma television.

I was told to simply use a 47K resistor (approx) in line in order to drop it to an appropriate level. I was alternately warned that the television's audio amplification might be using a bridge circuit with no common ground and I would need to install isolation transformers in line as well as the resistor.
TRANSF 10K AUDIO CTAP/2K CTAP - TY-142P

Will a mulitimeter tell me all I need to know off of the terminals? Or would a bridge circuit still read zero ohms between COMM and ground at the speaker's terminal? Anybody here game to describe a simple circuit for this (identifying the appropriate taps on that xformer should it be req'd?)

... in case you're curious....

...no I cannot use the television's existing analog or spdif optical outputs. Neither of them (inexplicably) respond to volume/mute from the remote control, and both of them (inexplicably... how hard would it have been to correct this at the factory?) exhibit lip sync issues that the internal speakers do not (these were, thankfully, correctly adjusted to sync with image processing delay at the factory). There is a lip sync adjustment in the television's commands for the spdif output, but the volume level is still not controllable via the remote, and my amplifier does not have optical inputs or its own remote control. I can get an optical/analog converter for only 22bucks at monoprice, but will still have the volume control issue. A simple stereo male RCA pigtail tapped to the internal speakers solves all of these issues very simply.

So would, of course, a new AV receiver with optical inputs. Minimum 200 dollars. Another 150 dollars for entry level speakers. Wires everywhere. Room for those speakers. Clutter. Another remote control. Another user's manual to learn. Not an option at this point.

[Pano]

Even if it's bridged, you could just put a 600 ohm resistor in each leg and not worry about it. Worst case the ground of your preamp will be 600 ohm from the low bridge leg. Not going to hurt anything.

You do want to check for DC, tho. If there is DC between the speaker lines and ground, you'll need a DC blocking cap. It's important to check. I did a mod like this on my circa 2001 JVC CRT TV. Worked like a charm. No cap needed in that case.

[peace brainerd]

Thank you very much for that. Can I infer from some of this that anything in the range of approx 600 to 47K resistance is appropriate to this task?

And regardless of whether, or not, there is DC between the speaker leads and ground, and I may not measure it properly to begin with, would it hurt to simply be on the safe side and put a DC blocking cap in there anyway?

What size cap? A cap passes AC and blocks DC. So simply put the cap in between the lead that showed DC voltage and ground to keep it from circuiting? Or put the cap in series with the resistor to protect the circuit at (from) the end?

Sorry. I'm embarrassingly bad at these fundamentals.

While I'm embarrassing myself.... is it important which leg I put the resistor on? they should go on the ground leg...no? If the ground is not common in the set, but is so in the device I'm connecting the RCA plugs to, then it's that circuit that needs the resistor to keep it from being completly bridged?

If the "common" lead in the RCA pigtails are connected to the negative tabs on the speakers, and the television's amplifier output is not common ground, and there is a chance that the negative terminals at the device I'm plugging into are merely bridged, this is evidently where the circuit needs a resistor? I realize the resistor here is being used primarily to lower the signal from speaker level to something more appropriate to the AUX inputs on the amp. But it's also doing duty as protection in case the television's amp is not common ground?

[Mooly]

I'll just add a CAUTION to all this and that is that older TV's (CRT types) often have a live chassis irrespective of mains lead polarity. Because someone, somewhere will think "thats a good idea, lets feed the TV into the hifi".

If you have such a TV then you can not just add an R/C feed. You must use a suitable AUDIO isolation transformer.

As a generalisation, if the TV has scart sockets and AV inputs and so in it's probably fine but make sure first.

If it has just one aerial socket then it probably isn't.

[westom]

Originally Posted by peace brainerd

Can I infer from some of this that anything in the range of approx 600 to 47K resistance is appropriate to this task?

No. Don't screw around. You do not know if the TV has a 'floating ground'. A floating ground can create human safety problems, create ground loops, and other problems.

Another has defined an excellent solution - galvanic isolation. That transformer. You have zero reasons to not do that.

I am particularly happy to see you found a rare person who actually knows about floating grounds and how TVs can work. Take his advise - I say bluntly.

[Pano]

Originally Posted by Mooly

As a generalisation, if the TV has scart sockets and AV inputs and so in it's probably fine but make sure first.

That's good advice. Many old TVs did not have a safe ground. My JVC had fixed level audio out via RCA and also A/V inputs, so at least that part of the chassis was not a live ground. It was not a shock hazard. Check the speaker leads to see if they float or if one is at ground.

A 2-4uF cap will be fine for just about anything. You can go as low as 0.5uF without much harm to TV sound. If the negative leg of the speakers is common to ground (antenna connector, A/V connectors) then you need only put your cap and resistors on the + leg. If it's a bridged amp with DC on it, put a cap and resistor on each leg. A transformer is a very good option for a lot of reasons.

[Mooly]

Originally Posted by westom

I am particularly happy to see you found a rare person who actually knows about floating grounds and how TVs can work. Take his advise - I say bluntly.

Well it used to be the day job

[cbdb]

Did you check the tv menus for a switch for the line out to be controlled by the tv volume ?