need to add headphone jack to an LCD TV with NO outputs

Discussion in 'General Electronics Chat' started by vladh, Mar 9, 2013.

  1. vladh

    Thread Starter New Member

    Mar 9, 2013
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    Greetings,
    I have a small project that I need help with. I have a new LCD TV that has NO outputs of any kind, no RCA outs, no headphone jacks - nothing. I need to build a headphone connector into this TV (it's for a senior who lives in a care facility, she can not have the speakers on but can use the TV with the wireless headphones if she could connect them to the TV). Ideally I would like to have the jack automatically switching off the speakers when the headphones are plugged in, but a separate switch coulld also work. Now here's the problem: the speakers inside a TV do not seem to be connected to a common ground wire. The headphones jack is the three-wire connector where one wire is shared (ground) between the two channels. The speakers inside TV are 8Ω, 5W small speakers. I would greatly appreciate guidance on how to make this work and what parts are needed.
    Many thanks in advance.
    P.S.: No I cannot use another TV - this one is unique because it plays movies from a USB key and is 25" in size. Any other model that can play from USB is 40" and larger and that's too big for where it is going to live.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,991
    3,737
    Some newer TVs are using H-Birdges for speaker output so there is no ground and more power can be had at a lower voltage. I cannot think of an immediate solution of how this can be done.

    Also, is there a capacitor in parallel with the speaker connection? An option may come soon.
     
  3. vladh

    Thread Starter New Member

    Mar 9, 2013
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    Here is the picture of the board where speakers connector is (circled red). It seems there are some components connected in parallel with speakers - the black ones with "100" printed on them - I do not recognize them (I haven't done any electronics in 20 years).

    [​IMG]
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,357
    The big black things are inductors. The output amplifier is a class D amplifier and requires the inductors to filter out the switching signal.

    You will need a stereo jack with switches on both channels.
    You will connect L+ and R+ to the headphone jack, with a 100μF capacitor in series with each channel.
    Take a GND signal from off the board to the GND of the jack.

    Could you provide a clearer photo of the 48-pin chip or tell us what is written on the package?
     
    Last edited: Mar 9, 2013
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  6. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    What are the chances p1104 is an auxiliary output?
     
  7. vladh

    Thread Starter New Member

    Mar 9, 2013
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    No idea. How would I find out without frying the TV?
     
  8. sheldons

    Active Member

    Oct 26, 2011
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    nononooooooo.......what model tv set are we dealing with?if its a standard lcd with a smps unless you know exactly what you are doing you may risk a nasty surprise shock wise-you need to remember the chassis is always live-if you wish to connect a jack use an audio isolation transformer....that way you are in no danger of getting a shock.give me make and model and ill show you how to do it safely......if the set is the type that runs of 12v dc(not an smps 12v type) supply for example its ok to add a jack with no tx
     
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  9. vladh

    Thread Starter New Member

    Mar 9, 2013
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    It seems to be saying NTP-7500L:

    [​IMG]
     
  10. vladh

    Thread Starter New Member

    Mar 9, 2013
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    It's LG 24MA31D
     
  11. sheldons

    Active Member

    Oct 26, 2011
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    ok im just having a look thro my manuals-more important does it use an smps as the main supply for the set?
     
  12. vladh

    Thread Starter New Member

    Mar 9, 2013
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    I am not sure how to tell. The power supply says the following:

    Model: LCAP16B-A
    output: 19V, 2.1A dc
     
  13. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    so is it a separate supply for this-if so post a pic of it if its an smps type you need audio isolation transformers when adding your headphone socket (two of them are needed)-and the jack needs to be a stereo type
     
  14. vladh

    Thread Starter New Member

    Mar 9, 2013
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  15. vladh

    Thread Starter New Member

    Mar 9, 2013
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    Ok, so I posted the picture of the power supply - is it an smps power supply? What's my course of action if it is and if it isn't?

    Thank you
     
  16. sheldons

    Active Member

    Oct 26, 2011
    616
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    ok that is very much a switch mode powersupply-what you need to do is obtain two audio isolation transformers-have a look online there should be plenty about and you need to connect the transformer between the amplifier output and the headphone socket,i have a schematic which is the next model up to yours but not sure about the audio stage being the same as yours,due to the design of this models op stage if you ground either side of your two speakers the audio ic fails ....i should have an answer for you shortly.
    what you could do is switch the set off,disconnect your smps and pull your speaker socket out of the board and just connect one side of your meter to the ground on the pcb and the other on the L- and R- in turn and check for continuity......we need to make sure of this before we go to the next stage to avoid any expensive mistakes....in the meantime ill have a further trawl thro the manuals i have im sure ive got it because that model rings a bell-im sure i had a dead one a while back.
     
  17. vladh

    Thread Starter New Member

    Mar 9, 2013
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    Thanks Sheldons. I checked the connections and neither side of the speakers is connected to the ground. I will try buying the transformers tomorrow - what should I look for? any particular specs?

    Your help is greatly appreciated.
     
  18. vladh

    Thread Starter New Member

    Mar 9, 2013
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  19. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    no reason why they wont work,usualli i scavvange them from my spares dept that i have lying around
     
  20. vladh

    Thread Starter New Member

    Mar 9, 2013
    11
    0
    Ok, so I went to a couple of local component supermarkets and found that the staff was useless, stocks less than impressive and I got not parts and no help there. Frustrating. Looks like I will end up ordering one online. So I need to ask the gurus: please help me locate the right transformer at a good supplier who can ship fast to Toronto, Canada. What do I need to look for in a transformer? What parameters does it need to have, i.e. 8Ω primary coil, 1:1 ratio; what else? Can anyone give me a link to a part that will work?

    Thank you
     
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