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Bypassing TV's internal speakers to home-made aux input

Discussion in 'Audio' started by hedgehog90, Dec 16, 2017.

  1. hedgehog90

    hedgehog90

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    Sep 1, 2012
    I've inherited a TV with good picture and sound, but absolutely atrocious sound. I want to hook up some old PC speakers up, while retaining the ability to change the volume with the remote.

    The TV has an aux input but it's not very good. It has a separate volume setting in a menu somewhere and sound still comes out the internal speakers.

    I want to bypass the internal speakers completely to a female aux input which I can plug my PC speakers/headphones into.
    Figured it would be pretty easy to bodge something together.

    This is what I found inside:

    [​IMG]

    Seems very simple, each speaker (ringed) receives 2 wires from the board, just like 2 phono cables.
    I only unplugged the white (left) and red (right) wire from the speaker, the black cables are firmly clamped on.
    I wired a female RCA/phono wire to each of the speaker wires, which I connected to my headphones, but the audio was drowned in hiss on both sides.
    However, if I just wire the left or right individually, I hear it perfectly clear and crisply through my headphones, it's only when I have both that I get tons of hiss.
    And if I connect both female phono wires to one side then I also get a perfectly crisp sound (albeit mono).

    Can someone explain what's going on and how do I fix it for perfect stereo?
     
    Last edited: Dec 16, 2017
  2. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Is that a SCART socket I see on the rear panel? (above the antenna input). If so, why not take the audio from there?
     
  3. Alec_t

    Alec_t

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    710
    Jul 7, 2015
    There also seem to be RCA stereo sockets on the rear panel. Is the volume from there controllable with the remote?
     
  4. hedgehog90

    hedgehog90

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    Sep 1, 2012
    I just happen to have a 2-way SCART to RCA so I tried it out. I was surprised to find if I plugged my headphones in it worked! But unfortunately the sound is a bit delayed and I have no way of adjusting the AV sync, and it also means I can't use the scart socket for the input of my (rather outdated) Sky box.

    Unfortunately it only receives, doesn't output :(

    Still wondering why left+right = lots of hiss, but just left = fine, just right = fine, left+left = fine, right+right=fine.
    For some reason when I combine the two as god intended I get a bloody load of hiss.
     
  5. BobK

    BobK

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    Jan 5, 2010
    The outputs are probably bridged, which means neither of the wires are at ground. When you connect both channels to RCA jacks, you are shorting two wires together since each RCA jack has one side going to ground.

    Bob
     
    Harald Kapp and JWHassler like this.
  6. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Hi Bob,

    While posting on an electronics forum, I have very little understanding of circuitry...
    What is the solution?
     
  7. Audioguru

    Audioguru

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    585
    Sep 24, 2016
    Since you don't know what you are doing then don't do it.
    There might be a severe electrocution hazard if the TV does not have a power transformer. The circuit might be connected to one of the electricity wires.

    Since the speakers are tiny they cannot produce low audio frequencies so the circuit probably does not give them any low audio frequencies. You don't want that.

    I get hifi line levels to my amplifier from my cable TV decoder box. Then my TV remote is not used for anything.
     
    Harald Kapp likes this.
  8. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Well I haven't electrocuted myself yet! I make sure it's fully powered off before fiddling with it.

    As I said, when I connect either left or right individually to my headphones the sound is perfect, I don't think there's any limiting taking place.
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    Don't do it! If your TV does not have a power transformer then its wiring might be connected to one of the electricity wires in your house. If you connect the TV speaker wires to your stereo then the metal case and speaker wires of your stereo might also be connected to one of the electricity wires which is extremely dangerous because one of the stereo products might be grounded and you will be electrocuted if you touch both products at the same time.

    Your headphones have 3 wires because they have one side connected to the other side as the common ground.
    But the TV speakers probably have 4 completely separate wires with no common connection between them, the amplifiers are called "bridged", also used in cars. Your headphones probably wrongly shorted the black speaker wires together. Usually that short would destroy the sound amplifier in the TV.
     
  10. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Some more info:
    - The TV in question is a Panasonic TX-L32EM6B.
    - It has an internal transformer.
    - The internal speakers are listed as outputting 5 watts each.
    - It's a fairly cheap TV which I inherited, if it breaks it's not the end of the world.

    Surely there's a way to wire a 2 channel bridged amp to a standard stereo output socket (2 RCA/3.5mm).
     
  11. BobK

    BobK

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    Jan 5, 2010
    There is a way.

    You would need to use an audio isolation transformer between each speaker output and the headphones. After the Isolation transformer, you can connect one wire of each to form the common without shorting the wires from the speakers.

    Here is an example of what I am talking about:

    Audio isolation transformer

    Bob
     
  12. hedgehog90

    hedgehog90

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    Sep 1, 2012
    That looks good, but I've read lots of negative comments saying it badly affects audio quality, particularly bass.
     
  13. BobK

    BobK

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    Jan 5, 2010
    Probably true if you are talking about high end equipment. For this application, not so much.

    Bob
     
    Last edited: Dec 17, 2017
  14. Audioguru

    Audioguru

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    585
    Sep 24, 2016
    If there is no household electricity on the speaker wires when the AC electrical plug is plugged in one way and the reverse way then you do not need another transformer. An "audio" transformer probably cuts high and low frequencies.

    I told you that your headphones have a common ground wire for both channels that you connect to the TV's circuit ground, then feed the red speaker wire from each channel through a coupling capacitor to each headphone signal wire. You might need resistors (220 ohms?) in series with each headphone to reduce the maximum loudness.
    But the TV speakers are so small that they produce no low frequencies so the amplifier circuit in the TV probably does not produce any low frequencies. It might not even produce the highest frequencies played by tweeters.

    My TV and my sound system for it are not "high end" but the audio is truly high fidelity and sounds great.
     
  15. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Ah... I've just tried the USB functionality on this TV to try listening to some music and comparing the outputs. Audioguru was right in his assumption that the signal going to the speakers is really lacking in higher frequencies.
    When I was watching TV it was unnoticeable, but it's very noticeable with high-fidelity music.
    Looks like the builtin headphone socket or digital optical output are my only course of action now...
    *sigh*
     
  16. BobK

    BobK

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    Jan 5, 2010
    Really? The isolation transformer would work perfectly well, all you have to do is place it between your already wired jacks and the headphones / speakers. It is not going to make your audio any worse than it already is in a cheap TV audio circuit. I had to use one of these between by computer output and stereo system because of ground loop problems and it did not noticeably affect the sound.

    Bob
     
  17. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Ok, but when I listen to 1 channel via the speaker wires to 1 channel on my headphones, it has significantly reduced fidelity compared to the built-in headphone output.
    Like Audioguru suggested, I think the signal passed to the speakers has a reduced range of frequencies.
    That said, I've just tried connecting a speaker from some old hi-fi equipment (simply wrapped the wires around the internal speaker's + and - nodes) to one of the internal channels and played some music from the USB - it sounded surprisingly ok. The same method via the headphones it sounded muddy and occasionally crackled (I noticed it on a cymbal crash).
    I will also note that any bodge with the internal speaker wires makes the volume settings significantly louder than they should be. The volume goes from 0 to 100, but I don't see any scenario where I'd want to put it past 20.

    Maybe an isolation transformer will improve things, but I highly doubt it'll provide the same quality as the built-in headphone output.
    It's such a shame Panasonic didn't provide the user the choice to use the headphone socket as the primary sound output. So many people complaining about it online, no solutions though.
    Bloody stupid design.
    If they released the source code I might have a chance of fixing it with some tweaked firmware.
    I've requested the source through an online form, there's no chance it'll work but I figured I might as well ask.
     
  18. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Last edited: Dec 17, 2017
  19. BobK

    BobK

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    Jan 5, 2010
    Yes, any of those looks fine.

    One problem I have with TV shows that I watch from BBC is that I swear the enhance the amount of bass and treble, probably because they expect the TVs to have such bad sound. When played on my good sound system, this results in the musical score being outrageously loud compared to the voices, which then get lost.


    Bob
     
  20. hedgehog90

    hedgehog90

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    Sep 1, 2012
    From another forum, someone said this:

    As I previously noted, if it's a bridge output you can't join the two black lines, as they are not really ground, but likely biased at a DC level.
    The output of a bridge amplifier looks rather like a differential signal with the opposite polarity AC signal (to ground) on each output
    .​

    Can someone explain in plain English what they meant and if it's at odds with what you understand to be the problem?
    It should be okay to join the black lines after the ground loop isolator, correct?
     
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