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

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

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  1. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Not a fan? :)

    All I've got to hand is a little bag of 100 ohm metal film resistors. I'm guessing they're rated for 0.25 W.
    If I run a few of these in series for each signal wire should I expect them to fail?
    Also, if I use coupling capacitors as well, can I expect the output voltages to quarter? Would you recommend using both the coupling capacitors and a ground loop isolator?
     
  2. BobK

    BobK

    7,674
    1,684
    Jan 5, 2010
    Using coupling capacitors and referencing one side of the bridged output to ground will produce 1/2 the voltage on the output. Going into the same load, they will produce 1/4 of the power. But the ear response is logarithmic. 1/4 of the power will not sound 1/4 as loud. Perhaps not even 1/2 as loud.

    You do not need capacitors if you use the transformers.

    Audioguru is giving you the audiophile point of view. If the sound quality of your TV was audiophile quality and the sound was going to $1000 speakers, yes you would hear a large loss of quality. Given the poor quality sound, going into PC speakers, you will not notice much difference.

    The reason the headphones sound better through the headphone jack might be because they are driven by a headphone amp that is lower power but higher quality than then the amp for the built in speakers.

    Bob
     
  3. Audioguru

    Audioguru

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    Sep 24, 2016
    $1000 speakers? The sound system I use for my TV cost me $25.00 brand new and it has distortion so low that my audiophile ears cannot hear it. It is advertised at having 150W (fake Whats) of output but inside it the power transformer is rated for 9VAC/1.1A (9.9 Watts). I figure 1.5W goes to each satellite mid/tweeter and 4W goes to the woofer.
     

    Attached Files:

  4. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Still no capacitors! Grr.

    Here's something interesting - I tried using Audacity instead of the crappy Windows sound recorder to record the sound out from the builtin headphone socket this time, and the waveform was all between 0-1, when it should it have been -1 to 1, so it looked a bit odd in the program, half the size and offset by 0.5. Is that normal?

    It may be a side effect of using the line-in on my laptop, but when recording from my bodge-job aux input, the waveform in Audacity appeared to be in stereo and in the middle... but it's way too loud.
    So perhaps once I've got some capacitors and resistors I'll have something better than the built-in headphone socket.

    I asked before but I'll try again - my tiny 100 ohm resistors, shall I try hooking these up when I get my capacitors or shall I expect a lot of smoke? If the speakers are rated for 5 watts, does that mean I need resistors that can take 5 watts? Mine are rated for 0.25 watts. Should I get some higher power rated resistors?
     
  5. Audioguru

    Audioguru

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    Sep 24, 2016
    You need to learn about Ohm's Law.
    If the output signal voltage to a speaker is 5VAC then when the speaker is 5 ohms the current is 5V/5ohms= 1A and the power is 5V x 1A= 5W.

    But if the speaker is 10 ohms then the current is half and the power is half at 2.5W.

    We do not know the output signal voltage from the TV amplifier and we also do not know the impedance of your headphones. If the output signal voltage is 5V and if your headphones are 32 ohms then the current in each headphone is 5V/32ohms= 0.156A and the power is 5V x 0.156A= 0.78W which will blow them up and probably make you deaf. 0.1W is pretty loud. To make 0.1W in a 32 ohm headphone then the voltage must be 1.8V and its current will be 1.8V/0.056A. Then a resistor in series with a 32 ohm earphone would be (5V - 1.8V)/0.056A= 57 ohms.

    If you use your 100 ohm resistor in series with a 32 ohm headphone and the output signal from the amplifier is 5V then the total is 132 ohms and the current is 5V/132ohms= 0.038A. The power in the headphone is (0.038A squared) x 32 ohms= 0.046W and the power in the 100 ohm resistor is (0.046A squared) x 100ohms= 0.21W and a 1/4W resistor will get pretty hot if the signal is continuous. If the headphones are not loud enough then try two 100 ohm resistors in parallel then connect them in series with each headphone.
     
  6. hedgehog90

    hedgehog90

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    Sep 1, 2012
    Sorry for departing without thanking all of you for your help. I really appreciate all the advice I've gotten here so far.

    Even though I ordered them I decided not to bother with the capacitors, instead I just hooked up my old PC speakers (and subwoofwer) via the ground loop isolator to my DIY RCA sockets.
    I've got the manual volume control for the amp/speakers set to about 1/4, which I think gives it the TV an ideal volume range.
    It sounded great so I figured job done. Why continue to fiddle about when I've got the desired result?

    There is a new problem though...
    When putting it together, I made sure to insulate (with insulating tape) wherever the was a join between 2 wires.
    I had the speakers on full volume while the TV was unplugged, I noticed the hum got quieter by insulating the joins instead of leaving bare wire somewhat exposed.
    I considered the hum at full volume to be acceptable, it was inaudible at 1/4 volume unless I held one of the speakers to my ear.
    However, when I plug the TV into a wall socket without turning on the mains / TV's on switch / standby, they hum very loudly.
    But as soon as they receive a signal from the TV (when it's on / not on standby) the hum is a lot quieter.
    Here's a recording I made with my phone (louder - TV off, quieter - TV on & muted):
    https://clyp.it/iyjjrkmz
    If I put the speakers on max and put the TV on mute, I get a bit more hum than if the TV was unplugged, but no where near as loud as when the TV is off.

    I'm sure you're wondering if I only notice this when turning off the TV, why don't I just turn the speakers off at the same time?
    The problem is the TV and speakers are situated in my bedroom and I rely on the TV's sleep timer.
    I've tried this a few times with the speaker volume at the preferred 1/4 level, and every morning I woke up and immediately noticed the faint humming noise. It's quite disconcerting.

    For now, I've set the speakers to about 1/8 of the max volume, whereby the speaker hum when the TV is off is barely noticable, but I've noticed if I turn the volume up loud (via the TV's volume control) it get's a bit distorted, so it's not ideal.
    Remember that the audio signal from the TV travels through a Ground Loop Isolator. If I remove the GLI and hook the TV directly to the speakers' amp the hum is even worse, and audio from the TV is also affected (distorted and generally a bit rubbish sounding)

    I've also noticed the tone and volume of the hum changes slightly when I remove the aerial cable from the back of my TV when powered off.

    I suspect part of the problem is the fault of the PC speakers/amp, as I briefly tried some different PC speakers (which I'd prefer to keep attached to my computer) and the hum wasn't as bad.

    Any ideas? If the problem is present when it's not powered, would the capacitor trick work?
    Everything I've read online suggests it's sign of a ground loop, but then why doesn't my GLI resolve the problem?

    Thanks and happy new year everyone.
     
  7. Audioguru

    Audioguru

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    620
    Sep 24, 2016
    I think the hum is caused by the wires between the TV and the input of your PC amplified speaker system are not shielded audio cables where the shields block hum pickup from all around you. When the TV is turned on it has a very low output impedance that shorts out most but not all of the hum then when the TV is turned off the unshielded wires are antennas that pickup the hum.
    Tape? WE solder wires together then insulate the joints with shrink tubing. Why don't you?
     
  8. hedgehog90

    hedgehog90

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    0
    Sep 1, 2012
    I've separated the RCA wires & GLI as far away from any other wires with this in mind, it doesn't help unfortunately.
    Can lack of shielding explain the behaviour whereby the hum massively worsens when the TV is plugged in, even though the mains socket is turned off?
    When the TV's on, the hum almost completely disappears. If it was a shielding problem wouldn't I hear the hum when it was on too?

    Have fun with your shrink tubing and soldering, I'm a trash person and I prefer trash solutions ;)
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    Since you do not use normal shielded audio cables that everybody else use then your wires are "antennas for interference pickup" which guarantee hum pickup, especially when the TV is turned off. I already explained that "When the TV is turned on it has a very low output impedance that shorts out most but not all of the hum".
    RCA plugs and jacks are made for shielded audio cable.
     
  10. hedgehog90

    hedgehog90

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    Sep 1, 2012
    The RCA cables are short and appear to be quite well shielded (thick plastic jacket, wires wrapped in foil). They were the best ones I could find laying around without ordering some online.
    The internal wires however (which originally led to speakers on either side) are lengthier, thinner and have no shielding as far as I can tell.
    I can't replace all of it as all 4 wires lead to a small plug on the mainboard, but assume I replaced most of it with shielded cable, that should greatly reduce the hum then, right?
    Maybe I could try wrapping the internal wires in aluminium foil... Would that help in theory?
     
  11. Audioguru

    Audioguru

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    Sep 24, 2016
    All audio products come with shielded audio cables for connecting products together, but speakers do not use shielded cables. Sine I have a box full of shielded audio cables I have not tried to use Mickey Mouse aluminum foil and try to connect it to the ground wire.
     
  12. hedgehog90

    hedgehog90

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    0
    Sep 1, 2012
    It's nothing to do with shielding. I've tried different wires, I've tried applying more shielding, it doesn't make the slightest bit of difference.
    If I plug the 4 speaker wires directly into where they should go, I get hum when the TV is turned off. This is built into the motherboard, they obviously didn't design it with speaker customization in mind.
    I'm certain it's nothing to do with wire insulation or interfering signals.

    As the 2 black speaker wires appear to be common ground, I've tried joining these and the 2 RCA grounds, essentially copying this:

    [​IMG]

    And then attaching a crocodile clip to this and probed around the motherboard with the other end in an attempt to stop the humming / find a ground.
    If I attached it to mount points, the solder point you mentioned on the sides of the RCA jacks, or practically any metal on the case, the hum would appear to be no different as to when it's plugged in directly to the socket on the motherboard.
    The only places on the board that I've found I get zero hum was the power socket and a nearby fuse seating.
    As I have little to no idea what I'm actually doing, I wasn't foolish enough (or curious enough) to turn on the TV.

    I'm just about ready to call it a day on this.
    Even so, any ideas?
     
  13. Audioguru

    Audioguru

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    Sep 24, 2016
    You show a shielded audio cable from the TV low impedance output to the low impedance headphones. The low impedance does not pickup hum and interference.
    You are assuming that the balck speaker wires are gronded together but we keep telling you that the balck wires are probably bridged and must NEVER be grounded or connected together. Also when bridged the red speaker wires have DC on them that must NEVER be connected to speakers or headphones.

    Then I think you are severely overloading the TV amplifiers shorting the black speaker wires and are overloading the DC on the red speaker wires causing the power supply to produce the overload hum.

    Another problem is that headphones are against your ears. Although the little TV speakers cannot produce the low frequency of hum wouldn't you hear it if the TV speakers are better and are against your ears?
     
  14. hedgehog90

    hedgehog90

    29
    0
    Sep 1, 2012
    I'm not using headphones to test for hum, I'm using PC speakers (2 speakers and an amp / subwoofer - a single powered unit).
    Again, the hum from these speakers only appears when the TV is turned off, ie - the TV is plugged into the wall but the mains switch is turned off or the TV is in standby mode.
    I'm not sure you've fully understood that.
    It also produces a small amount of hum if it isn't plugged in at all.
    The moment the TV is turned on, the hum disappears completely.
    The built in speakers do not hum when the TV is off obviously so I cannot test that, however if I plug an amp and speakers into the same wires I hear the loud humming noise.
    How can I be overloading speakers when the audio signal is coming from an unpowered motherboard?

    Also, if I plug the PC speakers into the TV's builtin headphone socket, there is no hum whatsoever (on or off) as you'd expect.
     
    Last edited: Jan 6, 2018
  15. Audioguru

    Audioguru

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    Sep 24, 2016
    I guess the TV is built to produce hum from its internal speaker amplifiers when it is in standby mode and its amplifiers are not powered.
     
  16. hedgehog90

    hedgehog90

    29
    0
    Sep 1, 2012
    Yup, it would appear that's just how it's built.
    So I've been trying the built-in headphone port instead for the last 2 days.
    Besides the frustrating problem of volume adjustment, there's also another problem whereby the speakers (or the headphone socket) just conk out completely. It requires that I manually toggle off/on the speakers or sometimes they'll come back to life after a few minutes.
    Instead of carry on with this I think I'm going to call it a day and just invest in a soundbar.
    Very frustrating.

    Many thanks to Audioguru and the rest. Your help was much appreciated.
     
    Last edited: Jan 7, 2018
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