how to put a signal level tap on my television's speakers?

Discussion in 'General Electronics Chat' started by frascati, Jan 3, 2012.

  1. frascati

    Thread Starter Member

    Jun 1, 2010
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    Basic question is.... what devices/parts do I need inline with a speaker level output to drop it to signal level (ie, appropriate for use with aux inputs on an amplifier)? I also want to be highly confident that such use will not harm or put excessive load on the originating amplifier. The longer story is....

    Astonished that the new plasmas and lcd television sets provide no control to the external audio jacks, neither analog or spdif (optical), via the remote control.

    Mine is a Samsung PN50C550.

    I want to simply add a small powered subwoofer under the television stand. I also want to send audio to a small amplifier that is used by a member of the family that is hearing impaired.

    There are quite a few options for this using the external audio jacks. Non of which will allow me to still use the tv remote for volume/mute contol. This would be bad enough, but an additional problem is that the delay sync is not balanced between the screen and the external speakers. It can be adjusted for optical output via an on screen slider from 10ms to 250ms but the analog out cannot be adjusted. If you want to use an amplifier that has neither optical input, or dedicated remote control, you are apparently just out of luck with these sets.

    I could solve both of these hurdles, lip sync with the screen since internal speakers and screen are already synced by the manufacturer, and volume control via the remote, with a simple tap on the internal speakers to a dual male rca pigtail out of the back of the set . The amp that feeds these speakers may not be powerful enough to feed external speakers. But the powered subwoofer inputs as well as the little amp I'm using for powering headsets for hearing impaired are signal level inputs anyway.

    The television's just beyond warranty, so I'm not worried about removing/replacing eight panel screws in the back to expose the internal speaker's terminals.

    What do I put inline with the speaker tap and the signal level inputs of the subwoofer in order to...
    ....match the level expected by the subwoofer and aux input rca jacks on the small radio shack amplifier?
    ....and be quite confident that I'm not overdriving the audio amplifier within the television set?

    Thanks so much for any advice.
     
    Last edited: Jan 3, 2012
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Instead of tearing apart the TV, why not get a $149 5.1 home theater set to add to the Fiber Out/TOSLINK jack?

    The line out you have is a 3.5mm Headphone jack, which the volume should control, the other line outputs aren't meant to go to a receiver.

    The other option most TV's have is programmable remotes, so the volume up/down would turn your amplifier on your 5.1 system volume up/down while disabling the internal speakers.

    Simply tapping off a speaker could cause all sorts of problems with impedance mismatching to get a line level output, an audio isolation transformer of some sort would be required for isolation as well, so a DIY audio solution doesn't destroy the TV.

    You only SEE 8 screws, if it is a LCD TV, there are probably several dozen before you get to where you need to be.

    --ETA: Found some reviews, I guess the internal speakers sound very crappy, "like a 1970's transistor radio".

    Specs:
    The TV seems to have been designed for an external amplifier, many plasma displays do not even have internal speakers.

    Get the 5.1 system and a fiber cable for the TOSLINK (make sure the 5.1 system has TOSLINK input, most do). Don't buy a "Monster Cable" TOSLINK fiber, they are about 5x overpriced. You should have pretty great sound for about $150 to $200 that will stay with you when this TV dies.
     
    Last edited: Jan 4, 2012
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    What if you snag the signal before the amplifier? between the DSP/preamp and the amp IC?
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I was thinking two Radio Shack audio isolation transformers, putting the 1kΩ side in parallel with each speaker, but getting that to line level would require a good preamp.

    Depending on how comfortable the OP is with circuit design, soldering, etc, the home theater route may be faster, cheaper, and sound a lot better, since I googled the model number and saw a lot of complains about sound quality from internal speakers. Not to mention a few extra boxes and wires hanging out of the TV, and when this one dies (lifespan of 5 years planned), he'd have to start over anyway.
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I was just saying that because if I remember right, when I was playing around with my TV's audio circuit, I probed those signals with a PC speaker headphone (R!f@@ style) and that was a good line level signal. I believe it went up/down with volume change and mute. could be wrong.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I fully agree.

    I know you, I, and most of the "regulars" here could do it.

    We don't know what the OP's abilities and physical tools are, nor do we know his education. All we know is his desire to get better sound, and a willingness to try to tap into a speaker, be it good or bad.
     
  7. frascati

    Thread Starter Member

    Jun 1, 2010
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    Thank you very very much for the input here. A few points then.

    The internal speakers really are not bad at all. One person's opinion perhaps, but in my living room it's the only one that counts. Another of my (admittedly not widely shared) opinions is that experiencing video content via a 50 inch screen is not much enhanced by surround sound. There's just a cognitive mismatch there. I'll just wait for affordable eight foot LCDs before investing in sound that compliments the experience.

    However, the addition of a small decent quality subwoofer makes a huge difference even at very low levels. But it's mitigated considerably by that sync issue. It must be at least a couple of hundred ms off since you can actually distinguish a doubling of individual beats when a single bass beat appears in the audio content. Also, when headphones are plugged into the analog outlet directly, there is audible echo between them and the internal speakers. For this reason, there is distracting echo as well via the headphones used by the hearing impaired family member.

    This analog, 3.5 mm outlet is not controlled by the televisions's volume/mute control. Neither is it adjustable for sync.

    The optical output is adjustable for sync, from 10 to 250ms using a GUI fader. But it also is not controlled by the television's volume/mute.

    Can anyone tell me why, pray tell, this is the case?
    Is there a logical reason why the manufacturer would have set this up in this fashion? I understand that it's not at all uncommon for modern sets. It severely limits the owner's ability to make full use of these outputs.

    There is an 85 dollar box from B&H photo, with stereo analog RCA in/out and a rotating selector for 0 to 200ms. I can use that with the analog output on the set to fix the sync, but am still left with the volume control problem, and I'm a cheap bastard, and 85 dollars is steep for three dollars worth of components assembled in china.
    [​IMG]
    http://www.bhphotovideo.com/c/product/760361-REG/AV_Toolbox_AP_411_AP_411_Lip_Sync_Corrector.html

    There is a 22 dollar box from Monoprice, with SPDIF (optical) in, stereo RCA out that will allow me to use the SPDIF output of the television as well as the spdif lip sync adjustment onboard the television set. This is perhaps the most elegant and cheapest solution. Still unable to adjust volume remotely, but since I generally keep the television at approximately the same volume regardless of content, I can adjust the volume of the little subwoofer on the subwoofer itself to satisfaction and be happy with it. The little radio shack integrated amp for the hearing impaired member of the family has it's own volume and tone controls as well and sits next to a chair.
    [​IMG]
    http://www.monoprice.com/products/p...=10423&cs_id=1042302&p_id=6884&seq=1&format=2

    The 149 dollar 5.1 HT receiver remains a last resort. If it were 25 dollars at goodwill or garage sale I would still hesitate. Next comes 150 dollars worth of speakers, room for placing them, wires connecting them, one more remote (or reprogramming my own at the very least which is far from certainly possible), and another owner's manual to agonize over. I really hate adding that many layers of complexity and clutter for the simple matter of adjusting the lip sync for a hearing impaired member of the family (the most essential need here) and adding a small subwoofer to supplement the internal speakers.

    A signal level tap on the internal speakers would after all have been an extremely elegant solution all the way around, addressing every need I've mentioned, if only I'd discovered that it could be accomplished with a few discrete caps and resistors and a stereo RCA pigtail. But it sounds like that is not the case.

    So the 22 dollar toslink/analog converter it is. Along with a 2 dollar three foot toslink cable (got to love Monoprice!).

    Unless any further ideas arise in this thread.
     
    Last edited: Jan 4, 2012
  8. crutschow

    Expert

    Mar 14, 2008
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    The possible problem with a speaker pickoff is that the speakers may be powered by a bridge circuit and thus not connected to a common ground. For this reason if you want to connect to the speakers I would recommend using a small interconnect type audio transformers such as this for isolation. You can experiment with the connections, using only one side of the center-tap or reversing the input and output to get the proper voltage level out.
     
  9. frascati

    Thread Starter Member

    Jun 1, 2010
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    Is it possible to confirm whether I'm dealing with common ground or bridge circuit with a simple multimeter? Or is Common always going to show zero ohms regardless of whether it's a bridge circuit?

    And could you clarify this for the sake of application? I was advised on another board to simply use a 47K resistor in line and be good with it. When I mentioned your reservations about the possibility of a bridge circuit without common ground I was pretty much ignored. I'd still need the pair of 47K (approx) resistors in addition to the isolation transformers, correct?

    Is there really no good consensus on something as elementary as this on an electronics board? I would think that a "speaker pickoff", or "tap" as I referred to it, whether of common ground or bridge circuit would be pretty basic electronics 101 to many of the contributors here. Am I wrong?
     
    Last edited: Jan 5, 2012
  10. crutschow

    Expert

    Mar 14, 2008
    13,013
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    With the TV unplugged from power you can measure from each of the speaker terminals to ground (such as the common on the analog output). If one of the measurements is a few ohms or less, then it is not a bridge circuit. In that case you could use the resistors as isolation and forget the transformers.

    With the isolation transformers you won't need the resistors. That would be a belt and suspenders design.

    The reason it is not basic is because we don't know how the speakers are driven and connecting to the speaker outputs is something that is not often done. If we knew what the speaker circuit schematic was, then there would likely be more of a consensus on the answer.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    It is pretty simple, once the design of the amplifier is known.

    The other problems that pop up relate to if it is a "traditional" amplifier, Class A-B, or a Class D-G amplifier. The latter of which is more sensitive on loading and impedance.

    If you already have access to the speakers, you can try, I'd suggest getting an audio isolation transformer from Radio Shack to tap in with, using the 1k side of the transformer, in parallel with the speaker leads.

    That way you are providing minimal impedance/load change to the TV amplifier, and will get a small signal out with an isolated ground. The isolated ground is important to prevent noise and keep your new additional amplifiers from shorting out the TV.

    The 47k resistor would bring the level down, but wouldn't isolate the ground. I tend to play on the safe side with such things.

    What have you chosen for the sub amp/sub and for the extra speakers for the hearing impaired listener? I'd suggest a pair of decent computer speakers for the hearing impaired chair, and the sub is your choice.

    The price for the HT system I mentioned above for $150 does include the speakers and sub, but not the optic cable. They do sound great for the price.

    However, if you are going to spend that much on a sub alone, the bass will be better if it isn't filtered out of the standard TV speakers to prevent distortion.
     
  12. sheldons

    Active Member

    Oct 26, 2011
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    beats me why people would buy and use a set when it has such problems-you never had this sort of problem with a crt set-which is 1000 times more reliable and provides better picture quality and will last for years before failing,and when it does is a simple component level repair and not a throw away scrap set.
    do you have a chassis type number as i have several manuals which relate to different chassis fitted in this plasma set?-as regards your fault i believe there is a software update which will cure this
     
  13. frascati

    Thread Starter Member

    Jun 1, 2010
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    Hey, back off man.... despite Carl's byline I'm believe in the running here for resident curmudgeon. You're crampin' my style:):)

    I actually have no regrets replacing my 25 year old Zenith CRT with this 50 inch plasma last Christmas. I'm glad that I made such an extreme jump instead of getting a bigger screen CRT in the interim. Breathtaking. And probably saved a couple of thousand dollars on a 50 inch plasma just by waiting a few years... prices dropped dramatically. But yeah, lot's of bugs to be ironed out yet and none of this stuff, as you point out, is likely to be cost effective to fix if it goes south. But I'm already on the treadmill man. Five years from now I'm gonna want that 8ft LCD like I can't live without it!

    I updated to the latest firmware, 1016.1 about six months ago. My television reminds me of new firmware updates via the web and asks if I wish to update. I've been in and out of Samsung tech service on this and they cannot help me. As a matter of fact, no one that I've connected with via tech help at Samsung even realizes that you can adjust the lip sync of the optical outs on this set from the user controls. In fact they all have assured me that you cannot!!! That is without question incorrect.

    Samsung support typically (i do not exaggerate) knows less about your product than can be learned from your own user's guide. That includes supervisors. I have little faith that a software update might correct this since googling the issue suggests that is has been common to many models and manufacturers of modern LCD and plasma televisions for over six years, and has been inexplicably ignored by the manufacturers.





    [​IMG]


    [​IMG]

    I'm not wholly incompetent with circuits, but I will admit to being often perplexed by grounds, ground loops, etc.

    If a stereo signal source may not be using a common ground, and the device to which you wish to send a stereo signal has a common (bridged in fact) ground, then this is a problem.. no? So the resistor would need to be in line with the ground leg in order to protect against a full "loop". No?
     
    Last edited: Jan 5, 2012
  14. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    In a case where your audio is floating/neither speaker side is grounded, and connecting to a sub where the input is grounded, the 47k resistor will essentially minimize or prevent damage. This isn't the same as saying "it will work perfectly and sound good".

    The only way to get the signal from the speakers to another amp is to use an isolation transformer, which will provide floating ground outputs.

    In the event of 60/120Hz "hum" in the sub, the transformer wire acting as shield should be grounded to eliminate that EMI.
     
  15. sheldons

    Active Member

    Oct 26, 2011
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    so you have updated with memory stick the latest update software wise and it hasnt cured it?im searching out the manual for this set i seem to remember after updating one of these sets a while back i also had to replace a panel to cure it(ssb)but that was a different model...im not at all impressed with reliability of either lcd or plasma sets against a crt set as you can see(from a servicing point of view)but it pays me a wage every time i get a plasma wanting replacement sustain boards and drivers so cant complain.
     
  16. sheldons

    Active Member

    Oct 26, 2011
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    i cannot upload the manual because of upload size restrictions here ....if you want it ill have to email it to you it is a winrar file
     
  17. sheldons

    Active Member

    Oct 26, 2011
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    heres a few pages from the manual
     
  18. frascati

    Thread Starter Member

    Jun 1, 2010
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    Well, I'd all but given up on the possibility of finding a means of adjusting the analog audio sync via the controls, but maybe there is hope yet. Still would not be able to adjust the volume (unless I find an "enable" option for that under "advanced" as well. But that's asking perhaps for too much good luck.

    Mute 1 8 2 Power.
    I'd discovered that from the afsforum.com.

    It opens the service menus with...

    Option
    Control
    SVC
    ADC/WB
    Advanced

    Under "control" there is a "sound" option that allows adjustment for "speaker delay" but I'm assuming that this applies to the internal speakers. It is set to zero. I presently have no issues with this adjustment so I'll leave it. Unless I can determine whether it in fact, for some reason, applies to the analog outlet separately. Naturally, there is no good source of information on this since consumers are not supposed to be in these menus. Also, I am completely loathe to change any settings in here until I'm 110 percent confident of what I'm doing. Is this information in the manual you have?

    There is some possibility that other audio adjustments are available under the "Advanced" tab (last one listed above in the opening menu) but I cannot access it and cannot find out in the forums how to either. I think some models accessed this using 0 0 0 0 in sequence but mine does not. I was hoping that the first page you linked would indicate the code to access "advanced" but I did not see it. I will PM you my email address so you can attach the manual. Gmail should be able to handle the size without a problem.

    Thank you VERY much for your kind offer. One method I've used to share larger files beyond a forum's size limits is to use a site like mediafire.com. It's really straightforward. Just upload the file like you would a photo to photobucket and then provide the url link in the forum post for others to access it. Of course that also might expose one to some risk of property rights violations for something like a service manual? I don't know.

    Thanks again. Will PM you.
     
    Last edited: Jan 7, 2012
  19. sheldons

    Active Member

    Oct 26, 2011
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    unless you know exactly what you are doing its very easy firmware wise to mess your set up.ill email you the manual i have for this model
     
  20. frascati

    Thread Starter Member

    Jun 1, 2010
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    Thank you Sheldons for that. I did not find anything relevant to this in the service manuals. I also did not find any mention of access codes for the "advanced" section of the menu. That's kind of strange.

    In the interim here I ordered and received in the mail that optical analog converter from monoprice.
    [​IMG]

    Plugged it in. It worked!! Happy day!! And in the television's menu under SPDIF settings it was surprisingly easy to adjust out the echo simply by ear. Sliding the fader between 10ms and 100ms there was a very clear cancellation at 80ms. Wow.

    Hold on a minute son... the dark chaotic muses have not finished with you.....

    Start changing television channels. The television only seems to want to deliver audio to the optical port on perhaps 75 percent of them. A mystery. Worse... my dvd blu ray player, and my nextflix delivered via that dvd blu ray player, and my HTPC.. essentially anything connected via hdmi does not get its audio delivered inside the television set to the optical port. It's not the optical/analog converter at fault since I can plug it directly into the blu ray player's optical port and it works fine.

    So I spent the better part of the morning with tech support at samsung and netflix. No remedy.

    So the converter goes back to monoprice, and I guess I order that 85 dollar analog lip sync device to use on the analog outputs since I already know it does provide sound for all sources.

    Unbelievable. Just unbelievable.
     
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