Duplicated tv speakers audio and now there's noise when I connect something - Page 1
Forum section: electronics / beginners

author: azigta
author_rank: Regular Contributor
timestamp: June 06, 2017, 06:24:07 pm
content: Hi, recently I putted an hdmi tv decoder to my mother's tv, and the thing is this last christmas my brother and I bought her some wireless headphones and she was using them until now. She can't use them anymore because the tv doesn't have audio output when you connect something via hdmi.So I figured out, well, I just solder some cables to the tv speakers, and I adapt it to a female jack, so then I can plug the wireless emitter base there and problem solved. I know how to do this, and in fact it works, only that I hear a horrible soft but loud "bzzz" kind of sound along with the tv audio (so this indicates that the cables are not mixed up, right? if it was shorted (example l/r channel connected to ground) I wouldn't hear anything but a loud low tone, right?)The speakers on the tv are rated 8ohm 15w, and I forgot to put a switch to disable the speakers in order to hear just through the jack and not from both the tv and the jack, does anyone here think this could be the problem? The noise appears when I connect just a jack adapter (6.35mm jack to 3.4mm jack) , with no device attached to it.Maybe there's something wrong with what I did? I don't think I made any mistakes but I can't find a solution for this problem... The tv speakers have a big terminal and a small terminal, the big one being the ground and the small one being Left or right channel, and I soldered those to the matching tabs on a TRS 6.35mm female jack, using the pinout reference in electrodroid. So It was pretty easy to do, no idea what could be wrong there... Maybe the cables that run from the speakers to the jack are too long? Or the cables I used are not adequate for this? (just regular thin copper wire with insulation like 2mm thick) Or should I solder those cables in the pcb instead of in the speakers ? Just thought it would be easier to solder it to the speakers but I have come up with this idea right now, that might be the problem. But again, no idea.Also, if I put a switch to switch the audio output through the speakers or through the female jack, could I do that with a simple shaft toggle switch or would I have to do it via some circuit? I would appreciate a lot if anyone could help me with thisThanks

author: Audioguru
author_rank: Super Contributor
timestamp: June 07, 2017, 02:14:01 am
content: You should not connect anything to the TV circuit because it might not have a transformer to isolate its circuit from the AC electricity which might kill a user with a shock.Since the speakers are powerful then their level is much too high to feed a wireless transmitter, use an attenuator.The amplifier for the speakers might be "bridged" using an amplifier for each speaker wire so you must know how to find a ground for the attenuator.

author: sokoloff
author_rank: Super Contributor
timestamp: June 07, 2017, 02:46:34 am
content: I'd get something like this and be done with it:https://www.aliexpress.com/item/4K-Digital-HDMI-input-to-HDMI-3-5mm-Stereo-Audio-Output-Extractor-Decoder-1080P-black-for/32773235274.html?Monoprice or Amazon will also have something along the same lines.

author: sokoloff
author_rank: Super Contributor
timestamp: June 07, 2017, 02:50:12 am
content: For not much more money and much faster shipping, Amazon hasthis one with twin RCA jacksorthis one temporarily on sale for $13 with a 1/8" stereo mini jack(also Prime).

author: azigta
author_rank: Regular Contributor
timestamp: June 07, 2017, 01:30:45 pm
content: Quote from: Audioguru on June 07, 2017, 02:14:01 amBut could that explain the noise appearing when I connect just a jack adapter with no wireless transmitter or any speaker attached to it?Quote from: Audioguru on June 07, 2017, 02:14:01 amI will check continuity with the ground cables going to the speakers and see if they have separate grounds. For now I will measure that, and put a 3PDT switch to select between audio going to the speakers or to the jack, in case it's common ground for both spakers. I never heard of an attenuator so I will also read about that, thanks

author: azigta
author_rank: Regular Contributor
timestamp: June 07, 2017, 01:35:14 pm
content: Quote from: sokoloff on June 07, 2017, 02:50:12 amQuote from: sokoloff on June 07, 2017, 02:46:34 amYeah thanks, I'm trying to do this using the stuff I already have but I will consider buying that If I don't succeed with this project.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 07, 2017, 11:58:43 pm
content: As almost all headphones have a common ground, and almost all stereo speaker systems (including the ones in tv) are driven deferentially, I can only assume you simply connected the grounds of the speakers together. That is the cause of your problem. I see why you did it though, as not all tv's have headphone sockets or analogue audio out (like that ghastly old SCART standard).The HDMI audio extractor looks to be the best idea. But if you want to use the signal from the speaker then I think you will have to use isolation transformers, one for each channel with their grounds commoned, then perhaps low value voltage dividers to reduce the amplitude. Thats the cheapest way I can thing of getting 'headphone out' from tv's speakers. But probably not the safest, or easiest.

author: tooki
author_rank: Super Contributor
timestamp: June 08, 2017, 12:50:08 am
content: If it's a modern TV, then the audio amplifier is almost guaranteed to be a class D amp, and unless specifically designed for headphones, those things have differential speaker outputs that cannot be joined directly. But it can be done using a couple of resistors, as shown on this page:http://www.edn.com/design/analog/4313726/Add-headphones-to-a-Class-D-amplifierI've tried it and it worked fine, might be worth the cost of 6 resistors to try.

author: azigta
author_rank: Regular Contributor
timestamp: June 08, 2017, 12:28:41 pm
content: Quote from: Buriedcode on June 07, 2017, 11:58:43 pmRight, that's why I joined the grounds together, I have only worked with headphones and I always join the grounds of both speakers together, however, if joining the grounds causes that bzzz sound in the tv audio, I wonder... Why does it only do the noise when I connect a jack adapter? The grounds are soldered together before I insert the jack adapter, so it should do the noise without connecting the jack, if connecting both grounds is the problem. Right? Now this is getting interesting xDQuote from: tooki on June 08, 2017, 12:50:08 amHi, I will look at what IC does the audio come from and verify it's a class D amplifier. In case it is a class D amp, would those same resistor values work ? Or does it depend on the IC that outputs the audio? How could I find the correct values for the resistors? I have plenty of 1/4w resistors, would those do? Thanks