2 audio channels to 3.5mm jack - Page 1
Forum section: electronics / beginners

author: vladio
author_rank: Newbie
timestamp: November 20, 2023, 07:31:44 pm
content: I have a newer Samsung tv in the bedroom that has no headphone connector or bluetooth. I tried an optical converter but I believe there's some type of DRP that mutes the audio on certain apps (Discovery+, Vudu...). Thinking there was only one speaker I bought a switched 3.5mm headphone jack and, yesterday, tried to install it. Upon opening the TV I realized there's 2 speakers, both on separate channels. I tried to tie both grounds into the ground on the switched jack but got heavy static. Just wondering if this is possible.Thanks for your time!

author: magic
author_rank: Super Contributor
timestamp: November 20, 2023, 07:55:09 pm
content: The speakers are probably driven by bridged amplifiers - the + side is driven normally, the - side is driven by an inverted signal, not grounded. This doubles available voltage swing (alternatively: reduces required supply voltage) and enables operation without negative supply. It's a common trick to simplify the power supply of the TV or other device at the cost of more complex amplifier electronics, which are still very inexpensive and probably integrated fully in one chip.This precludes connecting the negative terminals together.To get external audio output, you would need to intercept the signal at the input to the amplifier chip. Further amplification (or at least unity gain buffering) may prove required to drive relatively heavy loads like headphones.editAlternatively, "balanced" headphones with separate negative connections to each channel could be used. This is an audiophool gimmick available on some high-end headphones, or it could be DIYd on cheaper ones - the cables often have separate grounds, only connected at the plug.A speaker amp may still end up producing too high volume and/or noise for comfortable headphone use.

author: macboy
author_rank: Super Contributor
timestamp: November 20, 2023, 08:25:06 pm
content: I suspect the optical converter didn't work because the TV was trying to send Dolby Digital over the optical S/PDIF connection, when the program being played provides such. This is perfectly valid, and is accepted by any A/V Receiver made in the last 25 or so years with S/PDIF inputs. A simple DAC can only accept uncompressed stereo PCM audio, not Dolby Digital or DTS encoded. There should be a setting in the TV sound menu to output only PCM or Stereo or something similar. Definitely don't choose "5.1" or "surround" or "best available", etc. The TV will automatically decode DD or DTS material into stereo PCM for the DAC.

author: vladio
author_rank: Newbie
timestamp: November 21, 2023, 05:25:33 pm
content: Quote from: magic on November 20, 2023, 07:55:09 pmThanks, Magic! I appreciate your explanation!Quote from: macboy on November 20, 2023, 08:25:06 pmThanks, macboy! I did check last night and it was set to PCM, Dolby was greyed out. I didn't have the DAC plugged when I tried, I'll try again tonight with optical plugged in.

author: EPAIII
author_rank: Super Contributor
timestamp: November 21, 2023, 07:13:29 pm
content: They do make four pin, 3.5mm telephone plugs and jacks. Just be sure you mount it with insulation from the chassis or any other ground inside the set. And you will have to do your own headphone wiring. It will be a special set of headphones just for that TV.But then, the speakers will be on while you are using the headphones. That is usually not what is wanted. Another way to do this would be to use two of the telephone jacks with switched normals. One jack for the left channel and the other for the right. There would be no connectionsto the sleeve terminals. Move the speaker wires to the tip and ring contacts in the jacks and the normals would go to the speakers. Be sure to preserve polarity on the speaker wires as you move them. That way the speakers are silent when using headphones.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 21, 2023, 08:00:02 pm
content: What kind of ocnnector do these speakers out have? Is it just a stereo 3.5mm?Another option would be a differential amplifier - taking balanced output of the speakers to single ended, with an opamp per channel, so both outputs are referenced to a common ground - meaning you can plug in your headphones.But there are caveats:1) You would need a ground reference for these balanced outputs - that is you would need L+, L-andGND (and the same for the right channel), thats why I asked about stereo jacks, simply using L+ and L- doesn't provide a ground reference so would require a transformer (decent audio transformers are expensive and more hassle than they are worth).2) The opamp would have to be able to drive headphones - so 50-80mA output, or you would have to follow the differential-to-single ended circuit with an actual headphone amp. High drive opamps are common so might as well just use one.3) It'll need power, which is just another hassle. I did something similar for a neighbours TV and used the USB connector for 5V for the amp (and a TS922 dual opamp for the headphone amp).So ultimately, probably a lot of hassle, but its a solution, albeit a complicated one.

author: Thunderer
author_rank: Regular Contributor
timestamp: November 21, 2023, 10:18:49 pm
content: As already pointed out, the newer TVs have class-D amplifiers, so the outputs are bridged, no common point from one channel to the other.A simple solution is below. I do not know about the quality of the circuit. Most likely it will kill any trace of damping factor for the amplifiers, so not a good solution for direct connection for headphones (you will need a buffer amplifier). But, it is good if you feed the signal to an amplifier, its input impedance will not play an important role.But, the most elegant solution is something like INA2137 :https://www.ti.com/lit/ds/symlink/ina2137.pdf?ts=1700605262564&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FINA2137, plus a buffer for direct connection of headphones.

author: magic
author_rank: Super Contributor
timestamp: November 22, 2023, 10:35:31 am
content: Not only output impedance increases, but also "ground" impedance does and hence you would see increased crosstalk between channels.And if you add buffering, you might as well pick up the signal from before the power amplifier chip. The only downside is that you need to find it on the PCB, it's no longer a matter of just tapping into cables.

author: vladio
author_rank: Newbie
timestamp: November 28, 2023, 03:53:04 pm
content: I really appreciate the help, thanks everyone!I did some playing around and was able to get audio through the DAC I bought.Macboy's suggestion helped. I had to cycle the tv off/on after the optical was plugged in to get where I could turn off Dolby.Thanks again all!