Adding Extra Line Out to TV - Page 1
Forum section: electronics / projects

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 18, 2021, 08:38:14 pm
content: Hi All.I have a Sony KDL-40W5500. I have a problem with the way the line out of this TV functions.The problem is that the Line Out is always controlled by the volume control. This is a pain because it means that when I want to use my HiFi speakers (such as when watching a movie) I have to go into the menu and mute the TV speakers manually.Now on my newer TV in the other room I can set the line out to be a true non-volume controlled line out in the settings menu, then I simply just press mute on the TV and turn on my HiFi amp. However the older TV does not have this option. If I mute the TV the Line Out is also muted.I have had a look at the service manual and notice that the Line Out is driven by the 'AMP1' section of a CXA3785R. It appears this IC has a I2C programmable gain control so I assume this is where the 'volume' is controlled from?I am thinking there is a way I can tap the signal out from the AMP1 inputs.I can see the input pins labelled AMP1INL_P and AMP1INL_N and also AMP1INR_P and AMP1INR_N.The data sheet of the CXA3785R shows what appears to be a simple balance (positive and negative) audio input into an op-amp which is outputting an unbalanced signal.This is as far as my understanding really goes.I am wondering, could it work taking a wire from the AMP1 inputs to this IC and tapping them into another pair of op-amps (TL072 or similar) for a line-out that isn't affected by the volume control?Then I could just add a panel 3.5mm jack socket to the rear of the TV and be happy with a real Line Out!What do you all think? Could this work?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 19, 2021, 01:30:16 am
content: Have you tried S/PDIF or HDMI ARC output? Note that you might have to select "raw S/PDIF" to get the unprocessed stream.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 19, 2021, 10:22:31 am
content: Yes I have! The TV does have an optical Toslink output, however unfortunately this is also controlled by the volume of the TV!This TV does not appear to have HDMI ARC support ( I have tried with the other room's soundbar)If the optical did work i would need an converter anyway seeing as my amplifier is from 1977 and only has RCA inputs anyway.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 19, 2021, 10:32:48 am
content: This is what I have worked out so far regarding the CXA3785R. (see attached images).I have coloured the inputs for the LINEOUT amp in green and the outputs red.If the AMP1 inputs are indeed just analogue signals then sureley I can just solder a ribbon cable to them and add an extra op-amp IC to buffer the extra socket?What I do not understand is the inputs PWM_LI and PWM_LD / PWM_RI and PWM_RD.What is the I and D in LI LD / RI RD?

author: rdl
author_rank: Super Contributor
timestamp: June 19, 2021, 01:39:10 pm
content: Why do you even use the TV speakers?

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 19, 2021, 01:48:47 pm
content: To prove your theory you’ll have to test it.Hook up a scope to the AMP1INL signal and see wherever the volume or mute controls have an effect on it.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 19, 2021, 03:13:02 pm
content: Quote from: rdl on June 19, 2021, 01:39:10 pmWell mainly I just use the TV speakers when I am just watching something like the news or just browsing youtube. It is an older TV and they are fairly decent as far as TV speakers go.Plus the speakers are 1980s and it would wear them out unnecessarily using them all day pointlessly.I only use them when watching a movie or something like that. Or just when I play some music.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 20, 2021, 04:53:40 pm
content: Quote from: paul_g_787 on June 19, 2021, 10:22:31 amSee if there's a setting in the menus to output raw S/PDIF, that will bypass the internal audio processing.QuoteTime to upgrade? It should be pretty easy to get a digital amplifier without HDMI or even with 1080p HDMI for cheap because they're "outdated".Or if you're using an external player, maybe use the audio output on that?