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Adding Extra Line Out to TV

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paul_g_787:
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?

NiHaoMike:
Have you tried S/PDIF or HDMI ARC output? Note that you might have to select "raw S/PDIF" to get the unprocessed stream.

paul_g_787:
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.

paul_g_787:
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?

rdl:
Why do you even use the TV speakers?

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