Feedback wanted - TV Fixed Line-out Volume Control

[mgf897]

Hi All,

I recently tried to connect some external amplified speakers (PC speakers) to my Samsung TV. The sound was MUCH better but there was no way to control the volume without getting up and turning the volume knob on the speakers.

I've drawn up a schematic for a volume control board to accept the 3.5mm fixed audio line out from the TV and provide a variable line out signal to the external speakers. The micro will control the digital pots using the IR signal from the TV remote. The board will be powered by USB which is available on the back on the TV.

Concerns:
* I have no isolation on the audio, will I have problems with ground currents?
* Have I completely missed something/made an obvious mistake?

Any feedback/critique is appreciated

Thanks,

Matt

 

[linuxworks]

digital pots often suck for audio.

look into using the pga2311. great chip, also has zero-crossing detection which the digipot solution will never have.

 

[mgf897]

Thanks for the recommendation linuxworks, zero-crossing was something I hadn't considered. The pga2311 certainly looks like it would simplify some aspects of the design

 

[linuxworks]

it has an onboard op-amp (buffer) for output. that's nice. still, I recommend 3 stages: prebuffering, pga chip, postbuffering.

postbuffering protects against reversing input and output jacks and blowing up the chip. plus, its nice to have to decouple the chip from the outside world.

prebuffering, since you want to give the pga chip's input a fairly low and predictable Z. an input OA can also limit the input swing that the pga sees (oa should run slightly lower in rails than pga) and that tops bad pga behavior.

(ob disc, I'm working on a pga/arduino board; you're welcome to give it a try.)