Samsung LS27D390HS-ZA - Manual (Page 30)

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Connecting and Using a Source Device
2
2 Connecting and Using a Source Device
2.2.2 Connecting to Headphones
Do not connect the power cable before connecting all other cables.
Ensure you connect a source device first before connecting the power cable.
1
Connect an audio output device such as headphones to [ ] port on the product.
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This function is only available in HDMI mode.
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The sound could only be heard when using the HDMI-to-HDMI cable.