Sony STR-DA3400ES Operating Instructions Manual page 60

Types of 2CH mode
x 2ch Stereo
The receiver outputs the sound from the front
left/right speakers only. There is no sound
from the subwoofer.
Standard 2 channel stereo sources completely
bypass the sound field processing and multi
channel surround formats are downmixed to 2
channel except LFE signals.
Note
No sound is output from the subwoofer in the 2ch
Stereo mode. To listen to 2 channel stereo sources
using the front left/right speakers and a subwoofer,
select "A.F.D. Auto".
This receiver will generate a low frequency signal
for output to the subwoofer when there is no LFE
signal, which is a low-pass sound effect output from
a subwoofer to a 2 channel signal.
x 2ch Analog Direct
You can switch the audio of the selected input
to 2 channel analog input. This function
enables you to enjoy high quality analog
sources.
When using this function, only the volume and
front speaker balance can be adjusted.
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