If you look at the schematic, you will see that it is activated by a leaf switch on the jack, which is opened automatically regardless of the plug that is inserted. Stereo or mono jacks open the leaf switch when inserted. ...
Yes, of course that's true, Gary. The internal speakers of the S910 are muted, regardless of whether a stereo (TRS) or mono (TS) plug is plugged into the Phones jack.
However, my previous note relates to something else: When you plug a
mono (TS) plug into the stereo headphone jack, the right channel of the headphone output (which is at the RING contact of the TRS jack) is
short-circuited to ground via the sleeve contact of the mono (TS) plug. And this means there is a risk of damaging the output stage of the
headphone amp.
Therefore, as I said, I recommend using a
stereo (TRS) plug (the three contacts of which are isolated from each other) or a TRS headphone
adapter plug, as shown in the picture above.
Best regards,
Chris