Thank you, you are right!
I guess that means one should not plug in the mono jack adapters shown in Section 34.24 for the front PH outputs because that creates a short for the right channel. For the rear outputs it seems that these mono jack adapters are ok.

Thank you very much, that makes it clear to me on how to proceed.
Maybe that subtle but important difference between outputs at the rear and outputs in front could also be mentioned in the manual?
The manual explains already a lot of details, I am happy to read and learn from the manual.

It‘s all there, in full detail, i.e. manual page 67:
“ 1/2 TRS (rear)
Output: 6.3 mm TRS jack, impedance balanced”

Of course one needs some technical background, that has no space in the already very comprehensive user manual, to understand what that means.
Or you have to Google it:
https://www.soundonsound.com/sound-advi … -balancing

Impedance-balanced Send: Increasingly, manufacturers are using impedance-balanced outputs. The signal is still being transmitted differentially, but only the hot wire carries the (full level) signal. The cold wire is arranged to have the same impedance to ground to ensure proper common-mode rejection. This approach has fewer components and so is cheaper to implement, but it also ensures the correct signal levels are maintained regardless of whether the destination is balanced or unbalanced.

> Additionally on page 69, Phones 1/2 output is described "as output 1/2 TS" while it is really "as output 1/2 TRS".

Maybe the manual is not updated as the impedance balanced resistor is only available on the FS R, not previous model.

> @KaiS: From what you are saying, I have understood that impedance balanced outputs (at the rear) can be shorted, while unbalanced outputs (in front) cannot be shorted. Thank you.

For the impedance balanced output on the back, the ground and - phase (pin R and S) can be shorted because there's nothing driving the - phase. R is simply a resistor connected to ground.

For HP out 1/2 or 3/4 on the front, the  ground and independent right channel (pin R and S) are driven actively so they cannot be shorted.

once you get the two facts, the rest are no-brainer.

That‘s true, and both use the same 6.3 mm TRS plug.

But - the main difference:
The 2 Front Outputs are for 2 pairs of STEREO headphones.
The 2 Rear Outputs are the separated left and right Line Outputs.
All are labeled as such, and have different output power to suit their purpose.

This is so common that probably nobody ever thought that it needs a dedicated explanation.

So:
• Connecting amplifiers, active loudspeakers and such: Rear
• Connecting headphones: Front

The 2 headphones outs can be used as 2 additional stereo line out even unbalanced, when using a properly wired cable.

But that’s a special case.

For balanced better use the rear XLRs (or even TRS on the later models), to keep ADI-2 Pro’s flexibility.
Headphones Balanced Mode incorporates some limitations, as both DAC chips are blocked for this.

No, look here some posts above:
https://forum.rme-audio.de/viewtopic.ph … 96#p182196

The Ring of the TRS plug is connected to Ground through a resistor (in later models, early models are TS single ended) that equals the output impedance of the Tip which carries the signal.

You get the full benefit of common mode rejection, but not the double voltage of true balanced (which is too hot for most applications anyway).

It’s all there, you just have to choose the right connector for your application.