I am poor at English. I don't know the subtle nuances. There may be a misunderstanding. In that case, please correct it.
Thank you for the thorough and thoughtful reply. It's interesting to know that the 4.4 jack can be used this way. I'm more a novice on these things... When you describe your "2x xlr", it's also equivalent to a balanced 3 pin xlr connection, including your jump ground line to the 3.5mm jack, no? ...Just want to be sure I'm thinking about this correctly... Also, if I'm correct, the commercial 4.4M TRRS-to-dual 3-pin-XLR would have also worked, albeit at more expense, to provide a balanced connection, wouldn't it?
First, the A50s ground and XLR pin1 must be connected.
If the sleeve of the A50s's 4.4mm jack is already grounded, a perfectly balanced output is possible with just the cable shown.
(However, neither your Amazon Link cable nor my cable seems to have a shield at the bifurcated part. I think it's only the drain wire.)
If the sleeve of the A50s's 4.4mm jack is floating, you will need to connect the sleeve of the 4.4mm plug to the ground of the A50s by some means.
In the case of DMP-Z1, the sleeve of the 4.4mm jack was floating, so I grounded the sleeve of the 4.4mm plug via the sleeve of the 3.5mm jack.
Also, I infer that the headphone jack's level and impedance are equivalent to an active speaker's line-in?
I don't think the line input impedance of an powered speaker is equivalent to the A50s output impedance. I think it is extremely large when compared.
If you connect the headphone output and the line input, the output level and input level will of course be the same.
How did you measure THD+N? I wouldn't have thought to do that. What is the practical risk in a setup like this, mostly noise?
The output of the 4.4mm (+ 3.5mm sleeve) was measured by AP.
Back when I posted my question, I decided instead to use inexpensive but unbalanced RCA-to-XLR connectors and use my dac/amp as a switchable pre-amp for headphones and speakers, rather than have to unplug-and-plug at every switch. Plus, when I thought about it, I didn't really need a balanced connection for just a short-run line out to active speakers (cross my fingers on ground loop). I'm still learning how to think through my priorities on things like this...
Usually that's fine. I also use it a lot.
But this cable-only conversion is also a compromise.
For reference, one example of the best conversion when connecting from an unbalanced output to a balanced input is shown below.
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You can't tell what the difference will be in the case of the A50 until you try it.
The A50s has a high performance balanced output. I think the theoretical best bet is to take advantage of this.