The problem with the 2.5mm and 3.5mm TRRS connections is that there is no grounding connection. Thus when you try to tie a balanced output from a portable DC powered device to an AC based home piece where there is a true earth ground associated with it, there can be a ground loop issue.
This can introduce hum into the signal path thus the reason Astell & Kern developed their dual connector (3.5mm and 2.5mm) block for connecting their portable players to home systems. The 3.5mm acts as the ground connection. The 4.4mm TRRRS is a brilliant connection choice in my opinion as it is smaller than 1/4", 4 pin XLR or dual 3 pin XLRs. I hope more companies adopt it. It is more robust than the 3.5mm and 2.5mm. The tabs in the 4.4mm female jack are also large enough to help retain a good contact force over the male plug. The 2.5mm is terrible at this and even 3.5mm jacks get loose over time.
I hope this helps clarify it a bit more. Please let me know if you have any additional questions.
Thank You.
Drew Baird, P.E., CEO
Moon Audio
106 Brady Court
Cary, NC 27511
919-649-5018
drew@moon-audio.com
http://www.moon-audio.com
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