*3.5PRO is a new standard proposed by the China Audio Industry Association Headphone Division and is said to be adopted by several Chinese manufacturers in the near future as an improvement over the original Hifiman proposed 3.5mm TRRS balanced plug. It is essentially just the same 3.5mm TRRS (wired the same way) with an extra plastic ring molded at the sleeve (shown in red on the picture above). The 3.5PRO socket will be more recessed to accommodate the extra plastic ring. The problem with Hifiman original proposed 3.5mm TRRS is that a single-ended 3.5mm TRS plug can be mistakenly inserted into the same socket and causes shorts. But a single-ended 3.5 TRS or normal 3.5mm TRRS plug will not be able to fully insert into the 3.5PRO socket and thus making sure no shorting out will occur. On the other hand, a headphone with the new 3.5PRO plug will still be compatible with an old 3.5mm TRRS balanced socket found on Hifiman and some LH Labs’ devices.
PDF can be found here.
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