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Im having a hard time wrapping my head around the neutrik xlr/trs combo jacks and if they can be connected for my usecase .

In my rack I have a 3.5mm trs input jack that can be either line or mic level (it’s an ATEM extreme switcher) . My plan is to break this out into a patch panel where I have two combo jacks . What I want to achieve the following

  1. Plug in two balanced XLR then these should combine to LR on a 3.5mm trs plug
  2. Plug in two balanaced TRS then combine as for 1.
  3. Plug in two unbalanced TS , use L and R to 3.5 mm plug directly
  4. One stereo TRS , just patch LR channel as for 3

Is this setup possible to achieve without using a mixer?

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The XLR contacts can be hard-wired for your use case, 1&3 all to ground (S on the output), 2 of the first to T of the output (left), of the second to R (right). This is an "unbalancing" connection.

Use case 1–3 would be covered with connecting TRS of the combojacks with 231 of the XLR parts.

However, use case 4 requires combojacks with normalizing contacts. Those work with the TRS part of the combojack, are called TN/RN/SN and are connected to the corresponding T/R/S when no TRS/TS plug is in the combo jack.

When the left TRS combojack is occupied and the right isn't, the left R needs to be disconnected from ground and instead connected to the output R. When the left TRS combojack is occupied and the right is, the left R needs to be connected to ground. The right plug only opens connections we can depend on, it doesn't create a ground connection. So we cannot really achieve that differentiation when working with a balanced signal like a dynamic microphone. This would only work with a differential or pseudodifferential connection (the kind a mixer produces) that does not mind if we keep the ring of the TRS plug entirely unconnected because live and return signal are electrically independent outputs.

In that case, use of the normalizing contacts can fit your problem. But the fully general case with possibly coupled live and return signals is not covered.

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  • Thanks a thousand for this excellent answer . Heated up the iron today and build me this setup , worked like a charm
    – Johan
    Oct 17, 2023 at 17:48

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