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1/8" to XLR

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Dan Richardson:
I have a cable wired 1/8" stereo to 2 XLRs, for plugging my whatever into a desk that only has XLR inputs.
Someone mentioned that phantom power will blow up an iPhone's output stage.
That would be bad.
Yes, I can turn off the phantom on most desks, but things happen.
I don't want to carry around a pair of DIs just for this application.
Is there a safe way to wire this?
What happens if I leave the XLR pin 1 open, and wire pin 3 to shield and pin 2 to signal?
Is this a reasonable electronic solution?
Are there boards with phony balanced inputs where this will cause issues?

Geoff Doane:

--- Quote from: Dan Richardson on February 01, 2011, 07:20:54 PM ---What happens if I leave the XLR pin 1 open, and wire pin 3 to shield and pin 2 to signal?
Is this a reasonable electronic solution?
Are there boards with phony balanced inputs where this will cause issues?

--- End quote ---

That should work, although the two pins-3 will be common to each other (still shouldn't be a problem that I can see).

If you run into an unbalanced XLR input (very rare these days, I think), it shouldn't be a problem either, as long as the XLR is wired pin-2 hot.  If it isn't, it just won't work.  It shouldn't damage anything.

Building an adapter with a 10 μF, 63V cap in series with pin-2 might be an even better idea.  Connect pin-1 and -3 to the shield of the TRS.  This might cause current to flow from pin-3 to -1, but that's only a problem with transformer inputs (could magnetize the transformer), which are also rather rare these days.

GTD

Gareth Marsh:
I made the same cables, and used to use them with inline transformers to prevent this happening.

One day, I was on a system that had far greater issues and the transformers were used somewhere else. House techs had global phantom on (without any input needing it), I hooked in my macbook without thinking, and that was the end of the headphone output.

I have just been very careful since then, but I have thought about blocking caps, or even if I could cram a transformer into an XLR shell somehow (this would also help with noisy laptops).


Gareth

Josh Bennett:
are you connecting this in a way to get stereo? XLR(L) pin2 to 1/8" tip, pin3 to 1/8" sleeve; XLR(R) pin2 to 1/8" ring, pin 3 to 1/8" sleeve. That would work against phantom damage, but will insert any ground noise from your laptop or other device directly into the signal stream.

Frank DeWitt:
What your suggesting should work OK on battery powered devices. such as a I-pod. (Keep the output low, as earphone output is close to line level.

I can imagine some strange things happening with any powered device such as a laptop.  (If both are properly grounded, then you have reconnected pin 1)

A DI box provides protection and ground isolation and gets the level right for a mic in so it is a very nice solution.  If you only need Mono you could build a 3 resistor summing cord,  http://www.rane.com/note109.html  Then you would just need 1 DI box, and some of them are quite small.

Disclaimer, I build and sell DI boxes.

Frank   http://lbpinc.com/DI.html

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