Blocking phantom on XLR inputs

[Stuart91]

I've just bought a new digital console, and one limitation is that there are no jack or phono inputs, everything is XLR.

For "walk in music" etc. I need to have a minijack available at the console. The obvious answer is a simple minijack to dual XLR lead, but I'm a little concerned that at some point phantom power will get accidentally applied. (Imagine, for example, a visiting engineer loading a show file etc.)

 

The other option is a stereo DI, but even the smallest ones will be a bit annoying kicking around in the case. And they are likely to get nabbed for use elsewhere.

 

What I'd ideally like is a compact way of isolating the +48v from the minijack. Looking around for potential circuit designs, I can find this:

 

 

It's clearly intended for protecting mics on a balanced connection. My connection will be unbalanced, so I could make just one half of the circuit and potentially fit it inside an XLR connector. Is that likely to work, or have I overlooked something?

[david.elsbury]

I made one a few Ye ars back with an XLR male to female barrel where I ripped out the female XLR and glued in a compression gland; and I fitted a Neutrik Nte1 transformer inside to isolate the signal. Fitted a captive 3.5mm cable and all good.

[Stuart91]

That's a good shout - it hadn't occurred to me that a small transformer would be an option. And they're cheap enough that I can buy a couple to experiment with.

[henny]

https://cpc.farnell.com/pulse/pls00550/ground-loop-isolator-3-5mm-to/dp/AV25544

[paulears]

That's a damn useful device from CPC - thanks for the link.

[ojc123]

https://cpc.farnell.com/pulse/pls00550/ground-loop-isolator-3-5mm-to/dp/AV25544

I think I'll be having a couple of these! Thanks.

 

E2A And a couple of these too. My link

Edited November 17, 2019 by ojc123

[Brian]

That's a good shout - it hadn't occurred to me that a small transformer would be an option. And they're cheap enough that I can buy a couple to experiment with.

 

As long as you aren't passing DC through the winding you'll be OK. If you are then, depending on the size and construction of the transformer, you run the risk of saturating the core which'll reduce your headroom and introduce distortion.

[Stuart91]

https://cpc.farnell.com/pulse/pls00550/ground-loop-isolator-3-5mm-to/dp/AV25544

I think I'll be having a couple of these! Thanks.

 

E2A And a couple of these too. My link

 

A very good find, looks like it'll be ideal for the purpose.

 

I suppose it's just as well that CPC have 400-odd of them in stock...

[NeilW]

Not quite as cheap as the CPC unit but our standard unit in hire stock is these PC balance boxes from Interspace

 

https://www.interspaceind.com/av-audio-products/product/28-pcbb²/category_pathway-41.html

[Stuart91]

Those look like very sturdy units. I imagine they'd be ideal for use on stage, but I'm not so keen on the idea of one bouncing around in a dog box with the mixer.

[sameness]

Those look like very sturdy units. I imagine they'd be ideal for use on stage, but I'm not so keen on the idea of one bouncing around in a dog box with the mixer.

 

Velcro it down then?

[abbotsmike]

Those look like very sturdy units. I imagine they'd be ideal for use on stage, but I'm not so keen on the idea of one bouncing around in a dog box with the mixer.

 

Velcro it down then?

 

Stop throwing the mixer down stairs?

 

While they are sturdy, I can't see one being able to inflict significant damage to the back of a mixer. Also, by the wording of visiting engineer, is this going into a venue anyway?

[kgallen]

https://cpc.farnell....m-to/dp/AV25544

E2A And a couple of these too. My link

 

In case anyone needs to know...

 

Both of these are dual 600R-600R transformers. XLR pins 1 and 3 are connected. XLR shell is not connected to pin 1.

 

On the mono version, the two transformer outputs (L and R) both connect to pins 2 and 3 of the single XLR. On the stereo version, each transformer secondary connects to pins 2 and 3 of the respective XLR.

3.5mm stereo plug to transformer bubble is 75cm, 2-core+screen, transformer bubble to XLR male(s) is 75cm twin 1-core+screen.

Transformer bubble is a metal case with screw affixed end-caps (so you could take it apart) and the label has a circuit diagram per the description above.

 

DC continuity measures 80 ohms between 3.5mm T or R to S. On the stereo version, XLR pin 2 to pin 3 measures 98 ohms, and between XLRs pin 1 to pin 1 measures <1 ohm. 3.5mm S does not have continuity to XLR pin 1.

Edited November 21, 2019 by kgallen

[sunray]

I like these:

https://cpc.farnell.com/av-link/201-086uk/ground-loop-isolator-rca/dp/AV26879

The case is 2 steel sections that pushfit together then covered in heatshrink. I have often opened them to fit different cables, especially in fixed systems.