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Author Topic: disabling 48v on XLR  (Read 1946 times)

Ivan Feder

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disabling 48v on XLR
« on: January 14, 2018, 04:31:26 am »

Hi PSW,
I am looking for a quick way to make an adapter that would not pass 48 v when I plug in a mixer output into to my sound card.
I have a Focusrite Scarlett which when the 48 v button is on, sends to both inputs. I don't want to send 48v to the mixer output as this could fry it! I am using Smaart with a mic on input one and a ref signal from the console on input two.
Thanks for your advice,
« Last Edit: January 14, 2018, 06:40:18 am by Ivan Feder »
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Scott Holtzman

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Re: disabling 48v on XLR
« Reply #1 on: January 14, 2018, 08:00:15 am »

Hi PSW,
I am looking for a quick way to make an adapter that would not pass 48 v when I plug in a mixer output into to my sound card.
I have a Focusrite Scarlett which when the 48 v button is on, sends to both inputs. I don't want to send 48v to the mixer output as this could fry it! I am using Smaart with a mic on input one and a ref signal from the console on input two.
Thanks for your advice,

You need to build a filter using a simple RC time constant.  Total resistance is twice the impedance so 1200 ohms.  You may get a bit of HF rolloff so see how it sounds.

One of theses in series with each of the audio leads.  https://www.parts-express.com/22uf-100v-electrolytic-non-polarized-crossover-capacitor--027-324

You could also put 100kohm resistor to ground upstream of the capacitors to bleed off any excess current without loading the circuit.



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Scott AKA "Skyking" Holtzman
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Mike Caldwell

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Re: disabling 48v on XLR
« Reply #2 on: January 14, 2018, 08:04:32 am »

Hi PSW,
I am looking for a quick way to make an adapter that would not pass 48 v when I plug in a mixer output into to my sound card.
I have a Focusrite Scarlett which when the 48 v button is on, sends to both inputs. I don't want to send 48v to the mixer output as this could fry it! I am using Smaart with a mic on input one and a ref signal from the console on input two.
Thanks for your advice,

Transformer isolation would do it. Since your using it for measurement you want to get a good transformer.

Todd Friemuth

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Re: disabling 48v on XLR
« Reply #3 on: January 14, 2018, 08:49:21 am »

Is going XLR to TRS an option for you? I use an Eridol UA-25 interface, not a Focusrite, but that is all I do to get around the 48V issue. The TRS inputs don't get phantom on my UA25.
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Ivan Feder

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Re: disabling 48v on XLR
« Reply #4 on: January 14, 2018, 08:59:52 am »

Transformer isolation would do it. Since your using it for measurement you want to get a good transformer.
thanks for your answer, a simple 600/600 Ohms transformer would work?
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Ivan Feder

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Re: disabling 48v on XLR
« Reply #5 on: January 14, 2018, 09:03:54 am »

Is going XLR to TRS an option for you? I use an Eridol UA-25 interface, not a Focusrite, but that is all I do to get around the 48V issue. The TRS inputs don't get phantom on my UA25.
Hi, I think going to TRS is not an option as the inputs on the Scarlett are combo XLR/TRS
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Scott Holtzman

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Re: disabling 48v on XLR
« Reply #6 on: January 14, 2018, 09:24:03 am »

thanks for your answer, a simple 600/600 Ohms transformer would work?

Freq=0 Inductive Reactance=0 so yes current transfer will be 0 however I would still use the DC blocks to keep from loading the interface considering the low DC resistance of the windings on the transformer.

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Pete Erskine

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Re: disabling 48v on XLR
« Reply #7 on: January 14, 2018, 10:08:30 am »

Hi PSW,
I am looking for a quick way to make an adapter that would not pass 48 v when I plug in a mixer output into to my sound card.
I have a Focusrite Scarlett which when the 48 v button is on, sends to both inputs. I don't want to send 48v to the mixer output as this could fry it! I am using Smaart with a mic on input one and a ref signal from the console on input two.
Thanks for your advice,


No transformer needed, just non polarized caps in each side - pin 2&3
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John Roberts {JR}

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Re: disabling 48v on XLR
« Reply #8 on: January 14, 2018, 10:39:23 am »


No transformer needed, just non polarized caps in each side - pin 2&3
Yes capacitors are cheaper than a transformer. For that size (22uF or larger) you will probably use polar caps so orient the + leads toward the higher voltage source. Note: caps are in series with pins 2 and pin3, leave pin 1 hard connected.

JR
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Mike Caldwell

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Re: disabling 48v on XLR
« Reply #9 on: January 14, 2018, 11:06:36 am »

Yes capacitors are cheaper than a transformer. For that size (22uF or larger) you will probably use polar caps so orient the + leads toward the higher voltage source. Note: caps are in series with pins 2 and pin3, leave pin 1 hard connected.

JR

If you go DIY with a little finesse your could probably fit the caps into an XLR connector, if there's not room to fit two caps in one connector put one on the appropriate connections at each end of a short DC blocking cable or build it into an XLR barrel connector.
You can by DC blocking XLR barrels as well.

I was assuming you may have a iso transformer laying around to save building or buying something.
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