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Connecting coaxial source with XLR mic input of Scarlett 4i4

anli

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Hi!

The aim is to connect coaxial source to 4i4 mic input. So I'm going to solder an adapter. Would it be correct to connect this way

coaxial ground -> XLR ground and one of balanced line (say, negative)
coaxial signal -> another XLR balanced line (say, positive)

?

I have not seen 4i4 mic input schematics, so just am guessing.
 

sergeauckland

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Yes that's exactly right. Just make sure you turn off phantom power as your source may not appreciate 48v up it!

...they don't like it up 'em...

With thanks to Cpl Jones...

S
 
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anli

anli

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Thanks!

Yes, I have thought about film capacitor, but have not got it with sufficient nominal. So, I'm going to try electrolytic capacitor (with plus -> hot) and, at this case, do use +48V.
 

sergeauckland

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Thanks!

Yes, I have thought about film capacitor, but have not got it with sufficient nominal. So, I'm going to try electrolytic capacitor (with plus -> hot) and, at this case, do use +48V.
Sorry, no idea what you're referring to. You don't need a capacitor, just turn off the 48v phantom supply.

S
 
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anli

anli

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Sorry, no idea what you're referring to. You don't need a capacitor, just turn off the 48v phantom supply.

S
I mean accidental case of unintended turning phantom power on. With serial electrolytic capacitor only short limited (phantom power has got large output impedance) impulse will go to source to charge the capacitor. And, well, especially for small signals electrolytic capacitor must be polarized, and phantom power will do just exactly it.
 

DVDdoug

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RANE has a reference showing wiring all of the common balanced to unbalanced cables/adapters.

I assume you your "coaxial" (RCA?) connection is line level and I assume you know that a line-level signal is around 100 times stronger than a microphone signal (depending on the loudness of the sound and the sensitivity of the mic, etc.). The "instrument" input is "closer" to line level, and I see the Focusrite also has "pad" to reduce the signal. With some interfaces you have to use a TS/TRS plug as the instrument input but I don't know about this particular one.

Also "coaxial" often refers to S/PDIF digital so I hope you have an analog signal...

For the DC blocking capacitor - The microphone input impedance is 3K and you're making a high-pass filter (DC is zero Hz). This filter calculator says you need a 2.6uF (or higher) capacitor if you want to go-down to 20Hz. Usually that's going to be an electrolytic.

You should also have a resistor on the input-side (the filter input, not the Focusrite input) between signal and ground. With nothing connected the voltage can "leak-up" and possibly damage whatever you plug-in before that voltage discharges. That can be 100K or so. (You can't measure that leakage voltage because the multimeter will discharge it and it will be gone before it displays anything,)
 
OP
anli

anli

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Oct 20, 2021
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RANE has a reference showing wiring all of the common balanced to unbalanced cables/adapters.

I assume you your "coaxial" (RCA?) connection is line level and I assume you know that a line-level signal is around 100 times stronger than a microphone signal (depending on the loudness of the sound and the sensitivity of the mic, etc.). The "instrument" input is "closer" to line level, and I see the Focusrite also has "pad" to reduce the signal. With some interfaces you have to use a TS/TRS plug as the instrument input but I don't know about this particular one.

Also "coaxial" often refers to S/PDIF digital so I hope you have an analog signal...

For the DC blocking capacitor - The microphone input impedance is 3K and you're making a high-pass filter (DC is zero Hz). This filter calculator says you need a 2.6uF (or higher) capacitor if you want to go-down to 20Hz. Usually that's going to be an electrolytic.

You should also have a resistor on the input-side (the filter input, not the Focusrite input) between signal and ground. With nothing connected the voltage can "leak-up" and possibly damage whatever you plug-in before that voltage discharges. That can be 100K or so. (You can't measure that leakage voltage because the multimeter will discharge it and it will be gone before it displays anything,)
No-no. Source is just IEC711 replica with coaxial jack.
Yes, discharging R would be useful, thanks for the idea!
 
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