Grounding and Rane Note 110

I made a cable to connect my Kilpatrick PHENOL, which has unbalanced outs, to my recording interface (MOTU 16A) which has balanced inputs and outputs.

I’ve read Rane’s note #110 about interfacing unbalanced lines with balanced a few times and thought that I’ve internalized it well at this point. I followed no. 14 on their diagram and made a TS to TRS cable, but when I plugged the cables in I got a pretty nasty 60Hz hum at -60dB. At the same time TS-TS cables work fine and their noise floor is at -110dB!

The PHENOL is using a wall-wart and is therefore un-earthed and floating. This means that its enclosure (and therefore its audio ground) is one big antenna. Feeding that antenna to the balanced line’s “-” injected all that noise into the inputs of the interface. According to a friend, Rane note #110 is only applicable when both devices are earthed, and that makes sense.

 

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