Isolating external sound interface

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Hi,

I have recently purchased an EDIROL UA-1EX for my laptop as the onboard sound is dreadful.. full of powersupply noise. The interface suffered the same fate when i plugged it in directly to the USB port. I decided to try to power the device from an external source. I had an old zip drive supply laying around (5vDC, 1A capable) and a spare USB extension cable.

I stripped a bit of insulation back on the extension cable and cut the ground and +VCC wires. I soldered the wires from the device recepticle end to the zip supply leads. fully insulated the connections, fully insulated the ends of the supply leads coming from the laptop, added some heatshrink on the whole thing to neaten it up.

After doing a few checks with a multimeter for continuity and voltage i plugged the powerd cable into the laptop... no bang, all good 🙂 Then i plugged in the external interface. It powered up and was recognised as before. Started playing a few songs and SUCCESS!! no dirty noise when i moved the mouse or anything 😀

Used it on the desk like this for a few days worked great. Moved it to another room and the device wouldnt be recognised.. powered up though. Tried the device straight in USB port and it worked fine. After several rebuilds on different cables and even on prototype board i couldnt replicate my original success. Until i added the ground from the laptop USB supply to the powered cable circuit, still using the zip supply for the +5V though.

Now the device would be recognised and worked again yay! except the addition of the GND introduced the dreaded PSU noise in again of course 😡

Has anyone done anything similar? any tips i could try? it's doing my head in :dead:



Thanks for your time!
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