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I have a USB microphone that I'd like to supplement with more power than my laptop USB port can provide. What I did was cut the power lines on the USB cord and fed in an external power source through those lines. I then tested the power line connectivity by plugging in my phone, and it charged (meaning power lines are good), then tested D+ and D- lines with a multimeter, both of which showed a very small amount of resistance. However, when I plug in my microphone and external power source, my computer does not recognize the device. I even reconnected the power lines back to their original wire and tested it, and it worked fine, so I believe there is no problem with the wiring or connectivity. From my knowledge, it seems to me my computer should be recognizing the device because as long as the device is powered, the USB line is pulled up and the computer will recognize there is a device present. Is there anything I am missing?

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    \$\begingroup\$ Did you keep the laptop Ground connected to the microphone? \$\endgroup\$ – Peter Bennett Jun 11 '20 at 4:07
  • \$\begingroup\$ No, the microphone is using ground from the power supply. \$\endgroup\$ – Matthew So Jun 11 '20 at 4:32
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    \$\begingroup\$ All grounds must be connected together to provide a common reference. What do you expect to gain by using an external power supply? \$\endgroup\$ – Peter Bennett Jun 11 '20 at 4:42
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You can't disconnect ground from laptop, even if you feed extra 5V supply. The USB data still needs ground reference. Good thing is the USB is not damaged but it might have been damaged by equalizing supply leakage currents via data pins.

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