is it safe to split a USB cable to a peripheral device? separate power and data? - Page 1
Forum section: electronics / projects

author: chirality
author_rank: Contributor
timestamp: July 29, 2018, 10:58:16 pm
content: Hello all,I have an audio device (Roland Boutique synthesizer) that has a well known ground loop noise issue when powered by the USB port from a host computer or USB hub. The issue has been well documented on internet forums, etc.See here:https://www.gearslutz.com/board/electronic-music-instruments-and-electronic-music-production/1043565-roland-jp-08-noisy-output-if-usb-connected.htmlOne work around to reduce noise involves powering the device via a USB wall wart PSU. However, by doing so, you can only communicate to the synth via a 5-pin DIN MIDI cable and not the more common USB port.Anyway, I was wondering, can a USB cable be split so that the power and data are separated -- power from the wall charger and data to/from the USB port on the computer or hub? For some reason, I'm thinking that the power portion would work but that the data lines to the computer might not be recognized or grounded/shielded properly. I'm too scared to test rig it and damage something...Thoughts?

author: tsman
author_rank: Frequent Contributor
timestamp: July 29, 2018, 11:10:30 pm
content: Quote from: chirality on July 29, 2018, 10:58:16 pmNo. You need the ground connection. The data lines are referenced to ground which you're not connecting. The data lines are a differential pair but also uses single ended signalling where both lines are pulled low together.Buy a USB isolator if you need to power it from a separate PSU but still use USB. The cheap ones are only Low/Full Speed though so if your device needs High Speed then it won't work. I suspect it is only Full Speed though.

author: ajb
author_rank: Super Contributor
timestamp: July 29, 2018, 11:12:49 pm
content: Not a good idea. Even if nothing blows up it's not likely to be reliable.A better solution is probably to use a USB isolator. Maybe one likethis, as it would be pretty easy to remove the DC-DC converter from the board and tie in a suitable wall adapter (or even another USB pigtail to use a USB wall wart) for the power to the device.

author: DimitriP
author_rank: Super Contributor
timestamp: July 29, 2018, 11:22:30 pm
content: Fighting with Universal Serial Bus when you have a dedicated Musical Instrument Digital Interface available seems silly.Maybe it isn't but it seems so

author: chirality
author_rank: Contributor
timestamp: July 29, 2018, 11:31:47 pm
content: Quote from: DimitriP on July 29, 2018, 11:22:30 pmI do prefer using MIDI whenever possible.Sadly, these Boutique synths seem to lack System Exclusive so saving the settings or patterns from the synth can only be done via USB to my understanding.

author: DimitriP
author_rank: Super Contributor
timestamp: July 29, 2018, 11:33:15 pm
content: It gets to be a little more fun than simply splitting or filteringhttp://andybrown.me.uk/2015/07/24/usb-filtering/the USB power :http://www.analog.com/en/analog-dialogue/articles/digital-isolators-in-medical-and-industrial-apps.htmlhttp://www.analog.com/en/products/interface-isolation/isolation/usb-isolators/adum4160.html

author: JS
author_rank: Frequent Contributor
timestamp: July 30, 2018, 03:13:56 am
content: Can't you break the ground loop on the other end? Breaking ground loops in audio lines is a quite usual thing to do, you could use transformers or differential receivers and use the ground as a reference, you could use an external box with receivers powered from a wall wart and reference the ground of it to the audio output. Noise isn't expected to be really high so a box working from reasonable rails could keep it's input between rails.JS