USB ground loops

[Andy Burns]

I have a portable speaker, with a rechargeable battery, it takes in power via a uUSB, it also offers power out via a USB type A.

I can play audio to it over bluetooth, or into a 3.5mm aux socket, the bluetooth quality isn't the best (no AptX) and neither is the range.

I have a chromecast audio, this also takes in power via a uUSB and has a

3.5mm aux output[*].

If I plug the chromecast aux into the speaker aux, and power it from an external USB charger, and stream music while the speaker is running on battery, all is good.

If the speaker is powered from one USB charger, and the chromecast is powered from another USB charger, all is good.

I've got two 13A sockets nearby the speaker, but need three devices plugged in.

However if I power the chromecast from the speaker's USB power out, even when the speaker is running on battery, I get all sorts of horrid "motorboating" and clicks like on an 1980's international phone call.

If I power both the speaker and the chromecast from a single USB charger with multiple outlets, or using a uUSB "Y" splitter cable, I get the same interference.

I presume the chromecast is spitting out interference on its power input, which the speaker objects to as it's relative to the audio ground?

I looked to see if there's such a thing as a USB isolator, they do seem to exist, but are north of £50 which seems excessive, If I pared-down a USB cable, so it only has the power +/- pins, no data pins and no shield, is that likely to help? or do USB sockets generally link the -pin to the shroud?

[*] Cunningly the same 3.5mm aux socket is also an S/PDIF optical output, which I suspect would help with isolation, if I fed it into a DAC (I have a spare) but then it doesn't help overall because the DAC would require yet another power source that I don't have a socket for, and would add to the spaghetti.

[Andy Burns]

I had to try ...

I can see the chromecast spitting red light down the fibre (which it turns on/off when it's streaming/silent) the DAC is powered by USB, but only silence comes out of its phono outputs, seems I had mis-remembered that the DAC has S/PDIF output rather than input :-(

Anyway, it proved that it would be *way* too much spaghetti, even if it had worked ...

[jrwalliker]

USB negative power is usually connected to the cable shield.

You could isolate the USB power with something like this: Farnell: 2747515 REC5A-0505SW/H2 Isolated Board Mount DC/DC Converter, Medical, 1 Output, 5 W, 5 V, 1 A It is almost certainly overkill and you can probably find something similar for a lower price. John

[Brian Gaff (Sofa 2)]

Yes sometimes you get rubbish on usb supplies. There is no answer other than use a very good usp powered hub thingy. I had this with a sound box getting whining and ticking from the computer usb. Just a simple usb psu did no good as the supply was not isolated, just beefed up a bit. I ended up with a several way usb with power supply, but the annoying thing for you is it it is the usb device which is generating the crap, so really that needs to be the device powered by the main unit and the good isolated hub powering the audio stuff. Another thing I've noticed is that if I start hearing the start of a whine again I remove all plugs give a squirt of cleaner on the plug end then work them in and out and its gone for some more months, leading me to think that the contacts are not plated or maybe very poorly. Brian

[Andy Burns]

I tried a good 60 watt USB charger (1x USB type C, 4x USB type A and 1x USB quickcharge port) it was as bad as the cheapy two port 2A charger.

Was trying to avoid clutter ... have decided on a method that has more clutter, but it is hidden!

[Andy Burns]

I think I'll just move the speaker to the other side of the room, where a 4-way mains power strip and multiple USB chargers can be hidden on top of some cupboards ...

[Pamela]

If you get interference with a splitter then wouldn't one (or both) of Chromecast and speaker be failing to pass on the true ground potential from the USB input internally to their respective audio connection?

As opposed to the Chromcast injecting spurious interference back through the USB power input.

Perhaps the two separate PSUs isolate their own mains grounds so as not close (permit) the earth loop which causes the motorboating, etc.

[Andy Burns]

Well, the interference only happens when speaker and chromecast are sharing a USB power source (cheapy 2x output charger, more expensive 5x output charger, or single output charger with splitter cable)

If one is on battery, or they're both on individual chargers, it's fine.