• Registered: 2012-05-23
  • Posts: 37

Topic: The USB Y-cable and providing enough power

I've used the USB Y-cable bundled with the Babyface for years in a way I only now realise may be a bad idea. My last computer didn't have enough juice to fully drive the interface, so I hooked up the other end of the Y cable to an old phone charger. Problem went away and the interface worked fine.

Recently, I switched computers and just hooked everything up the way it was. Today, I started thinking about how smart it really is to combine two power sources like that. The manual for the Babyface says you can use the Y cable with a power bank, but in my case it was a phone charger. Does this come with any dangers? Apparently the USB spec forbids this kind of cable.

  • From: Houston, TX
  • Registered: 2007-05-11
  • Posts: 1,191

Re: The USB Y-cable and providing enough power

Out of curiosity (and basic USB operation/not RME specific) - Seeing as this effectively "parallels" two unlike power sources with potentially varying +5v tolerances and ground potentials - what actually happens with power delivery to the device in this case?

I always assumed USB Y-Power-Cables were meant to be used with a common PSU in a PC, but bypassing the current limitations of the individual USB port (imposed by the chipset/controller) - but still effectively paralleling to an identical power source (the MoBo's onboard +5v regulators or the direct +5v from the PSU).

I assumed if you get any variances across the disparate +5v supplies, you'd get inter-PSU current flows and ground contamination?  Maybe DC power delivery is a lot simpler than I'm assuming in this regard? tongue

MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8

3 Reply by MC

  • MC
  • MC
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  • From: Germany/Thailand
  • Registered: 2007-04-06
  • Posts: 25,432

Re: The USB Y-cable and providing enough power

tigersoul wrote:

The manual for the Babyface says you can use the Y cable with a power bank

It does NOT say that for your use case, but for stand-alone operation.

tigersoul wrote:

Does this come with any dangers?

Yes. Those cables are wired in parallel for using two USB ports from the same (!) source. If you want to use an external power supply then a differently wired cable must be used that cuts power from the USB data port.

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2012-05-23
  • Posts: 37

Re: The USB Y-cable and providing enough power

MC wrote:
tigersoul wrote:

The manual for the Babyface says you can use the Y cable with a power bank

It does NOT say that for your use case, but for stand-alone operation.

tigersoul wrote:

Does this come with any dangers?

Yes. Those cables are wired in parallel for using two USB ports from the same (!) source. If you want to use an external power supply then a differently wired cable must be used that cuts power from the USB data port.

Yeah I realize that now, reading the manual closer. At that point in time I didn't read as carefully. Those cables should be friggin outlawed. Which they are, USB spec forbids them. Manual should be more clear about what you can and cannot do.

The question is what can happen here and whether I may have ruined something. Everything evidently works as far as it seems so far. Been googling but had little actual suggestions on HOW bad it is. Just people saying it's not a great idea.

From what I can tell in results, the power source with the higher voltage (which is the USB bus) will serve more of the current to the attached device. USB has current limitation so over drawing current should not be possible. Also, the power source with the lower voltage will become part of the load and have current flow back into it. There may or may not be protection from this depending on the quality of that power source. It was a Chromecast original dongle so not a cheap China charger for what it's worth.

Investigation so far thus seem to suggest the lower volt power source is the one most likely getting into trouble. On top of that of course a risk för signal disturbance. In fact, the reason I realized I had this still connected was the babyface hung when dealing with an ADA1000. Then I saw the adapter connected and went "oh shit". The babyface came right back on after disconnecting the extra power source and it seems OK. Not that I tested every imaginable feature yet but still.

More people must have made this mistake and I am yet to find a post about someone frying their USB port or interface but who knows...

5 Reply by MC

  • MC
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  • From: Germany/Thailand
  • Registered: 2007-04-06
  • Posts: 25,432

Re: The USB Y-cable and providing enough power

I also never witnessed issues. The electronics are current limited and sometimes even reverse current protected. The BF itself is not affected at all.

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2016-06-14
  • Posts: 1,142

Re: The USB Y-cable and providing enough power

Halfway off topic but maybe interesting anyway: I once destroyed an Apple Lightning USB adapter in a comparable setup. I used an USB Y-cable to connect an audio device (not RME interface but Sounddevices MixPre) to my iPad. So it was one end on the adapter, the other on a power supply ... bad idea, the adapter was roasted!

UCX - FF 400 - Babyface pro - Digiface USB - ADI-2 (original)
Mac mini M1 - Macbook pro - iPad Air2