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I am about to buy two USB-devices (a RIAA pre-amp and an iPad with the "Lightning to USB 3 Camera Adapter") and connect them to transfer audio using USB. The problem is that both these devices also are powered/charged via USB.

How can I power these devices using the setup described above? Is there something like a USB female-to-female that inserts power "from the side" into the circuit?

  • Which "both these devices"? One is "a RIAA pre-amp", I understand. What is the other USB device that needs the power over USB? – Ale..chenski Jun 7 '17 at 1:04
  • @AliChen An iPad with Lightning to USB 3 Camera Adapter inbetween. – d-b Jun 8 '17 at 14:38
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Is there something like a USB female-to-female that inserts power "from the side" into the circuit?

Yes, it's called a USB hub. 😊

USB hubs aren't just multi-way splitters; they provide USB bus power to downstream ports. "Self-powered" USB hubs (the ones that come with their own power supplies) usually provide USB bus power for "high power" 2.5W (0.5A @ 5V) devices on all downstream ports. Note that 2.5W isn't enough power to charge even an iPhone, much less an iPad. The USB chargers Apple includes with iPhones support 5W (1A @ 5V) charging, and the iPad chargers support 10W (2A @ 5V) or 12W (2.4A @ 5V) charging.

Even "bus-powered" USB hubs are expected to provide power to "low power" USB devices on all downstream ports. But low-power devices are only 0.5W (0.1A @ 5V). I would guess than any bus-powered audio pre-amp would be a high-power device.

How can I power these devices using the setup described above?

The Apple "Lightning to USB 3 Camera Adapter" you mentioned — the one that actually has a "3" in its name — has an extra female lightning port so you can plug in your iPad charger into that adapter, so you can keep the iPad charging while using the adapter.

Note that traditional USB does not support sending power into a USB host port, so simply hooking the USB Type-A connector of this adapter into a self-powered USB hub via an A-to-B (host-to-device) cable will not cause the iPad to receive power. Besides, even USB 3.0's notion of "high power" wouldn't be able to charge an iPad. The only way to charge the iPad with this adapter attached is to connect the the adapter's female lightning port to a 10W or 12W USB charger. Any lesser power supply may be able to keep the iPad from discharging as fast, but to charge it up while in use, you probably need the 10W or 12W charger.

If your pre-amp is USB bus-powered, you'll need to put a self-powered USB hub in between the Lighting-to-USB-3 adapter and the pre-amp.

  • Regarding the iPad's demand for power - I will run it the screen turned off (just using Garageband to redirect sound in to Airplay) so I think it would make do with just a little power. Charging the iPad via the L2U3CA's lightning port is a good solution for charging the iPad. – d-b Jun 7 '17 at 8:45
  • Now the RIAA amp remains. According to p 13 in this static.bhphotovideo.com/lit_files/61861.pdf it only requires 0,1A (5 V 100 mA). The iPad cannot provide that, can it? Would a USB hub work? How does a USB hub know that the sound from device A should go to B while sync is done between C and D and the file transfer between C and E? – d-b Jun 7 '17 at 12:25
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    @d-b I believe the Apple Lightning to USB 3 adapter does provide USB bus power for "low power" USB devices. The info you cited for your pre-amp meets that, so I believe you'll be able to get away without putting a self-powered USB hub between the two. – Spiff Jun 7 '17 at 19:20
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    @d-b Regarding your question of how USB traffic gets to the right devices, realize that a USB bus always has a host system that coordinates things, and in the setup I described, the iPad would be that host. The only devices that work are ones the iPad supports, and the iPad would take care of reading from and writing to each device. Devices on a USB bus each have their own address. USB supports up to about 127 devices on the same bus. – Spiff Jun 7 '17 at 19:26
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    @d-b One more clarification: The extra Lightning port in the L2U3CA is for charging only. This is because when the iPad is using the L2U3CA, the iPad has to act as a USB host (like a PC) not a device (like a peripheral or accessory). When in USB host mode, the iPad can't offer services to a PC host; so you can't, for example, sync your iPad to iTunes via Lightning while the L2U3CA is plugged in. Wi-Fi syncing should still work though. – Spiff Jun 7 '17 at 19:44
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To refine a part of Spiff answer, from appleinsider.com review:

In addition to a USB-A port, the camera adapter also includes a female Lightning port. This allows users to charge their iPad at the same time as using a connected device.

This gives a clear answer to the concern. The iPad is connected to this SPECIAL "camera adapter". The adapter ADAPTS power feed (through the female lightening port) to BOTH iPad, and to the downstream USB port. The adapter makes the iPad to be USB HOST, but forces the iPad to reverse the direction of VBUS, so it becomes a power consumer. This functionality resembles the so-called "accessory charger" concept from the USB Battery Charging specifications.

In conclusion, the lightening cable (connected to "camera adpater") will provide power to BOTH devices, iPad (as USB host), and a downstream (RIAA, camera, whatever) USB device, at the same time.

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