SD must get a hundred emails a day about this topic as it sure creates a lot of confusion...
The USB 2 standard allowed for USB ports to output a maximum of .9 amps. For a long time, this was the maximum USB devices would expect or deliver. Some vendors - ie Apple - created custom scenarios (like USB-A to lightning connector) that provided more than .9 amps and there were specialized variations like USB Power Delivery for charges, but .9 amps was the standard. With the USB 3.x standards, the standard was raised up to 3Amps. The problem is that you don't want users to plug in old devices to newer batteries or power supplies and have them blow up/cause a fire, etc. So standards were developed for cables to prevent this. By the specifications, a USB-a (USB 2) to USB-C (USB 3) cable is supposed to have pull up resistors which limit the amperage to - you guessed it - .9 amps. The Mixpre-3/6 wants 1.5 amps (5 volts x 1.5 amps = 7.5 watts). This is why SD's power FAQ says:
NOTE: A USB-A power source used in conjunction with a USB-A to USB-C cable cannot deliver the required 7.5W for full power operation.
This is also why they include the y cable. A majority of people have computers with USB-A ports. People buying Mixpre's are going to want to connect them to computers as audio interfaces. Their solution - provide a y cable with two USB-A ends ( 2 x .9 amps = 1.8 amps).
A USB-A power source used in conjunction with a USB-A to USB-C cable cannot deliver the required 7.5W for full power operation.
Some vendors on ebay and amazon, however, sell USB-A to USB-C cables that don't have the resistors in them. In this thread, some people have identified such cables. If your USB battery supports 1.5+ amps and you use one of these cables, you can get the full 7.5 watts. SD does not officially recognizer this option as it bypasses the standard (the fact that these vendors do this bothered a Google Engineer and he actually went through Amazon vendors at one point and certified whether they were following the standards). There is nothing wrong with using a non standard cable if you know what you are doing. Several people here do because they can use the USB-A batteries they had. Think about this from a vendors point of view, however, and you will realize it's not a good idea. Some people also use the dual dual USB-A cable and simply plug both USB-A ends into their battery. Based on comments I've seen this is not a good idea at all.
Fully powered versus under=powered Mixpre. The Mixpre does have a low power mode and it can be run with less than 7.5 watts with limitations. The FAQ U linked below explains the limitations. Your battery icon will show a orange meter not a green meter.
ps. Can we get a picture of a talentcell running a Mixpre via the dual USB-a cable , with only one USB-A end connected, showing s full green battery indicator? That makes no sense.
Sound Devices Power FAQ:
https://www.sounddevices.com/tech-notes/mixpre-3-mixpre-6-powering-optionshttps://www.extremetech.com/computing/115251-how-usb-charging-works-or-how-to-avoid-blowing-up-your-smartphone