[Darryl Ramm]

First thing is the “right” way to do this is UA Solo 610 should be connected to a TRS line level input on the Mbox. There is no phantom power on the TRS inputs.

But even if you are not doing this phantom power should not harm the output of another preamp. This is a valve preamp, it will have an output transformer with something around 50 ohms DC resistance. Phantom power will not deliver 48 volts to such a low resistance. If the phantom power was a perfect voltage source it would sink around an amp into the transformer and generate around 50 watts of power... which would fry stuff quickly... but phantom power is very low output impedance, and the MBox will source a few mA at most... so no it won’t damage the output transformer on the UA preamp.

The question then becomes what happens to phantom power on the Mbox when trying to drive a low impedance load like 50 ohms... the answer should be nothing, and a good phantom power circuit (starting with just a big old resistor in series) should current limit itself and should also be separate DC supplies to each mic input... so connecting one to a low impedance source does not disable others. In this case... all bets are off, without looking at an actual Mbox 3 circuit. And competently designed phantom power circuit should be able to survive a dead short.

You can try it and see, what’s the worse that happens is both phantom power voltages drop too low to power the mic, or maybe you cook the Phantom power on the Mbox 3, hey it’s only an MBox 3... or you know, you could just do this properly with an XLR to TRS cable...