Mbox Pro 3 Phantom Power On and Using DI
 

Hi all! I'm hoping someone can answer this question, as I'm cautious about damaging my equipment.

Using an Mbox 3, I need to connect a condenser mic and a DI to track an acoustic guitar with a pickup. These will be on the XLR inputs. However, if I turn on Mbox phantom power for the mic, how will that affect the DI, if power is being sent back to it through the XLR?

Will it cause the DI signal to overdrive the input?

Will the DI (UA Solo 610) will ignore the power and the signal not be affected?

Is it possible that the DI could be damaged from the 48v being sent back to it?

Thanks in advance!

Re: Mbox Pro 3 Phantom Power On and Using DI
 

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...

Re: Mbox Pro 3 Phantom Power On and Using DI
 

I thought about using a TRS connector, but I'm trying the get the benefits of the UA 610 and XLR signal.

Maybe the best solution is to use the UA 610 and its phantom power for the mic, and another (though lesser) DI on the guitar's pickup output. That way, I don't have to turn on the Mbox phantom power at all.

Unfortunately, I only have one UA 610, and that thing always finds the sweet spot when I track guitar or bass.

Re: Mbox Pro 3 Phantom Power On and Using DI
 

What nonsense you talking about? "benefit of XLR signal"?

If you want to use the Solo as a DI in this setup... You put the Solo output to line level. You take a XLR female to TRS plug cable and plug the XLR end into the Solo Output and the TRS plug into a line input TRS part of one of the multi-plugs on the Mbox 3. This is the "proper" way of using the Solo as a DI, which is what your question was. If you want to use the solo as a DI you don't want to be driving a mic level signal into the ****astic preamps on the Mbox.... even if you can't fully bypass the preamps from the MBox inputs, the line level inputs on the TRS inputs should let you get more color out of the Solo. And the TRS inputs don't have phantom power on them. If you want to use the Solo as a preamp for the Mic then use it for that, it might be the best choice. But that's a separate question that you asked and you don't need to do that for the reasons you seem to think.

Re: Mbox Pro 3 Phantom Power On and Using DI
 

Thanks for your responses Darryl, you seem to have a lot of information, however I only have TS guitar cables, not TRS. A balanced XLR is much preferred over an unbalanced TS, simply for its better signal to noise ratio.

I appreciate your input.

Re: Mbox Pro 3 Phantom Power On and Using DI
 

Maybe you can run the Solo preamp in line output mode and pad the Mbox preamp and turn the gain down and the gain staging will work as well as going into the line level inputs. Neither are great, and there is no way to get the Mbox preamp out of the circuit. But you would only know what sounds best by testing the different connections/gain stagings but I would not assume that mic level output from the Solo is where you should start, for any signal.

You have an Mbox and that external preamp... you should be using a TRS terminated cable into the Mbox if for no other reason than to do exactly what you are asking and avoiding spraying phantom power around. You should have just purchased that cable when planning to connect these devices.

Buy a new cable, cut off the end of an XLR cable and solder on a TRS plug (few $), or buy an TRS to XLR adapter (starting ~$5).

Re: Mbox Pro 3 Phantom Power On and Using DI
 

Just to add a little more knowledge, calling the UA 610 a DI is like calling Steve Vai an "average guitar player":D The 610 is a full-blown preamp, which happens to have a DI input(as many external preamps have). No matter what you feed IN to it, the output is a line level signal and that's too hot to feed into most mic preamps. Going into the Mbox via the TRS input is the proper way to use the 610. To use a standard DI box, then you would go in via the XLR input(but few DI boxes will have the fatness of the 610). As always, a quick read thru the manual for any DI or preamp will give you the proper info to get the best results:D