Connecting A DI/Pre-amp Into Mbox 3 Pro

[bdam123]

Ok, I've read a bunch of post and haven't reached a definite conclusion to this question yet.

I just want to know if connecting a DI/pre-amp into one of "Line Ins" on the rear panel will bypass the Mbox pre-amp circuit.

To my understanding this definitely could not be done with the older Mbox's. I am just trying to get a definitive answer to if its possible with the Mbox 3 Pro.




*If it is, why isn't any of this mentioned in the manual? Isn't it very common for producers and engineers to have their desired pre-amps and not want to add another gain stage to there signal via Mbox's built in pre-amp (which I'm assuming are not as quality as most high end pre-amps out there). Or is this just because they are assuming that anyone that would be that serious about their signal would just opt for something higher up like their HD I/O interfaces?

[Pinball Wizard]

Yes, connecting to the rear line input bypasses the Mbox pre. You can see a diagram of the path in this document....

http://www.avid.com/static/resources...logy_Guide.pdf

I must warn you about the unusual gain structure of the mbox 3 pro. Although the mic inputs are short on gain, the line inputs have an unusually high gain. I had to build a box with pads to use them.

All that said, IMO the internal preamps are very good. If you have a direct box that has some unique character that you want to use, go ahead...but for a pure, neutral sound the instrument inputs on the front are very hard to beat.

[bdam123]

Thanks for the document, that helped a lot.

I'm running a BAE 312a into the Mbox. There is a pad on the it that I can use. Since I now know that the Line In bypasses the pre-amps I'm going to do a sound test with my guitar and see what I like better.

Regardless, I use the 312a mainly as a vocal pre-amp so I would rather take advantage of that over the built in pre-amp on the Mbox.

Thanks again.

[Pinball Wizard]

Ok, I get it. I have a tremendous amount of experience with API 312 cards. I used to use them to build systems in Vero frames. They have their own qualities which are different from the pres in the mbox & I can certainly appreciate that you wouldn't want to go through the 312 & then pad it down to go through the mbox preamp. IMO, you should have an excellent setup.

I should also point out for anyone looking at the drawings in the link that they do not make the location of the fader clear. The fader only works on the mic (and instrument) preamp....it does not control the line inputs. The line inputs have their gains selectable by a switch only.....no fader. That might be a problem if you just have a straight API 312 card but the BAE preamp has a level control so there isn't a problem here.

[bdam123]

I did an A/B between plugging my guitar into the BAE then going in through the "Line In" of the Mbox compared to plugging my guitar straight into the Mbox's "Mic/DI". The difference is significant to me. The BAE has more clarity and punch. The Mbox DI's are dull in comparison.

I recorded the clips here

[PD-John]

Originally Posted by bdam123

I did an A/B between plugging my guitar into the BAE then going in through the "Line In" of the Mbox compared to plugging my guitar straight into the Mbox's "Mic/DI". The difference is significant to me. The BAE has more clarity and punch. The Mbox DI's are dull in comparison.

I recorded the clips here

Wow, big difference. Mbox sounds somewhat nasal and closed in comparison. Thanks for posting the clips!

[Pinball Wizard]

Thanks so much for taking the time to post the AB test for us. IMO there is just no question....the 312s sound great. Congratulations on having such a good sounding setup.

[bdam123]

Yea no problem. Thanks. My band is gonna be working on a new song this month and it'll be our first time using amp emulation in the box. Once its all ready and done I'll post it so you guys can get a sense of what the 312 does to DI guitars.

[blueflamejames]

I find it interesting some Mbox pro's have unusually high gain through the line level inputs. Mine has the exact opposite problem - the gain is so low I get barely any signal. Meters on my preamps indicate proper levels (or I can even crank them all the way to clipping) and I get virtually no signal through the Mbox into pro tools. Test recording reveals barely the tiniest ripple of a waveform. Whether it's set to +4 or -10 (though it should be +4 in my case) makes an insignificant difference. I got this within the last two weeks and although the other features are quite nice, it's simply unusable for recording.

[Pinball Wizard]

blueflamejames: Just to be precise, I made some measurements. To insure that everything was proper, I hung a Hewlett Packard 403B on the output of my generator. The meter and the generator agreed. I hope my measurements will be of some use to you.

Using the 1/4" line inputs on the rear of my mbox 3 Pro and looking at a mono audio track in Pro Tools, I get....

With the switch in the +4 position:
+4 dBu (from the generator) = -10 dBFS (in Pro Tools)
or to put it another way,
- 6dBu = -20 dBFS
That's 10 dB too hot for us here in the US or 8 dB too hot if you like to do -18.

Operationally, some times I like to plug in a field mixer to the line inputs. It doesn't work. A reasonable level on the mixer's meter produces a track that is too hot. ..........and there is NO line level control on the mboox 3 Pro.

Just for reference, I also measured the line inputs in the -10 switch position. My meter does not read out in dBV so I used this calculator:

http://www.sengpielaudio.com/calculator-db-volt.htm

I gives me -10dBV = 0.316 volts = -7.78 dBu
I got:
-10 dBV = -16 dBFS
That's not as bad as the +4 but still a little too much gain.

Also since I had all of the equipment out, I gave the RCA connectors a try. As I understand it, they should work with -10.
I got:
-10 dBV = -13.75
again, too much gain.

I also note you say the switch results in an insignificant difference. On a relative basis, I see a 6 dB change when I operate the switch.

Can anyone else verify these measurements? I would be interested in knowing if all the mbox 3 Pros are the same.