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Old 09-13-2013, 02:42 PM
bcent15 bcent15 is offline
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Default Are the Mbox 3 Pro line inputs too hot?

Here's my issue: I have an API lunchbox with a 512c preamp installed connected into a line input of my Mbox 3 pro via a single XLR female to 1/4" TRS cable (Lunchbox to Mbox respectively). The line input is set to +4 on the Mbox. When my input signal on the preamp reads 0VU, the input level on the Mbox and in Pro Tools 10 reads about -6dBfs. That's too hot, it should read about -18dBfs, correct? Depending on the system, 0VU should be at least somewhere between -20dBfs and -15dBfs. This is frustrating because this setup won't allow me to open up my preamp as much as I 'd like to. According to the Mbox technology guide, the line inputs of the Mbox 3 do indeed bypass the internal preamps: http://www.avid.com/static/resources...logy_Guide.pdf

Does anyone know what is up with this level mismatch?
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Old 09-16-2013, 04:39 PM
bcent15 bcent15 is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

bump
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Old 09-17-2013, 07:08 AM
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Ben Jenssen Ben Jenssen is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

I never used a series 3 mbox, but can't you adjust its input in the control panel?
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Old 09-17-2013, 07:16 AM
DMB13KOS DMB13KOS is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

I've had trouble getting a hot enough signal from a guitar amp line out into the Mbox line in.
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Old 09-17-2013, 08:31 AM
bcent15 bcent15 is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

Quote:
I never used a series 3 mbox, but can't you adjust its input in the control panel?
That only works for monitoring purposes, it doesn't actually attenuate the signal at the line input. Even if I was mistaken and the fader on the control panel is all the way up to zero, there still shouldn't be so much of a mismatch between what the preamp is putting out and what the mbox is seeing.
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Old 09-17-2013, 10:00 AM
Pinball Wizard Pinball Wizard is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

Yes, this is a problem. If you put +4 into the line inputs it comes out as -10 in Pro Tools. That's waaaaay too hot. I'm not sure what they had in mind here. Even if you are using a console insert point that runs at -2, you still hit PT at -16 which is a little too hot for my tastes. I built up two 8 dB resistive pads in barrels to use with the line inputs.

No, the front panel faders do not control the line input gains.

Related notes:

This is not a problem at all with the Eleven rack....+4 in gives -18 in PT just like it should be.

Also with the Mbox 3 pro & Mbox 3, the microphone inputs are just the opposite....they are low in gain. See

http://duc.avid.com/showthread.php?t=319700


btw, I packaged an old API 312 card onto a 500 rack panel & edge connector. I run it into the line input of the mbox 3 pro. I only use one of the secondary windings in this case & it works extremely well. IMO you should be able to obtain excellent results with what you have when you get the level issues straightened out.
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Edit Room: Pro Tools 8LE 8.0.5, Windows 10, HP Z400 Xeon 6 core 12 GB DDR3, Eleven R, UAD 2 PCIe card

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Old 09-18-2013, 08:20 AM
bcent15 bcent15 is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

Thanks, glad to know I'm not the only one who recognized this as a legitimate issue.

I'm assuming the pads you've built have no sonic impact on the signal, because you probably wouldn't be using them otherwise. But that would be my main concern if I were to purchase one such as this: http://www.sweetwater.com/store/detail/MAX10/

How difficult is it really to build a pad like the one you've built for a balanced XLR connection? Isn't it just a few resistors to solder? Of course you need to know the correct values, but I can't imagine that it would be overly difficult to execute.
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Old 09-19-2013, 02:04 PM
Pinball Wizard Pinball Wizard is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

OK First let me say that I do know that this is NOT the best way to do this. The right way would be to adjust the gain of the Mbox 3 Pro so that it is correct. Unfortunately Avid doesn’t publish the schematic. You could open it up and try to figure it out, but that is difficult and you could easily miss something and end up compromising the stability of the system.

So, that understood, on to the pads.

The Sweetwater pad that you linked is not really the best tool for the job. It looks like it provides a 150 ohm load. That’s dangerously low for many output stages. In general these days we want to have low impedance outputs looking into loads of several thousand ohms. Asking the source to provide current into lower impedance loads can increase distortion. (Although sometimes we do provide a little load to trick an opamp output stage into class A bias…..but that is another subject.)

There also is a Shure A15AS which has a higher input impedance. This should be ok but be warned that it is a T configuration and you will lose common mode rejection. If you are using short cables in a non-hostile environment, this shouldn’t matter and this may work out fine for you.

Building your own is certainly an option. That’s what I did. I used Switchcraft XL barrels. These have a little more room than the traditional Neutrik ones but I understand that there is a newer Neutrik style that I have not used but you may prefer.

You may find formulas or tables on the internet to calculate the resistor values. Forget about them…they won’t work. They are based on known (usually 600 ohm or 150 ohm) impedances. You probably don’t know the exact impedance of either your source or load and even if you did, you might decide to use different equipment in the future. Please understand again that changing the gain is the right way to do this. When you build a pad, it is only correct for one set of equipment…..you just need to be satisfied with that. You will find the values empirically. Clip lead together some variable pots, adjust them to suit, and measure the values. Then build it up with fixed resistors of those values.

The simplest configuration is like a letter “U” on its side. Two equal resistors will be in series with the source and one resistor will shunt across the output. Adjust the whole thing to get the loss that you want for your specific equipment. You will need more loss than me because you are probably running all secondaries of your output transformer. Here is the guidance….. You want to provide your source with a load of several thousand ohms. You obtain this with the series resistors. I would say at least 3,000 but 6,000 minimum is a traditional value. These series resistors will not have any loss at all until you add the output shunt. The smaller this resistor, the more loss. Just as a starting point, I ended up with 3,300 ohms for the series resistors and 5,000 ohms for the shunt. I did this so a Shure mixer that I have would read indicated zero (+4) and Pro Tools would show -18 dBFS. It also works well with my API 312 card.

Try to locate the pad close to the mbox 3 Pro. The impedance of this connection is higher than we usually like to run so it may be vulnerable to hum & noise pickup. Also, the values of the resistors are not critical but if you want to run long cables, it is very important to match the series resistors. A close match is much more important than the actual value. I usually get about a dozen and measure them with a meter picking out two that are closest.

Good luck!
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Main Workstation: Pro Tools 2018, Windows 10, HP Z440 Xeon 6 core Broadwell 3.6GHz, 32 GB DDR4, Omni, Sync HD, UAD 2 PCIe card

Edit Room: Pro Tools 8LE 8.0.5, Windows 10, HP Z400 Xeon 6 core 12 GB DDR3, Eleven R, UAD 2 PCIe card

Both systems old & previously Avid qualified
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Old 07-26-2015, 07:18 PM
Swamp Turkey Swamp Turkey is offline
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Default Re: Are the Mbox 3 Pro line inputs too hot?

Quote:
Originally Posted by Pinball Wizard View Post
Yes, this is a problem. If you put +4 into the line inputs it comes out as -10 in Pro Tools. That's waaaaay too hot. I'm not sure what they had in mind here. Even if you are using a console insert point that runs at -2, you still hit PT at -16 which is a little too hot for my tastes. I built up two 8 dB resistive pads in barrels to use with the line inputs.

No, the front panel faders do not control the line input gains.

Related notes:

This is not a problem at all with the Eleven rack....+4 in gives -18 in PT just like it should be.

Also with the Mbox 3 pro & Mbox 3, the microphone inputs are just the opposite....they are low in gain. See

http://duc.avid.com/showthread.php?t=319700


btw, I packaged an old API 312 card onto a 500 rack panel & edge connector. I run it into the line input of the mbox 3 pro. I only use one of the secondary windings in this case & it works extremely well. IMO you should be able to obtain excellent results with what you have when you get the level issues straightened out.
Hey PW,

I was about to purchase a Neve 1073LB to use with my Mbox 3 Pro when I saw this thread about the line inputs being too hot. I am not going to lie; this makes me extremely upset with Avid. I expected more from them. However, has something been done to relieve the problem since 2013?
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