[Pinball Wizard]

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!