MBox hum -- especially on "inst"

[billsonbob]

Thank in advance for your help on this one. It's driving me nuts.

I recently got the Mbox. When I first started recording, I started to detect a noise that I thought originated from a Guitar effects pedal. This started a long investigative process, in which I unplugged and plugged every single piece of equipment I have. I then did the same to every single piece of equipment plugged into my computer. I also switched from USB 1 to USB 2 ports, and switch from mainboard to PCI ports, but no change.

Last night I was doing the Ghostbusters thing...wandering around the room with my headphones on and holding the MBox in odd places. (Glad no one saw me!)

This is what I ultimately found: The noise comes when i]absolutely nothing[/i] is plugged into the MBox except the USB. It doesn't matter what USB it's plugged into. It doesn't matter if I disable the LAN. I hear the noise through either headphones or speakers. I've tried a bunch of other things I haven't mentioned. In short, the only variables that change the noise/hum are:

1) Setting the Gain to "inst" makes it really bad, especially when the gain is cranked. However, I still hear the hum faintly and still hiss on the other gain settings.
2) Moving around in the room changed how bad the hum is. The MBox seems to really start humming near my Yamaha Keyboard when it's plugged in (even when not turned on). It also begins to hum when I personally get near it (but don't touch it. Touching it cuts the noise.)
3) This is the best of all...putting my finger over the SPIDF input line pretty much completely shuts the hum up. The hiss remains, unfortunately. But the hum completely disappears.


To me this seems like a grounding/electricity problem, but I'm a total ignoramus about these things. So even if that's what it is I don't have a clue what to do.

SO, I have three questions, all related to this issue:

1) Is a power conditioner going to solve this, or do I just have a defective box?
2) Has anyone had better success using a higher-quality USB cable, such as a Monster Ultimate Performance USB? Could such a thing make a difference?
3) Is there some way to jerry-rig a ground for the SPIDF input?


(One side note just in case someone wiser than me think it is related to electricity: We just recently moved into this newlY built condo. I've kind of felt like everything is half-a#^ here, so maybe the power is just crap. I dunno.)

Muchas Gracias

System:
AMD Athlon 1800+, 1.54 Ghz
PT LE 6.4cs9
Win XP SP2
640 MB Ram

[Icemaster]

I have an MBox too and I wonder why it is doing that. When I use Protools the hum would fluctuate, but when I use it as a audio driver I don't hear the hum really. Its mainly when I am in PT itself. why only with the Mbox though and not other devices?

[billsonbob]

Mine does it regardless of what program is running. It makes the noise as soon as the computer to USB connection is established.

[tele_player]

Since maxing the gain control boosts the noise, we know that the noise is induced in the analog input section of the Mbox. One thing to keep in mind when testing noise this way is that the inputs should be terminated.

In other words, if you're using the 'inst' setting on the input selector, a guitar cable should be plugged in, and the other end should be shorted together, grounding the input. Shorting them through a 5k-10k resistor is even more appropriate.

Similarly, if you're testing the mic inputs, you should use a mic cable with the '+' and '-' pins bridged through a resistor of about 150 to 1k ohms.

This doesn't solve your problem, but it's at least correct methodology for observing it.

It could also just be caused by noisy DC power on the USB; some people have found that a powered USB hub cleans up Mbox noise.

[billsonbob]

Heh ... well I figured it out.

Essentially I was just freaking out unessecarily.

I took it over to my friend's $30,000+ recroding studio and plugged it into his Mac. Same noise.

So we opened it up. We tightened the ground wire. It made a small inprovement, but not much.

Then (to agree with your last post) we tried it plugged into a bass. There wasn't nearly as much noise. While there was still a slight hum when cranked, the peak area was so low so that the noise-to-sound ratio was pretty much fine to our ears.

Then, just for kicks, we tried the same experiments I had been doing with his very nice, very expensive pre-amp/compressor combo. (Sorry, can't remember the brand.) We cranked it. Heheh ... suprise! ... same noise.

I guess when I was trying to figure out my GNX noise, I got sidetracked by the the MBox's natural noise.

In other words, this is just electrical hum. I suppose if we ran it thorugh a thousand dollar sound conditioner, it would go away. My friend has a decent Furman sound conditioner, but it still didn't eliminate that noise. Some guy has been telling me that the new Monster Sound COnidtioner will eliminate everything..but honestly I doubt it's even worth it at the kind of gain ratios we're talking about.

I am going to invest $20 in a monster USB cable, though. I suspect it will make a slight improvement on the STN ratio. I'll give it a whirl and post my findings.

Now I've just got to figure out why my stupid GNX4 is givng me a clean signal from it's TRS out but a horrible hum from the XLR's, even when I change the GRND lift ... but that's a discussion for another board I guess.

[Icemaster]

I'll try putting it on a powered hub then and see what happens. Thanks