Pre-Amp Problem - Help!

J

JMAC

New member
Hi,

I recently bought a Behringer Ultra Gain Pro to use as a pre-amp when recording onto Pro Tools.
I have a very basic set up, PT8 with original Mbox but have recorded guitars and vocals no problem for several months.

Recently I bought a new condenser mic for vocals and knowing it would need phantom power, I hooked it up to the Behringer.

Problem is, there is an overpowering static buzz coming back from the pre-amp.
When I plug headphones into the output on the amp, the signal is nice an clear, but as soon as it's connected to the Mbox it goes to mush. And, I can't seem to get enough juice from the mbox to run phantom power to the mic without the pre-amp.

Any ideas what could be causing this buzz?

I think I might need to buy a separate phantom power for the mic regardless, but any help on the pre-amp would be much appreciated.

Thanks,
 
You almost certainly have a hum/earth/ground loop.
This is caused by the fact that both PC and Berry pre amp are earthed to the mains and there is a second earth path via the balanced(I hope!) interconnecting cable(s)

The most elegant solution is a transformer isolator such as the Art Cleanbox ll or if you are in UK give Orchid Electronics a call.

A far cheaper solution is to modify the line level leads twixt the boxes by disconnecting the shield from ONE end of the cable in each case.

I can post a diagram if needs be.

Dave.
 
There you go.

I assume you are using a TRS to TRS cable. If an unbalanced TS-TS, nothing changes, just insulate the shield/screen at one end (if running two cables do both of course!)

If feeding the AI with an XLR* just cut the screen connection to pin 1.

Note, there is a faint possibility with this simple getout that you can end up with RFInterfernce, not very likely but even if you do there is another trick we can pull.

*For the "jobbing" recording bod it is worth making up a couple of XLR to XLR "slugs" about 200mm long with screen dissed one end for use when lumbered with strange equipment at the local "yoof" club or wherever!
You lot don't know you are born! In addition to vastly more audio connectors and no real level standards we also had to cope with some seven different mains sockets back in the "good" old days of tape!

Dave.
 

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Not to say that ecc83 is steering you wrong. It's not a bad idea to have at least a couple of lifted shield cables in the studio. Do remember to label or mark them somehow.

But the first thing to at least try in the case of a ground loop is to plug both devices into as close to the same outlet as possible. The same power strip or "power conditioner" is usually as good as you can get, but it can help sometimes even if that means running an extension cord. I don't ever trust separate wall sockets. Both studio and live, I do everything I can to keep everything plugged into exactly one outlet through as few "branches" of power strips as possible. It doesn't always stop ground loops, but it's good practice, and essentially free.
 
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