MB500 buzz issue

floatingaxe · #1 05-02-2011, 01:27 PM

Very disappointing weekend.
I tried to hook up my MB500 using the integrated DI (pre-eq, ground lifted) and there was a bad buzz in the house mix. Enable the ground and the buzz goes away from the house, but is now in the cab. I did some testing and if I simply plug a XLR cable into the DI there is no buzz; grounded or not.
Plug the XLR into the system and the buzz is there. In the house if lifted, in the cab if grounded.
What's the deal? I really want to use the DI. The line out is post-eq/master so that isn't really an option.

Wes Whitmore · #2 05-02-2011, 02:32 PM

Can you try a test where a mixer and the amp are on the same wall power outlet and let us know what you find?

floatingaxe · #3 05-02-2011, 02:58 PM

Unfortunately, no. Well, not easily.
It's a fixed sound system in my church. Not portable.
The bass players usually go direct to a passive DI then to the amp and cab, but A. the built-in DI is supposed to provide a better/cleaner feed and B. I want to use the tuner out and mute on the amp.

Wes Whitmore · #4 05-02-2011, 03:20 PM

Is this the first time you have used your amp in this location and in this config?
I understand the desire to use the gk preamp and tuner out. Just out of curiosity, does the hum change when you go pre to post eq on the XLR out?
If you don't have a test mixer, maybe you could just take your amp up to the soundman and ask him to plug into the same 120v power as the FOH mixer, and run the XLR out directly into the inpu on the board and see if the hum goes away. That way, you will know if its a ground issue, or the amp. if you can replicate the buzz "on the bench", send it back.
Wes

floatingaxe · #5 05-02-2011, 03:57 PM

I didn't try post-eq.
I might be able to plug into the outlet where the board is plugged in.
It just seems odd that the buzz switches to the cab when the ground is enabled.

okcrum · #6 05-02-2011, 03:59 PM

Not unusual, if the DI out of the MB500 is not actually isolated. This is the way most DIs are. Try an inline transformer on the DI line. It should cure both your hum/buzz issues.

Wes Whitmore · #7 05-02-2011, 04:38 PM

Could you use an external DI to isolate between the amp and the mixer?

I have has some amps with weird grounding issues. I send them back until I get a good one.

**ddbassGA** · #8 05-02-2011, 07:17 PM

Welcome to the new GK.