Buzz

[lieperjp]

Well, it's time for me to stop lurking...

We've got a problem. I have a system set up (well, not me because I'm not running the system) for our talent show. We're getting a terrible buzz from the mains and the monitors. Here are some details:

I get a very small buzz from the mains when the mixer is sending no signal (i.e. off) - but a very noticeable buzz when the mixer is on.

I get a buzz from the monitors when the mixer is sending no signal. I get a louder buzz when the mixer is sending signal.

The equipment is:
Mackie SR32.4 VLZpro
Mains run through a Crown XLS802
Monitors run through a Crown XLS602
A variety of wireless mic receivers are connected.
There are no signal processors between the mixer and the amps, nor the amps and the speakers.

All the sound equipment is on the same ground. I have tried three different electrical systems in our building, so I really doubt it is the power. I've switched out cables, connectors, whatever I can. I've tried a different mixer. The buzz does not go away. I'm really hoping the problem does not lie within the amps, however, I am stuck and don't know what else to test. Please suggest something! Thanks for any help...

 

[Anonymous067]

Okay. First, start disconnecting devices until the problem goes away.

Does the problem still exist after all the inputs (mic cables) are disconnected from the back of the board?

Then try disconnecting the output of the board from the amps.

Does the hum exist when the board is completely disconnected?

Try soloing all channels if the problem goes away after you unplug all the inputs. Plug them in one by one and find out which one causes that hum to reappear.

It could also be a Gain Structure issue, I'm sure somebody more experienced and qualified to explain gain structure (somebody who isn't a liability issue) can elaborate on GS.

 

[DuckJordan]

Like Blah067 stated start with your mixer and work your way back, just because all the channels are muted do not mean that it wont send a signal, (i get a popping noise when i hit the pfl button on our board in our booth monitors even when the channel is muted), start with the inputs into the board, then disconnect the board from the amp (go amp side and disconnect those leads not the xlr output on the board).

If that doesn't work try and get a hold of a different amp that will run the speakers just to see if it may be the amp.

last if your cable runs next to a power line between board and amp you may get electrical interference if your not using shielded cable or if its not in a conduit.

Gain structure really should not create a buzzing noise (gain structure really refers to the feedback loops created by outlandish frequencies.)

 

[Anonymous067]

Well, all I'm going to add on to what we already said is that yeah...duh...PFL overrides the mute button...

 

[museav]

Gain structure really should not create a buzzing noise (gain structure really refers to the feedback loops created by outlandish frequencies.)

I believe you may be thinking of loop gain or of Potential Acoustic Gain and Needed Acoustic Gain (PAG/NAG). PAG/NAG relates to acoustic gain while loop gain can be acoustic or electronic.

System gain structure is simply the electronic signal level as it passes through the system signal path. Typically the goal is to set the levels through the signal path to optimize the resulting noise and headroom. Probably the most common approach is the concept of having all device inputs and outputs clip at the same time although in some cases a unity gain (constant level or what goes in is what comes out) approach may be appropriate. One common result of poor gain structure is to raise the noise floor via having too low a signal level and then adding excessive gain to compensate with the result of audible hum and noise.