Angelo..
The culprit in clubs and restaurants is most often the things being shared on the line you are using.

For instance, fluorescent lighting or lights regulated by dimmer switches can cause a buzz. If the line serves any heating or cooling appliances, even electric coffee pots, a buzz or dip or spike could easily result.

Short of buying some kind of filter, your best bet would be to try find a plug on another circuit. It would be ideal if all outlets around the staging area were clean, but this is seldom the case. Howver, if the owner is bothered by the buzz, he might be inclined to have a clean line installed.
Eddie