I have been a sound-tech for a few years. I have lifted many amplifiers and speakers and put them in their right position. I have miked a drum-kit and plugged cables into snakes and into mixers. I would even say I am quite experienced when it comes to live sound.

But there is one thing that really kills a show.

Hum.

That’s right. Hum is one of the things that really gets on my nerves. Just try sustaining the word hum for a while.

Hummmmmmmmmmmmmmmmmmm.

Annoying isn’t it?

True story. I was working on my own as a sound-tech one night. The band had called me at 2 pm and the sound-check was at 5. I also had to put the sound-system up. No problem, I can do that.

Not realizing these bands had around 24 tracks each and every band had a different line-up. Synths, computers, Glockenspiels, pianos, you name it. Everything that drives a sound-tech crazy when he’s trying to sound-check.

Finally I had the sound-system up and running and we were going through the sound-checks as normal. Everything seemed fine and we were kind of able to make these bands sound. I had no experience in sounding them before and their peculiar lineup made them it all the more harder.

When came show-time things started to go wrong. DI boxes stopped working, feedback liked to join the party way too often and although the bands sounded all right and I could more or less fix what went wrong on the way there was one thing that reared it’s ugly head that just would not go away.

Hum.

While frantically changing DI boxes and re-miking instruments on stage(all this while the show was going on, I’m fast….and invisible) something I did made the PA hum. And when you are mixing a concert with 24+ tracks you can’t go muting one and one to see which one is the problem. Luckily this happened before the third band went on so I could give a quick mute-run through the tracks to see which one it was.

Of course, it was one of the DI boxes.

You see, in this case I was suffering from a particularly bad case of ground hum. Ground loop hum is usually generated by a 50 Hz wave(60 if you live in the U.S.). In my case one of my DI boxes was somehow receiving it’s ground(or earth) from somewhere else causing a difference in voltage, resulting in the hum I heard.

Oh, please that’s always happening to me! Tell me the solution!

Oh, it is quite simple. When I finally figured out which DI box was generating said hum, I just went and flicked it’s ground lift switch. That way I disconnected it’s connection to the ground and thus eliminating the hum.

So next time you are frantically running around the stage figuring out where that hum is coming from, try checking if one of your DI boxes is causing it.

It may save your reputation.