There’s no better feeling than when you’ve set up your system and powered it up to find it lacking any noise. 

This is no major feat when you have your own electrical distribution, but when you’re relying on the house or festival distribution, things can be a bit more dicey.

The classic scenario when the system powers up with a hum is for the operator to declare “ground loop!” followed by muttering and cursing while digging in the accessories box to locate the power cord cheaters.

I find it funny how a device that’s supposed to be used to provide an electrical ground connection on older two-prong outlets is most often employed to lift the ground instead.

But I often see that the whole ethereal concept of a “ground loop” is a distraction from one big fact: not every hum is from a ground loop.

What seems to be nearly universal is that a lot of folks don’t understand that the existence of a ground loop is not actually the cause of a ground loop hum.  A ground loop is an existing condition that is exploited by the true problem, which is an electrical current flowing through the loop.

Just so we’re all on the same page, a ground loop occurs when there are multiple ground paths between two pieces of gear. Typically, these paths are through the electrical ground, the audio wiring shield, and through shared chassis contact when together in the same rack.

Carefully designing a system to not have ground loops is certainly a valid engineering endeavor, which goes to my primary point: treat the cause, not the symptom.

An Interesting Case...
I had one system upgrade project at a mid-sized church here in Washington state where the sound system had a buzz problem that historically had not been solved.

When the original system contractor and I first walked into the sanctuary, the buzz was immediately apparent. The contractor turned to me and said, “Oh by the way, we’re going to solve their noise problem too.” Which was to say, “You need to fix this because I can’t.”

I hemmed and hawed about how the problem could be from the transformer on the pole, but was actually just making stuff up out of thin air, aggravated about this additional time-consuming task being added to my already conservatively budgeted schedule.

But I got on with acquainting myself with the system. Much of it had been obtained as surplus from the 1986 World’s Fair in Vancouver. I was surprised to find a rack filled with Bryston amps, a few years of warranty left and working just fine!

What disturbed me was that everything at the amp rack was plugged into “cheaters” and Radio Shack power-line RF filters. Someone wasted a pocketful of cash to buy those, and it was simply very bad AC practice to boot. 

This led to fix number one: I removed all cheaters and filters, and the molesto mucho buzz was transformed into a simple hum. A step in the right direction.

No other obvious conditions that might be causing the hum could be detected at that point after eliminating the lighting dimmers as a source.

The Saga Continues...
Two days later, we were back, doing the first thing these projects often require: taking everything apart. As the week progressed and we slowly re-assembled the system, I got to a place where I could examine the power distribution. 

I strung an extension cord from one of the sockets at the amp rack to the front of house position, and then deployed a Wiggins (solenoid voltage tester) to check between the hot on the extension cord and the hot at the FOH outlets. 

As feared, it measured 208 volts, meaning that the two different power circuits were on differing phases. Time to root around in the breaker box…

The building had a modern electrical system, which is fortunate. I’ve run across some problems in systems that were on legacy electrical distribution, and short of violating code, there’s sometimes not a whole lot that can be done to fix a problem. 

Suffice to say that a dedicated electrical ground and a modern distribution system is imperative for safe, noise-free sound.

Four circuits in the breaker box were dedicated to the sound system, and sure enough, they were grouped together all in a column. This was a typical commercial three-phase breaker box with the phase alternating for every row. 

The shame was that I could tell by reading the written-over labels that originally, the sound system circuits had been on every third row. My guess is that a well-meaning electrician had thought it would be smart to group the circuits together during one of the church expansion projects.

So, fix number two: I returned the circuits to their original spacing.

It’s a good idea to have all the power circuits on an identical phase because power supplies can leak.  Float a piece of gear and you can usually measure a small voltage from the chassis to power ground. 

When the chassis is electrically grounded, this potential doesn’t magically evaporate, and under the right circumstances, it can actually help create a ground loop current. 

If there happens to be leakage from gear elsewhere in the ground loop and it’s of a different phase, an inter-chassis current can flow through the loop due to differing resistance in the multiple ground paths.

Having all the sound system gear on a common electrical phase minimizes the chance of having a leakage induced ground loop current.

In other words, less chance of current = less chance of hum.

Moving Right Along...
The original installation also included a remote power switch for the amp rack, which consisted of a key-switch on a panel at front of house that fed low-voltage AC from a doorbell transformer to the coil on a relay at the amp rack. Turn the switch, the relay closes and the amps have power.

The problem was that the electrical installer appropriated one of the shielded balanced lines running through the front of house to stage/amp rack conduit for the remote power switch.

I don’t care what kind of coupling was going on in that 80 feet of conduit because no matter how you look at it, what was happening in there was a sin.

Thus, fix number three: eliminating the remote power switch. We could have run an alternate line, but ultimately decided to skip the hassle and just do away with it.

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