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Friday, December 02, 2011

Accidental Electrician: Eliminating Dreaded Sound System Hum & Buzz

An epic tale of finding and eliminating a system's long-time and ongoing hum and buzz issues. Just who was the culprit?

There’s no better feeling than when you’ve setup your system and turned it on to find it lacking any noise.

This is no major feat when you have control of the electrical distribution, but when you don’t, 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.

In such situations, you gotta do what you gotta do to safely eliminate these nuisances if possible. But I often see that the whole ethereal concept of a “ground loop” is a distraction from one big fact: not every hum comes 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 only a condition that is exploited by the true problem, which is an electrical current flowing through the loop.

Carefully designing a system to not have ground loops is a noble engineering endeavor. But in my book, there’s no real reason to do so if you take the time to eliminate the actual hum sources and potentials.

Treat the cause, not the symptom.

One job I worked in the past was on a sound system with a noise problem that historically couldn’t be solved.

This system was in a mid-sized church in a community known as the “Home of the Hardheaded Dutch.”

(You may accuse me of racial stereotyping, but believe me, this is how they referred to themselves. And they were proud of it.)

When we first walked into the sanctuary, the system was on and I immediately heard a significant buzz. 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 having been surprised with this additional time-consuming task being added to my already conservatively budgeted schedule.

The church elder and de facto tech director we were working with was a retired Master Chief named Dave. He knew diddly-squat about sound systems, but possessed a good nose for BS. I could tell he wasn’t buying mine.

Pocketful Of Cash
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, still with 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. 

I removed them and the molesto mucho buzz was transformed into a simple hum. A step in the right direction…

After eliminating the lighting dimmers as the noise source, there were no other obvious conditions that I could immediately identify as contributing to the problem.

So I went home and whined to my wife about how I had been abused by having this additional issue dumped in my lap. She didn’t buy my BS either.

Three days later, we were back, doing the first thing these projects always require: taking everything apart. As we slowly updated and 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 used my trusty old Wiggins to check between the hot on the extension cord and the hot at the front of house outlets.

As feared, it measured 208 volts, meaning that the two different power circuits were on differing phases. It was 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. 

Well-Meaning Electrician
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 panel were dedicated to the sound system, and sure enough, they were grouped together all in a column. This was a typical commercial 3-phase 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 (i.e., a common phase).

So the original system installer (OI) had specified that the circuits needed to be all on the same phase. My guess is that some well-meaning electrician had thought it would be smart to group the circuits together during one of the church expansion projects. I returned the circuits to their original spacing.

Having all the power circuits on an identical phase is important for minimizing the possibility of an inter-chassis current in a ground loop.

Power supplies leak small amounts of AC to ground. If there happens to be enough leakage from gear in the ground loop, and it’s of differing phases, an inter-chassis current will flow through the loop great enough to induce a hum.

By establishing a common system power phase I minimized the ground loop hum potential.

Elsewhere though, the OI committed a serious faux pas. The original installation included a remote power switch for the amp rack, which consisted of a key-switch on a panel at FOH that fed AC from a 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 OI appropriated one of the shielded balanced lines running through the FOH to stage conduit for the remote power switch. For 90 feet, there was a line carrying 60 Hz AC in cramped proximity with all of the system audio wiring!

Removing the remote power switch eliminated this hum potential. We could have run an alternate line for it, but Dave didn’t care for the attitude exhibited by those who had keys (control) towards those who didn’t so he decided to just do away with it.

Sudden Problems
Next, the system was gotten to the point where the console was re-patched and turned on. There was a hum.

Examining the console, I began to see that there were some channels and sends marked “Bad” and “Don’t Use.” I began to grow suspicious. Dave said that one day they came in and the console was suddenly having these problems. My gut said the console had taken a surge during a lightning storm.

They had continued using it not realizing that there was more to the console problem than just non-working channels and sends.

Replacing faulty gear (and a lecture about surge protection) was the solution for this hum source. We snagged the console from the church’s youth room and patched it in. Ahhhh, schweet silence!  I declared, “This house is clean!”

Too bad it wasn’t so.

It was the afternoon of the project’s last day and there was a sound check scheduled with the worship team that evening. We had patched in numerous mics for vocals, piano, drums, guitar cab and DIs for acoustic guitar, bass and keys.

When I opened the channels and pushed up the faders, there was a hum. My heart sank and Dave now began to give me “the eye.”

It wasn’t on just one channel, but on almost every channel from the stage. Pull the faders down or mute the channels and the hum would go away.

So the problem wasn’t ground related because it was controllable. Phantom on or off made no difference. Dynamic mic, condenser mic, or DI also made no difference.

I hit the stage and started taking a closer look at things. I picked up one of the mic cables and was disgusted to find that I recognized the writing on the jacket. I had seen it once before.

This cable had a certain respected pro audio name on it that had no business being there. I don’t know if this cable was a manufacturing screw-up or possibly a counterfeit, but it had no valid use anywhere in a sound system. It wasn’t even shielded twisted pair.

I looked across the stage and counted upwards of two-dozen of these demon-possessed cables from hell.

Luckily, there was a decent pro audio store about 20 miles away. We bought out their stock and I cut up the old cables so no one would be tempted to salvage them from the dumpster and put them back into service.

The final fix was replacing the bad cables with good ones. For the first time ever, the system was noise-free and there wasn’t a single lifted ground or shield anywhere.

I suspect that the original issue was with the remote power switch, and as the other issues occurred, they contributed to the complexity of solving the problem.

It’s a shame that out of hardheadedness they put up with it for as long as they did. But I can well understand the lack of trust when it came to parting with their money over a seemingly unfixable problem that shouldn’t have been there in the first place.

BS comes in many forms. Sometimes as talk, sometimes as a bogus feature in an install.

Not many weeks later, the church decided to expand the project by replacing the ailing console, adding a personal monitoring system and investing in some surge protection.

I guess we’d earned the respect of the Master Chief. Hooyah!

Since his start 30 years ago on a Shure Vocalmaster system, James Cadwallader remains in love with live sound. Based in the western U.S., he’s held a wide range of professional audio positions, performing mixing, recording, and technician duties.

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