Keeping Quiet
Learn to handle hum and buzz with a critical look at system noise from the ground up
Continued from Page 2

I will start with ground contamination. Standard 120 VAC Power is delivered to the outlet from the breaker box as three wires—hot, neutral and ground—all traveling through a common jacket. Cable types include plastic Romex, flexible metal jacket BX and conduit (pipe). Romex has a dedicated ground wire. Metal-jacketed BX cable has a somewhat less-substantial, non-insulated ground wire in addition to using the jacket as a conductor. Metal conduit is also used to distribute ground. In the latter two cases, the ground connection is made via clamps to the metal jacket. Following the ground from breaker box to a common outlet, the connections along the way are not always as positive as we would prefer. Only when the outlet is screwed into a metal mounting box is the ground connection made. This meets code for safety, but its effect on the system noise is a different matter.

Figure 3. How standard and balanced power would look on an oscilloscope. Wiring color codes are also included.-white is neutral; black is hot, and green, the metal-to-metal connection, is ground. The annotated colors refer to neutral(blue) as return for the hot sauce(red). The ground wire(green) provides safety by making the chassis-to-earth connection
(CLICK FOR LARGER IMAGE)

A loose clamp leaves room for oxidation to build up, increasing the resistance and therefore increasing the possibility that intermittent noises will be injected into the wiring and into susceptible gear as appliances are turned on and off. Once conduit is buried under layers of sheet rock, you cannot go back and tighten every joint, so a dedicated ground wire provides long-term piece of mind by minimizing some of the variables.

As power demand changes over time, the neutral wire can also become noisy because up to three phases (120 V legs) use it as a return line, ultimately to ground. When power distribution is suspect, check all connections with the power off and tighten when loose.

There is also the matter of star worship. A star ground scheme involves running a separate ground wire from each piece of gear, typically to a copper spike (or plate) penetrating deep into damp earth. This is no easy task. Standard rack mounting provides an electromechanical link. Depending upon your perspective, this either defeats the star or serves as a good start toward ground distribution. For example, connecting a massive ground wire to each rack rail rather than to each piece of gear. Remember, I am only documenting here, not suggesting.

Star fundamentalists feel the rack rail is just another potential loop connection and would be happier with a wooden rail, leaving space between each chassis so they do not touch, attaching a ground wire to each chassis, flying shields at either input or output connectors and adding ground lifters to all the gear. Ultimately, this is both time consuming and illegal. Again, the solution starts with the gear—balanced inputs and outputs with pin 1 going directly to chassis.

The problem with attempting to distribute an isolated ground is that the ground wire is not so isolated; it is in the same jacket as the power cables. The more current that flows, the more the power cables induce hum and noise into what is no longer a clean ground. Guess what? All of your hard work and money went down the tubes, especially when the pin 1-to-chassis rule has not been followed.

To prove that the ground wire had become contaminated, my theory was tested in an installation where a video monitor showed hum bars when connected to a ground-isolated outlet. By simply routing the ground wire independent of the power wires, the visual hum was eliminated. To implement this solution it would first be necessary to find a code-legal way to distribute ground wires through a dedicated ground conduit. Because code varies from state to state, discuss the matter with a licensed electrician.

Remember that standard outlets make ground connection when screwed into a junction box, which is mounted to either a wood or metal stud. Metal wall studs can become a path to some other ground; plastic outlet boxes can insulate a standard outlet from a metal stud. Hospital-grade orange outlets are often used because the ground is isolated from the mounting hardware.

Note that balanced power does not induce hum into the ground wire. Balanced power starts by installing a power transformer with a pair of 60 V secondary windings—the two 60 V legs are 180° out of phase, just like a balanced audio signal. The junction where the two windings are joined is called the center tap, the null point or 0-reference that is tied to ground. Now, the hot and formerly neutral connections are each 60 V (60-0-60) with respect to ground and 120 V with respect to each other. Closely matching the two secondary windings minimizes the noise radiation.

Finally, there is the matter of the other wires. The preferred power outlet secures its wires under a screw or by a screw-tightened clamp. The undesirable outlet secures its wires by spring pressure. Either way is considered to code, but the latter can create problems down the road.

It is quite common (and code) for electricians to daisy chain multiple outlets, looping the power through two or more outlet boxes. The combination of potentially poor physical connections and high current will generate heat, further degrading the connections. Expansion and contraction over time increases resistance at every junction, thereby making each hot, neutral and ground connection a potential noisemaker.

Power wiring in such a state will become especially vulnerable to devices, such as air conditioners and elevators, with momentary high-current demands. In addition, some power supplies kick back noise into the power lines. As each device snaps online, the weak links will exaggerate both transient as well as continuous noises. These will be particularly hard to troubleshoot in a multi-tenant commercial building.

The local solution is to turn off the power at the breaker box and inspect all connections from the breaker box to the outlets. Tighten screws and rewire any outlets not using the screw or screw-clamp connections. Wires should be under screw pressure. A global inspection should include the point at which the electrical service enters the building.

Running a dedicated ground wire from a central point to each outlet may not eliminate system noise or fix bad gear, but at the very least, it provides a bit more confidence in the long-term integrity of the ground. Of course, as your system grows so will its problems. Start with a good foundation, and remember that power and ground distribution must also be inspected and maintained.

Ciletti spent 19 years chasing hums and buzzes in New York City. He now chases baby Luca in the Twin Cities area. Drop by www.tangible-technology.com for a virtual visit.

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