Jun 1, 2000 12:00 PM, Eddie Ciletti
Learn to handle hum and buzz with a critical look at system noise from the ground up.
For sound and video systems in stallers, voodoo and wives tales abound about how to achieve minimal system noise. By noise, I mean hums and buzzes, which are power- and grounding-related issues. Hiss, on the other hand, is a gain-structure issue not covered in this article. Troubleshooting noise can eat into profit margins for installers, technicians and end-users, ultimately consuming an unpredictable amount of time and money. This article is intended to help everyone who has ever been burned by a problematic system.
The lack of noise immunity in some equipment is an insidious problem - the primary topic of this article - and the industry's dirty little secret. Long before software and hardware developers started pointing fingers at each other, all eyes turned toward the installer when the system powered up for the first time humming a tune. At least bad analog gear has somewhat of a voice to express its unhappiness - hums and buzzes for audio and a vertically creeping, horizontal bar for video. Digital gear lets us know only when it is too late, but it has increased manufacturer awareness regarding noise immunity, yielding a heightened consciousness that has trickled down and into some modern analog gear.
The first part of this article will cover systems basics and what should happen. Later in the article, I will address the fixes that have been attempted to accommodate bad gear and why they do not always work. Some are valid as insurance policies; others are borne out of paranoid overkill and mostly unnecessary.
I could not have written a single word about system noise without experience in the field. I have admittedly made mistakes and am lucky to have a tenacious, curious nature. All this makes me knowledgeable, but I am not an expert. In a parallel universe, brains far more brilliant than ours have wrestled these thorny issues. You can find more detailed information from Bill Whitlock at Jensen Transformers (www.jensen-transformers.com), including a library of reference books for your suggested reading.
Another authority is Neil Muncy, who chairs the AES Standards Committee (SC-05-05) on the subject. You can find his condensed overview of suggested practices at the Rane site (www.rane.com/note110.htm), and honorable mention of Muncy appears at the Philip Giddings site (www.engineeringharmonics.com/papers/pin1.htm) specifically addressing the pin 1 issue (more on that later). At www.josephson.com/audiofaq, there is a wonderful FAQ compiled from the rec.audio.pro newsgroup by the late Gabe Weiner.
There are four solutions typically applied to circumvent system noise problems. It is not my intent to debunk what some may have embraced as religion, merely to shift the focus to the real problem, which involves the gear itself. The first solution is power distribution, an important part of every system's foundation. Before considering noise insurance options like balanced power, an isolated ground or star grounding, all of which require considerable effort and cash, it is important to remember how easily installation costs can exceed the cost of the gear. Few customers can claim that money is no object, and none of the aforementioned items are an absolute guarantee of long-term silence.
The second common solution for noise problems is the use of balanced gear and avoidance of troublesome and, most likely, unbalanced equipment whenever possible. The third fix requires the installer to tame the problem children with transformers or an active-balanced interface. Necessity being the mother of invention, the fourth, only the most tenacious, uses gear modifications to drive the evil spirits away. Some of these modifications are simple; some are not.
Many designers and installers would choose to start from scratch rather than perform surgery on a system with a preexisting condition. It may be hard to convince a potential customer of this approach, but the experienced traveler will avoid the dark road that leads to wasted time, lost profits and customer dissatisfaction. It is too easy to put the blame and the curse on a previous installation and installer, respectively, so avoid the habit of flaming the competition.
Let us examine the foundation of the problem. Power from a standard wall outlet is an alternating current (AC) at 60 Hz. At minimum, this hum radiates from the power cable at the moment a device is plugged in and turned on. Sound converted to electricity is also AC, but batteries produce direct current (DC). Note that most equipment ships with a longer-than-necessary power cable. In cramped spaces, power cable bundles make for a crowded, unserviceable installation. Belden makes an 18 inch (457 mm) IEC power cable (part number 17002A-B1-10) available from Newark Electronics.
Additionally, electronic equipment is typically built on a metal chassis that is referenced to the earth via the round prong of a three-conductor AC power plug. If you are barefoot, pregnant and standing on a slab of damp concrete, all properly grounded equipment is safe to the touch.
Hum and buzz are often attributed to the elusive ground loop. This phenomenon might more accurately be described as ground current in that no two pieces of gear can be exactly at the desired zero-voltage reference (the earth). When connected by audio or video cables, an extremely small amount of AC current flows in the shield and ground wiring. Inside each electronic device, leakage currents from power transformers induce an electrical charge on the chassis. Even though the unit is grounded via the power plug, the length of all power cabling (back to the breaker box) amounts to enough resistance so that no chassis can be held to exactly the same voltage as la terra firma. Because this difference in potential (a measurable voltage between any two chassis) exists in nearly all instances, you might think we face insurmountable odds, but in reality, ground current affects only flawed gear. The typical external fixes, such as running ground wires and flying the shields at one end (! of audio cables), solves the imm ediate problem; ground wires lower the impedance of the path to ground, reducing shield current. Disconnecting the shield at one end eliminates current flow, but neither is the long-term solution.
Properly designed balanced gear should tolerate shield current. Unbalanced gear, on the other hand, cannot. By design, a balanced input circuit is immune to most cabling noises by way of its common mode rejection ratio (CMRR). At minimum, CMRR sees noises common to both signal wires as out-of-phase and as such, cancels the unwanted intruders. Not all balanced outputs have signal on pin 2 and pin 3, but the source impedance at each pin must be the same for CMRR to be effective.
Another source of common-mode noise is a wall wart. In close proximity to a balanced audio cable, any transformer will induce hum into both the signal wires and the shield. For wiring that feeds unbalanced gear, pay strict attention to cable dress and run wires away from power transformers both obvious (wall warts) and hidden (in gear). Balanced gear can tolerate both common-mode (induced) noises as well as shield current noise.
Internal product grounding will be referred to as the pin 1 issue not only as it relates to XLR pin 1 but also other connectors ( inch, RCA, BNC). The correct approach is easily illustrated by observing the many inch connectors on a Mackie mixer, all of which are metal jacks on a metal chassis. This approach creates a low-impedance firewall that protects the internal, high-gain circuitry from external influences.
It was not so long ago that manufacturers chose connectors that were plastic-insulated from the chassis. Such a choice may be cost effective or even streamline the manufacturing process, but it undermines the internal ground scheme, decreasing the noise immunity of even good product designs making the resulting gear inherently vulnerable to any power- or transmission-induced (RF/TV) interference. The reason is simple. Printed circuit board (PCB) ground traces do eventually make contact with the chassis. If shields do not go directly to the chassis, the pin 1 ground current (and its noises) will infect the PCB ground and all amps referenced to it. Remember that a mic preamp can have 60 dB of gain, more than enough to amplify even the slightest ground noise. Higher ground and noise currents can also raise the copper's temperature, increasing its already high resistance and decreasing ground integrity within the unit.
I once received an e-mail regarding a mic preamp that picked up television interference (TVI) with a certain condenser mic and not another. Incidentally, TVI is often heard as a 59.94 Hz buzz, video's vertical sync rate, with harmonic variations that correspond to the transmitted video image. This was a clear case of pin 1 not being connected to the chassis at the point of entry. Resolving the problem initially seemed complex - how to fix the problem via e-mail without the customer opening the preamp to make any modifications.
Because condenser mics require phantom power, there is DC current flowing in the shield in addition to whatever external noise currents the shield acquires while trying to do its job. The phantom power requirements of condenser mics may vary, but all should be within the industry specification. The higher current requirements of this particular mic tugged on the ground enough to cause the preamp to detect the TV signal. In essence, piggybacked on to the DC current in the shield was the AC transmission of a local TV station, the most obnoxious of which was the vertical sync frequency.
To fix the immediate problem, I simply had the customer open the male XLR of the mic cable and wire pin 1 to a lug on the XLR connector designed to route the shield to chassis. (See Figure 1.) The customer was now happy, and I felt rewarded for the number of times I have attempted to improve a product's noise immunity on the bench. Time spent on equipment modifications may not always be profitable, but it sure is enlightening.
Perhaps you can now understand why, when customers ask about the relative merits of balanced power or any external ground scheme, I have difficulty giving a straight answer. I believe that if all gear were balanced with well-executed internal grounding, then no deviation from standard electrical power distribution would be necessary. That said, customers with enough cash to pay for noise insurance will reduce potential system noises with the aforementioned items. On the other hand, as systems grow in size, a multi-room or multi-floor facility with great cable distances, it becomes more difficult to implement and distribute a power and ground scheme. More important is to make good equipment choices. To installers who learn the hard way that certain gear is consistently problematic, please forward that information to the manufacturer.
There should not be any voodoo required to achieve low system noise, but mere knowledge of the noise devil does not solve the problem. In order to complete the circle of designer, installer, end-user and technician, we must all provide feedback to encourage manufacturers to take the lead.
A critical look
To sum up thus far, I have detailed the causes of hums and buzzes in audio and video systems, placing the blame on gear that falls somewhat short of being professional in terms of its inability to reject noise. Gear modifications might be the short-term answer to what ultimately should happen as part of the manufacturing process. Further, all of us have taken detours in an attempt to accommodate unhappy pieces of gear. As a technician, the worst that I have seen are user-installed systems where home-brew fixes include everything from the benign - rubber chassis isolators and plastic rack screws - to the illegal lifting of the third pin AC ground. All are band aids that make for an idiosyncratic installation. I will now focus on why various external fixes do not consistently solve noise problems. Hopefully, after finishing this article, you will have enough background information to assist those in the trenches who are moving too fast to understand or investigate the underlying! problems fully.
Regarding the implementation of electrical power, the rules and regulations of the electrical code vary regionally. I am no expert. Finding a knowledgeable local electrician who is sensitive to the needs of a multimedia system is key. Using a ground adapter as a ground lifter is, by code, illegal. No matter whether the gear, the signal wiring or the power distribution is at fault, every temporary fix will eventually become an intermittent noise problem while also creating a potentially life-threatening situation.
The outlet-ground connection is simply to protect humans from the risk of shock (see Figure 2), and despite all the fuss about the dreaded ground loop, consider how may potential loops are created as soon as a piece of gear is rack mounted, plugged in, then connected to another piece of gear. It would seem as if we are doomed at the start and that meeting code and achieving a quiet system are disparate goals, but that is not the case.
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.
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 degrees 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.
Ever notice that when flying at 20,000 feet (6,100 m), you have no sensation of motion? I do not mean turbulence or steering but the actual speed at which you are traveling. To a certain extent, this analogy can be applied to ground-related noises. So long as every device is flying on the same ground (no matter how noisy), no device sees the noise. You might say that this is the ground plane. You are allowed to laugh or groan.
Here is another analogy. With one foot on a building floor and another on an elevator floor it is obvious that the elevator is not as steady as the building. This correlates to what happens when a direct box inadvertently links the system ground (via the recording console) with a bass amp plugged into a utility outlet that is not connected to the officially sanctioned ground. The point here is to translate what we already know - the sound of hum and buzz - into what actually happens when two grounds are not at the same potential or cleanliness, relative to the earth or to each other.
Tie yourself to two horses facing opposite directions and the above analogies will start to make sense. The idea is that differences between two devices puts stress on their connective as well as internal wiring. The differences come from voltage and noise variations. The problems come from gear with poor internal grounding and from wall warts placed too close to unbalanced wiring. Even a small audio system can have problems that start when multiple power strips are haphazardly plugged into different wall outlets. Distribute power from a single outlet, and chances are most of the noises will go away. This test is safe assuming that you know the system will not exceed 75% of a 20 amp breaker (15 amps) and that nothing else is on that circuit.
Connect the first two power strips to a standard wall outlet, and to those power strips, connect the other power strips in the most symmetrical way possible. You can also use a two-to-six outlet adapter that screws into a standard wall outlet; all strips then plug into that. Remove all ground-lift adapters. The emphasis here is to use a single outlet.
Power up slowly so as not to overload the breaker, and, if possible, use a current probe at the breaker box to monitor demand. The system should now be significantly quieter. If not, find all the wall warts and make sure none of the audio cables go near them. This is only a test. If the tree solves the noise problem, and the breaker is not overloaded, you have a temporary fix. Finding the problem gear is not quite as easy, but the clue is to look for plastic-insulated jacks and products with unbalanced inputs.
An awfully big build up for what seems like a rather simple but temporary solution, right? When a customer asked for this noise reduction tip, it did not help because there was one piece of gear that had to be plugged into a convenient outlet rather than follow the discipline of using a single outlet. So much for science.
Most of the noise was a combination of using unbalanced sources and cabling (MIDI modules) to a mixer that hummed with so many cables connected and faders down. Taking advantage of balanced inputs and cabling would have helped tremendously, even with unbalanced sources. The downside to affordable technology is that it makes a professional installation seem rather expensive.
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