Hum and buzz are perhaps the most mysterious of all the problems that can beset a sound system. Unfortunately, even experienced professionals often don't understand the true origin of such noises and may fall victim to myth and misinformation. Many so-called "cures" for these problems inadvertently create electrocution and/or fire hazards.
The equipment in most audio systems is connected to and powered by utility AC power. If we could listen to the power line, the 120 volts AC at 60 Hz would be heard as hum. Line voltage also contains significant energy at odd harmonics of 60 Hz, which is heard as buzz. This is an entirely normal condition. Although seductively appealing, the notion that so-called "noisy power" is to blame for audio noise problems is simply not true. We will hear the power line noise only if there is a way for it to enter the audio signal path.
The audio signals processed by the equipment are routed from one device to another via interconnecting audio cables. The signals will accumulate noise as they flow through the system and, once contaminated, no process can remove the noise without degrading the original signal. Therefore, noise must be avoided throughout the signal path. Hum and buzz will enter the signal path only where there is an unintentional coupling with the power line.
How Does the Coupling Happen?
It is a fact of life that there
will be harmlessly small voltage differences between the safety ground terminals
of any two AC outlets. In equipment with grounding (three-prong) AC plugs, safety
ground is connected to the chassis and audio connector ground terminals. Therefore,
when two such pieces of equipment are connected by an audio cable, the small
voltage difference will be impressed across the ends of the audio cable. The
voltage difference will be lowest (under a few millivolts) between outlets that
are physically close together, as on the same outlet strip, and highest (up
to a few volts) between outlets that are physically far apart or on different
branch circuits.
It is also a fact of life that harmlessly small (under 750 micro-amps) "leakage" currents flow from the power line to chassis and audio connector ground terminals in equipment with non-grounding (two-prong) AC plugs. Therefore, when this equipment is connected to any other equipment by an audio cable, the leakage current flows in the length of the audio cable.
Common Coupling
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An audio cable, along with a portion of equipment circuitry at each end, is called an interface. There are two basic kinds of audio signal interfaces. Unbalanced interfaces use two wires (generally a single wire inside a grounded shield) and are widely used in consumer and so-called "semi-professional" equipment. Cables are most commonly terminated with either "RCA" or tip/sleeve phone plugs. Unbalanced interfaces are extremely susceptible to noise problems. As shown in the drawing, power-line leakage current flow in the grounded conductor (shield) causes a small voltage drop, according to Ohm's law. For example, 300 micro-amps of leakage current flowing in a 25-foot cable whose shield has a resistance of one ohm will create a (buzz) voltage of 300 micro-volts. This buzz voltage will be directly added to the signal seen at the receive end of the cable. Since the impedance (resistance) of the shield is part of both the audio signal circuit and the power-line leakage current circuit, this coupling mechanism is called common-impedance coupling. In the example, since the nominal consumer signal level is about 300 milli-volts, the buzz will degrade signal-to-noise ratio to about 60 dB. When unbalanced interfaces are used in systems having grounded equipment, noise problems can get much, much worse. For these reasons, unbalanced interfaces should be avoided altogether whenever possible.
Balanced interfaces normally use three wires (a pair of wires inside a grounded shield) and are the hallmark of truly professional equipment. Because the audio signal is carried by the wire pair and ground currents are carried by a separate wire, the balanced interface is theoretically immune to the ground voltage difference and leakage current artifacts of the AC power system. Practical balanced interfaces have limitations but, with rare exception, are orders of magnitude more immune to hum and buzz. It should be noted here that some equipment has a designed-in defect (internal common-impedance coupling) called the "pin one problem" that can mimic symptoms of other problems.
Locating the Coupling Point
Troubleshooting hum and buzz problems
can be frustrating and time-consuming. The following test method is simple to
do, easy to understand, and requires no test instruments other than ears. In
all steps, write down results - imperfect memory can waste a lot of time.
Ask questions and gather clues: Was the system ever noise-free? Under what conditions does the noise appear? Was new equipment added? Was the system re-configured?
Experiment with controls: Use the equipment's own controls, and some logic, to provide additional clues. For example, if the noise is unaffected by the setting of a volume control or selector, then it must be entering the signal path after that control. If the noise can be eliminated by turning the volume down or selecting another input, then noise must be entering the signal path before that control.
Sketch a system block diagram: Show all interconnecting cables and indicate unbalanced equipment inputs or outputs. Generally, stereo pairs can be indicated with a single line. Note grounded (three-prong AC plug) equipment and any other ground connections such as to cable TV or a satellite dish.
Troubleshoot with Test "Dummies"
These special test adapters can
find the noise coupling location. Dummies for both balanced and unbalanced interfaces
are easy to make from standard connectors and resistors, wired as shown.
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Always work backwards: Unless clues suggest otherwise, start at the power amplifier and move "upstream" in the signal path toward the signal sources.
VERY IMPORTANT: Remove any "ground lift" devices such as three-to-two-prong AC plug adapters and/or replace any three-prong plugs that have had their ground prong removed. A fault in one piece of equipment with a disabled safety ground can render an entire sound system a lethal shock hazard. Courts have held that the person who defeats safety grounding is legally liable if an equipment failure causes shock, electrocution, or fire. Only UL-approved equipment originally supplied with a two-prong AC plug is safe to operate without safety grounding.
Testing the Interfaces (balanced or unbalanced):
| Step 1 - Unplug the cable from the input of Box B and plug in only the dummy. |
Output quiet? No - The problem is either in Box B or further downstream. Yes - Go to next step. |
| Step 2 - Leaving the dummy in place at the input of Box B, plug the cable into the dummy. |
Output quiet? No - Box B has an internal "pin one problem." The hummer test can confirm this (see www.jensentransformers.com/as/as032.pdf for details). Yes - Go to next step. |
| Step 3 - Remove the dummy and plug the cable into the input of Box B. Unplug the other end of the cable from Box A and plug it into the dummy. Be sure the dummy doesn't touch anything conductive. |
Output quiet? No - Noise is being induced in the cable. Re-route it to avoid interfering fields. Yes - Go to next step. |
| Step 4 - Leaving the dummy in place on the cable, plug the dummy into the output of Box A. |
Output quiet? No (unbalanced) - The problem is common-impedance coupling. Install an isolator in the signal path. No (balanced) - The problem is shield-current-induced noise or SCIN. Replace the cable with a different type or take steps to reduce current flow in the shield. Yes - The noise is coming from the output of Box A. Perform the test sequence at the next upstream interface. Repeat as necessary until problem found. |
Reducing Coupling in Unbalanced Interfaces:
1. Use the shortest
cable possible. Longer cables increase the coupling impedance.
2. Use cables with heavy, braided copper shields. Low resistance in the grounded signal conductor is the only property of a cable that significantly affects noise coupling. Contrary to the beliefs of many audiophiles and claims of many exotic cable manufacturers, inadequate shielding is NOT what causes most noise in unbalanced cables. Cables with only foil shields and drain wire increase the coupling impedance.
3. Maintain good connections. Contact resistance is part of the coupling impedance. Connectors left undisturbed for long periods can develop high contact resistance. Hum or buzz that changes when the connector is wiggled indicates a poor contact. Use a good commercial contact fluid and/or gold-plated connectors.
4. Remove any unnecessary ground connections. With rare exception, added grounds increase system noise. Of course, never disconnect safety grounding.
A device called a ground isolator will solve most hum and buzz problems, but it must be installed at the interface where the noise coupling occurs. A quality ground isolator virtually eliminates power-line current flow in an unbalanced signal cable and can vastly improve the noise rejection of most balanced interfaces. Commercial isolators are available for balanced or unbalanced audio, video, CATV, and digital signals. Since isolators can potentially degrade signal quality, only top-quality, fully specified isolators are recommended. See www.jensentransformers.com for full technical and application information. Isolators in the signal path are "silver bullets" to eliminate hum and buzz without creating safety hazards.
