Keeping It Quiet
Aug 1, 2001 12:00 PM, By Bill Whitlock
THAT UMPTEEN-THOUSAND-DOLLAR HOME THEATER system loses some of its allure when you turn it on and see interference bands moving across Julia Roberts' face and hear teeth-rattling static coming out of surround-sound speakers. While professional balanced interfaces largely preclude noise problems, most audiophile and home theater systems consist mostly, if not entirely, of audio and video equipment with unbalanced inputs and outputs, a danger zone for signals. In all but the smallest systems with very short cables, noise problems are likely to exist.
The ultimate goal of sound and image reproduction is realism, the kind that creates suspension of disbelief in the listener or viewer. And nothing breaks the spell like background buzz during a quiet musical passage or a suspense-filled moment in a movie. No matter how good the reproduction technology is, interference can really spoil the experience.
Because a reasonably quiet home listening room may have a background noise level of some 20 to 35 dB SPL anyway, many professionals believe that reproduced equipment noise below these levels will be masked and therefore inaudible. But in fact, on average, a listener can detect equipment noise some 15 to 30 dB below ambient room noise! Our ears and brain are very smart, using learned spectral signatures and directional cues to easily identify hum, buzz or hiss, even in the presence of other ordinary household noises.
Therefore, the dynamic range of a sound will need to be somewhere between 100 and 125 dB. Recent advances in converters and digital recording have already extended the dynamic range of CDs, and it's a safe bet that program sources will develop even wider dynamic range. Playing an ordinary CD with about 95 dB of dynamic range on a system whose residual noise limits dynamic range to 80 dB will bury the quietest passages in noise, painfully reminding the listener that he or she is listening to a mere recording.
Consumer expectations are high, and the ranks of “golden ears” are growing. For video, dynamic range requirements are a bit more relaxed. Even expert viewers cannot detect improvements in signal-to-noise ratio beyond about 50 dB.
WHAT'S MAKING NOISE?
Any signal accumulates noise as it flows through the equipment chain; and once it is contaminated, no process can remove the noise without degrading the original signal. Since the dynamic range of an entire system can be no better than its weakest link, noise must be avoided everywhere along the signal path. In most systems, the worst problem is not signal processing in the equipment itself but so-called pickup, or noise coupling, in the interconnect cables.
This noise is most often a mixture of 60Hz harmonics and other high-frequency noises that normally exist on AC power lines. Since it enters the audio or video signal path via ground wiring, this noise is usually referred to as ground noise. It should not be confused with random noise, which manifests itself as hiss in an audio system or granular movement (snow) in a video image. A predictable amount of this kind of electronic noise is inherent in all electronic devices and must be expected. Ground noise produces artifacts such as hum, buzz, clicks or pops in audio systems. In video systems, it generally produces horizontal bars (light or dark) or bands of specks that slowly move upward in the image.
The subject of grounding is widely misunderstood and plagued with nonsense rules. As a result, eliminating system noise often becomes a long series of experiments that finally end when someone says, “I can live with that.” At best, this method leaves a system vulnerable to a reappearance of noise when someone adds a new piece of gear. At worst, it results in an electrocution hazard and a legal liability when someone disconnects required safety grounds.
WHY IS EQUIPMENT GROUNDED?
Just the term ground itself can cause confusion. For this discussion, we need to talk about three kinds of grounds.
- Earth or Neutral Ground
Utility AC power systems require an actual earth connection to protect people from lightning. Lightning involves millions of volts and tens of thousands of amps in discharges from clouds to earth — it releases incredible amounts of power. Before modern standards like the National Electrical Code were developed, a strike to a power line would often just follow the power line directly into a building, killing someone or starting a fire. Power companies quickly realized that connecting one of the lines solidly to earth was a very good idea. It gave the lightning an easy and relatively safe path to real earth ground, before the lines entered a building. Virtually all modern electric power is now distributed over lines having an earth-grounded or neutral wire in addition to other live or line wires.
Electric companies supply most residential customers with a 3-wire service. One of these wires, which often is not insulated, is the grounded neutral conductor. This earth ground, along with those of your neighbors and other grounds at the power poles, provide lightning an easy path to earth.
- Safety Ground
Any AC-powered device can electrocute a user if it develops defects. Most electric devices have exposed metal parts. The transformers, switches, motors and other internal parts use insulation to keep dangerous currents from reaching external parts. But insulation can and does fail for various reasons, and often fails in a way that electrically connects the hot power line to the exposed parts. For example, if the insulation inside the electric motor of a washing machine failed, someone could be electrocuted if they touched the machine and a water faucet (grounded) at the same time. To prevent such a tragedy, most devices have a third wire connecting, or safety grounding, any exposed conductive parts to the third prong of the outlet.
The outlet safety ground is routed, through either a green wire or metallic conduit, to the main breaker panel where it connects to neutral. The circuit from the black or hot wire, through the defective equipment, returning to neutral via the green wire or conduit, is called the fault current path. With the safety ground in place, the potentially dangerous equipment failure simply causes a high fault current that trips the circuit breaker, safely and quickly removing power from that branch circuit.
Electrical safety is an extremely important issue. While nobody wants to intentionally create a lethal shock hazard, ignorance of how the safety system works can kill.
Never Use Devices Such as 3-Prong-to-2-Prong Adapters to Solve a System Noise Problem. These are sometimes even referred to as ground lifters. Remember that cables connecting equipment can also carry lethal voltages throughout the system if one lifted device fails. The judge in a liability lawsuit won't care about your hum problem.
Many appliances and electronic devices are supplied with 2-prong AC cords. Sometimes called double-insulated, these devices must be specially designed and certified to guarantee safety even when overloaded or experiencing an internal component failure. Such designs, common in consumer electronics, are generally advised only in relatively low-power equipment. Because they're not grounded, the parasitic capacitances of their power transformers form a voltage divider causing their chassis voltage to be a substantial fraction of the AC line voltage with respect to ground.
If their chassis, or the shield of their RCA output connector, is connected by a wire to safety ground, a leakage current up to about 0.5 mA will flow. When this current flows through a person, it causes a very mild, harmless tingling sensation. In any AC-powered system, the existence of these noise currents must be accepted as a fact of life. High-frequency power-line noise is created by power supplies in electronic equipment, fluorescent or dimmer controlled lights, and intermittent or sparking loads such as switches, relays or brush-type motors. Since the noise current is coupled through small capacitances in each piece of equipment, high-frequency power-line noise is coupled much more efficiently than the pure 60 Hz.
- Earth Connection
As I've described, a power engineer or electrician defines ground in terms of electrocution and fire hazard and earth connection. To an electronics engineer, ground is simply a common reference point that serves as a return path for various circuit currents inside a piece of equipment. It has nothing to do with earth ground! Sound systems in airplanes can operate quite well: Whether they are quiet or noisy depends on how the system is designed and wired, not whether it has a good earth connection.
NOISE CURRENT COUPLING
There's a common misconception that noise is something airborne, picked up by cables, which can, therefore, be cured with more cable shielding. In real-world AC-powered systems, small leakage or ground noise currents will always flow in any wire connecting two devices. This tiny voltage drop is actually what causes 99% of consumer system noise!
As shown
in Figure 1, when two pieces of equipment are connected via an
unbalanced interface, the noise current flows in the shield conductor
of the cable. Because the shield has impedance, a small noise voltage
drop appears across the length of the cable, according to Ohm's law.
Since the cable shield is also part of the signal circuit, the noise
voltage will be directly added to the signal at the receive end of the
cable, which is the sum of all the voltages in the loop from point A to
point C. Because the shield impedance is part of two circuits, noise
current and signal current, this mechanism is called
common-impedance coupling.
Consider this typical scenario. Two devices are connected by an RCA audio cable. Both devices have 2-prong power cords, and their power-line capacitances cause a 300uA, 60Hz noise curent to flow between them through a typical 25-foot unbalanced cable. The cable has a foil shield and a 26-gauge drain wire, making its resistance, or 60Hz impedance, about 1 ohm. Using Ohm's law, we calculate the resulting voltage drop to be 300 uV. With respect to the nominal consumer signal reference level of 300 mV, this noise level is only 60 dB. And the high-frequency noise may be even worse and more audible as buzz.
The magnitude of the noise current can be much higher if both devices are safety grounded (i.e., having 3-prong power cords). Referring to Figure 1, consider that leakage currents from all devices on a branch circuit cumulatively flow in the safety ground wiring. Since leakage currents for safety-grounded devices are not limited to the 0.5 mA allowed for 2-prong equipment, it is not uncommon to have leakage currents of 100 mA flowing in portions of the safety-ground wiring. Because of the impedance of the safety-ground wiring, this could result in well over 10 mV of noise voltage between the safety ground pins of two different receptacles.
And this can get much higher between two outlets on different branch circuits. For two safety grounded devices, this voltage will be impressed across the length of the signal cable shield and be directly added to the signal. For 10 mV of noise voltage, the noise would be only 30 dB with respect to reference level.
In a standard video interface, reference level is 1 volt peak-to-peak, including sync. The active range from reference black to reference white spans about 600 mV peak-to-peak. Since our previous noise voltage of 10 mV (rms) equals about 30 mV peak-to-peak, our signal-to-60Hz hum ratio would be 26 dB. This would cause a visible hum bar in a video display. Even higher voltage differences can result if one of the devices is connected to an outside ground point such as a separate earth ground or a CATV drop.
TROUBLESHOOTING
Weeding out power-line noises such as hum and buzz from an audio or video system can be frustrating and time-consuming. The following method is easy to understand and perform, and requires no test equipment other than ears. It points the way to solutions that are both safe and effective.
Inquire First
The success of any troubleshooting has a lot to do with how you think about the problem. First, don't think that, because you've done something a certain way many times before, it can't be the problem. Remember, things that can go wrong do! Sometimes, just gathering information will reveal the problem. Get as many clues as possible before you try to solve a problem. And write everything down. Imperfect recall wastes a lot of time.
Ask lots of questions. Here are a few good ones to start with:
-
Did it ever work right?
-
What are the symptoms that tell you it's not working right?
-
When did it start working badly or stop working?
-
What other symptoms showed up just before, just after, or at the same time as the failure?
Tinker a Bit
Use the equipment's own controls, with 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, it must be entering the signal path before that control.
Use a Visual Aid
Sketch a block diagram of the system. Show all interconnecting cables and their approximate length. Mark any balanced inputs or outputs. Generally, stereo pairs can be indicated with a single line. Note any equipment grounded via a 3-prong AC plug, and any other grounds such as cable TV or DSS dishes.
Ground Dummy Tests
The term dummy isn't intended to demean the person performing the test. It refers to special adapters that don't pass signal. Dummies allow the system to test itself and pinpoint the exact entry point of noise or interference. By temporarily placing the dummy at strategic locations in the interface, precise information about the nature of the problem is revealed. The tests can specifically identify common-impedance coupling in the cable, magnetic or electric field coupling to the cable, or common-impedance coupling inside equipment.
Dummies
can be made from standard parts and wired as shown in Figure
2. Since a dummy doesn't pass signal, it should be clearly marked
so it doesn't accidentally find its way into a system. Testing with a
dummy is described in the sidebar on page 42.
SOLUTIONS
Equipment manufacturers are seldom a good resource for solving
system noise problems. Their technical advisors will usually blame bad
grounding for hum and buzz problems. Some are so uninformed or careless
that they'll actually recommend disconnecting safety grounds as a
solution! 
When a system contains two or
more ground connections, whether through power cords to safety ground
or other grounded signal cables such as the CATV connection in
Figure 3, a ground loop is formed, which provides a
complete circuit path for ground noise current.
Because a voltage difference that is often substantial exists between the ground for CATV and the safety ground at the sub-woofer, a noise current will flow in the shield of all signal cables that are part of the loop. Common-impedance coupling then adds noise to the signal in these cables. In general, the noise added is directly proportional to the cable's length. This sample system would probably exhibit a loud hum regardless of the input selected or the setting of the volume control because of ground noise current flow in the 20-foot cable. The hum might be slightly louder if the TV input were selected and the volume were turned up because the same ground noise current also flows in the 3-foot cable. Because the ground loop is a series circuit, current flow can be interrupted by opening the circuit at any point. You might be tempted to open the loop with a 3-to-2 prong adapter at the sub-woofer AC plug, but that creates an electrocution and fire hazard for which you would be legally liable.
There are two basic ways to reduce common-impedance coupling in unbalanced interfaces:
Reduce Resistance
The shield circuit is the most common impedance. You reduce it by following these steps:
-
Keep cables as short as possible. Longer cables increase the coupling impedance. Even short cables can produce severe coupling if ground currents are high. Never coil excess cable length.
-
Use cables with heavy, braided copper shields. Cables with shields of foil and thin-gauge drain wires increase coupling impedance.
-
Maintain good connections. Contact resistance is part of the common impedance. Connectors left undisturbed for long periods can develop high contact resistance. Hum, or other noise that changes when the connector is wiggled, indicates a poor contact. Use a good commercial contact fluid and/or gold-plated connectors.
Reduce the Circulating Ground Current
Some tips for doing this:
-
Don't add unnecessary ground connections. With rare exception, additional grounds increase ground noise currents. Of course, don't disconnect required grounds to reduce noise current either.
-
Use ground isolators at problem interfaces. Commercial isolators are available for audio, video and CATV signal paths as well as for digital interfaces.
Figure
4 shows how a ground isolator breaks a ground loop. Since no
current can flow between the insulated transformer windings, noise
current can no longer add noise to the signal by flowing in the shield.
A transformer uses magnetic coupling to transfer signals from primary
to secondary windings with no electrical connection between them. A
ground isolator cannot remove hum and buzz if it's placed randomly in
the system: It must be installed at the interface where the noise is
coupling. This is easily determined by the testing outlined above.
High-performance ground isolators not only provide true audiophile signal quality but use internal shields to suppress ultrasonic and RF interference. Be aware that most audio isolators or hum eliminators don't use proper shielding, and many use tiny telephone-grade transformers that lose deep bass, cause distortion and create poor transient response. Beware of cheap products with scanty or missing specifications — signal quality is at stake!
An audio isolator is a safe solution for the ground loop of Figure 3. The isolator could be installed in the audio signal path either between TV and preamp or between preamp and sub-woofer. High performance should always be installed at the receive end of an interconnect cable.
Another safe solution is to break the ground loop by installing a CATV isolator at the antenna input of the TV. Although less expensive capacitive isolators are available, the ISO-MAX unit uses a wide-band RF transformer covering 50MHz to 1GHz to reduce shield current flow to levels by a factor of about 100.
If the TV in Figure 3 were driving a video projector having a 3-prong AC plug, the ground loop might cause hum bars in the display, especially if the video cable is long. Because the signal is baseband video (composite, component or S-video), different types of isolators are required.
COPING WITH FIELDS
Magnetic or electric fields can sometimes induce noise in cables. Electric fields are generated around wiring or devices operating at high AC voltages. Their coupling is prevented by a conductive outer shield, which completely surrounds and covers the inner signal conductor in cables. Braided shields provide 80% to 95% coverage, which is entirely adequate for all but extreme cases because electric fields are rarely a problem in audio or video systems.
Magnetic fields are likewise generated around wiring or devices operating at high AC currents. Building wiring, power transformers, electric motors and CRT displays are a few sources of strong AC magnetic fields. Increasing distance between signal cables and offending fields is the best cure for either electric or magnetic field problems. Ordinary cable shielding, whether copper braid or aluminum foil, has virtually no effect on audio magnetic fields.
Beware of Marketing Hype!
The only property of cable that significantly effects noise coupling is shield resistance. Coupling of even very low levels of ultrasonic power line noise can cause subtle signal spectral contamination in downstream amplifiers. Rather than agonize over which exotic cable makes the most pleasing small improvement, prevent the coupling with a ground isolator. Expensive and exotic cables, whether double or triple shielded, made of 100% pure unobtainium, or hand-braided by Peruvian virgins, have no significant effect on hum and buzz problems.
Bill Whitlock is president of Jensen Transformers, Inc. He has designed audio and video circuits and systems for 30 years. He lives in Oxnard, California, and can be contacted by e-mail at whitlock@jensen-transformers.com.
Audio RCA
Plug=Switchcraft 3502
Jack=Switchcraft 3503
R=1k•, 5%, 1/4 W Resistor
Audio 2C Phone
Same as RCA version, except use Switchcraft 336A and 345A adapters
For Video RCA
Same as Audio RCA, except R=75•, 5%, 1/4 W Resistor
For Video BNC
Same as Video RCA, except use MilesTek
10-01015 and 10-01016 adapters
FIGURE 2: Dummy construction.
TESTING THE SYSTEM
Each signal interface is tested using a 4-step procedure. If preliminary tests haven't narrowed the problem down to a specific interface or portion of the system, always start at the inputs to the power amplifiers (for audio systems) or the input to the monitor (for video systems) and work back toward the signal sources. Be very careful when performing the tests not to damage speakers or ears! The surest way to avoid possible damage is to turn off the power amplifier(s) before reconfiguring cables for each test step.
STEP
1
Unplug the cable from the input of Box B, and plug in only the dummy as shown above.
Theory: This test prevents any noise current that might otherwise flow in the cable's shield from entering Box B. It also effectively shorts the input. Therefore, any noise output must originate in either Box B itself or somewhere downstream.
Observe: Is system output quiet?
No: The problem is either in Box B or further downstream. Reconnect the cable and perform this test on the next downstream interface.
Yes: Go to next step.
STEP
2
Leaving the dummy in place at the input of Box B, plug the cable into the dummy.
Theory: This test retains the shorted input while allowing any noise current flowing in the cable's shield to enter Box B. Therefore, any noise output must be due to common-impedance coupling inside Box B itself or somewhere downstream.
Observe: Is system output quiet?
No: The problem is common-impedance coupling (a “pin-1 problem”) inside Box B or a device farther downstream. The hummer test can be performed on Box B to determine this. If the problem is inside Box B, have the unit repaired, modified or replaced with a functional — and functioning — equivalent. If the problem is not in Box B, reconnect the cable and begin the test procedure on the next downstream interface.
Yes: Go to next step.
STEP
3
Remove the dummy and plug the cable directly into the input of Box B. Unplug the other end of the cable from the output of Box A and plug it into the dummy. Do not plug the dummy into Box A or let it touch anything conductive.
Theory: By effectively shorting the far end of the cable, this test uses the cable itself as a sensor of magnetic or electric fields. The far end of the cable is left electrically floating to prevent any other currents from flowing in its shield.
Observe: Is system output quiet?
No: The noise is coupling into to the cable by induction. This is most often caused by a strong AC magnetic field near the cable. Sources of potent magnetic fields include high-current AC power wiring, power transformers, and TV or computer CRT displays. Electrostatic coupling is also possible, though rare, in cables that have a grounded outer shield. Re-route signal cables to avoid such strong 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.
Theory: This test prevents the output of Box A from driving Box B while allowing noise currents to flow from Box A to Box B through the cable's shield. Box B is effectively listening to the noise voltage at the far end of the cable caused by noise current flow in the shield.
Observe: Is system output quiet?
No: Noisy ground currents are being coupled by the common-impedance of the cable shield. Install an appropriate audio or video ground isolator at the input of Box B.
Yes: The noise is coming from (or through) the output of Box A. Perform these same tests on the cable(s) which connect Box A to upstream devices.
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