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SIGNAL PURITY

Jul 1, 1999 12:00 PM, Bill Whitlock


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Finding out how and where such power line-related noises as hum and buzz enter an A-V system can be a frustrating, time-consuming experience. All too often, these problems are solved by using dangerous ground-lift adapters and haphazard cut-and-try experiments, which usually end only when someone says, "I can live with that." The method described here is easy to understand, simple to perform and points the way to safe and effective solutions. Although I make no claim for inventing this procedure, I know of no similar technique ever published. It requires no test equipment other than ears, and even the underlying theory is simple.

Searching for clues A significant part of troubleshooting involves the way you think about the problem. For example, do not think that just because you have done something a particular way many times before that it cannot be the problem. Remember, even things that cannot go wrong do. The source of many problems either reveals itself or can be deduced if we gather enough information. It is important to have as many clues as possible before you try to solve a problem. Write everything down-imperfect recall can waste a lot of time-and ask lots of questions. Troubleshooting guru Bob Pease suggests these basic questions [Ref 1]:

* Did it ever work right?

* What symptoms tell you that it is 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?

Sketch a block diagram of the system. Show all interconnecting cables, indicating approximate length. Mark any balanced inputs or outputs. Generally, stereo pairs can be indicated with a single line. Note any equipment that is grounded via its three-prong power plug. Note any other ground connections such as cable TV or DSS dishes. Figure 1 is an example of such a drawing for a simple home theater system.

Use the equipment's own controls, along with some logic, to provide additional clues. If, for example, the noise is unaffected by the setting of a volume control or selector, 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.

The four-step program Special, easily constructed test adapters allow the system to test itself and pinpoint the exact entry point for noise or interference. By temporarily placing the test adapters at strategic locations in the interface, precise information about the nature of the problem is also revealed. The tests can specifically identify common-impedance coupling in unbalanced cables, shield current induced coupling in balanced cables, magnetic or electrostatic pickup of ambient fields by the cables, or common-impedance coupling inside defective equipment (a.k.a. the pin 1 problem [Ref 2]). The test adapters can be made from standard parts and wired as shown in Figures 2 and 3 for balanced or unbalanced interfaces, respectively. Because these devices do not pass signals, they should be clearly marked so that they do not accidentally find their way into a system.

Each signal interface is tested using a four-step procedure. If preliminary tests have not narrowed the problem down to a specific interface or portion of the system, always start at the inputs to the power amps (for audio systems) or the input to the monitor (for video systems) and work backwards toward the signal sources. Be careful when performing the tests not to damage loudspeakers or ears. The surest way to avoid possible damage is to turn off the power amp(s) before reconfiguring cables for each test step.

Step by step In the first step, unplug the cable from the input of Box B and plug in only the adapter, as shown in Figure 4. Theoretical basis: This test prevents any noise current, which might otherwise flow in the cable's shield, from entering Box B. It also effectively shorts the input. Therefore, any noise still present must originate either in Box B itself or somewhere downstream. Is the 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 the next step.

The second step involves leaving the adapter in place at the input of Box B and plugging the cable into the adapter as shown in Figure 4. Theoretical basis: This test retains the shorted input while allowing any noise current that might be flowing in the cable's shield to enter Box B. Therefore, any noise that appears must be due to common-impedance coupling inside Box B itself or somewhere downstream.

Is the system output quiet? No? The problem is common-impedance coupling (a.k.a. pin 1 problem) inside Box B or a device farther downstream. The hummer test [Ref 3] 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 properly deigned, functional 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 the next step.

In the third step, remove the adapter 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 adapter as shown in Figure 4. Do not plug the adapter into Box A or let it touch anything conductive. Theoretical basis: By effectively shorting the far end of the cable, this test uses the cable itself as an antenna to pick up noise from magnetic or electrostatic fields. The far end of the cable is left electrically floating to prevent any other currents from flowing in its shield.

Is the system output quiet? No? The noise is coupling into 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 television or CRT displays. Electrostatic coupling is also possible, but it is rare in cables that have a grounded outer shield. Reroute signal cables to avoid such strong fields. Yes? Go to the next step.

In the final step, leaving the adapter in place on the cable, plug the adapter into the output of Box A. Theoretical basis: This test prevents the output of Box A from driving Box B while allowing noise currents to flow through the cable's shield from Box A to Box B. In unbalanced interfaces, Box B is effectively listening to the noise voltage at the far end of the cable caused by noise current flow in the shield. In balanced interfaces, the inner conductor pair is acting as a magnetic pickup loop (it is effectively shorted at the far end) for any non-uniform magnetic field generated by the noise current flow in the cable's shield.

Is the system output quiet? No (unbalanced cable)? 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. No(balanced cable)? Noisy ground currents flowing in the cable shield are being inductively coupled into the inner signal conductors. This problem usually appears only with cables using a foil shield and drain wire. Use a cable with a braided shield or take steps to reduce ground current flow in the shield. Such steps might include measuring the voltage between the chassis of Box A and Box B. If it is more than 100 mV, check the AC outlets for proper wiring (neutral and safety ground connections may be swapped) or faulty connections. Yes? The noise is coming from (or through) the output of Box A. Perform these same tests on the cable(s) that connect Box A to upstream devices.

Too many grounds When a system contains two or more pieces of grounded equipment, whether through the power-cord safety grounds or other ground connections, a loop may be formed as shown in Figure 5, our example home theater system.

Because there is often substantial ground noise voltage between the CATV cable shield and the safety ground for the subwoofer, a relatively large noise current may flow in the shield of any signal cables that are part of the ground loop. In unbalanced interfaces, this current flow results in common-impedance coupling, which directly adds noise to the signal. In general, the amount of noise added is in direct proportion to the cable's length. This example 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 (6.1 m) cable. The hum might be slightly louder if the TV input were selected and the volume turned up because the same ground noise current also flows in the 3 foot (0.9 m) cable.

You might be tempted to break this ground loop with a three-to-two prong adapter, sometimes called a ground lifter. Remember that this adapter is intended to provide a safety ground (via the tab to the cover plate's mounting screw, metallic J-box and conduit) in cases where a three-prong plug is to be used with a two-prong outlet. Audio, video or other cables that connect equipment can also carry lethal voltages throughout the system if just one ground-lifted piece of equipment fails. Never defeat the safety function of the third prong on any equipment having a three-prong AC plug. This practice is both illegal and dangerous.

One safe solution is to break the ground loop by installing a ground isolator in the audio signal path from television to preamp as shown in Figure 6. High-performance ground isolators, like the Jensen ISO-MAX series, must always be installed at the receiving end of the cable.

Another safe solution is to break the ground loop by installing a ground isolator in the CATV signal path at the television as shown in Figure 7. Troubleshooting even large, complex systems can become manageable if testing always uses the technique of working backwards toward the signal sources.

Tips for unbalanced interfaces Keep cables as short as possible. Longer cables increase the coupling impedance. Serious noise problems are common with 50 foot (15.2 m) or longer cables. Even much shorter cables can produce severe problems if there are multiple grounds. Never coil excess cable length, and use cables with heavy gauge shields. Cables with shields of foil and light gauge drain wires increase coupling impedance. Use cables with heavy braided copper shields, especially for long cables. The only property of cable that has any significant effect on audio-frequency noise coupling is shield resistance, and you can compare this with an ordinary ohmmeter.

Maintain good connections. Connectors left undisturbed for long periods can develop high contact resistance. Hum or other interference that changes when the connector is wiggled indicates a poor contact. Use a good commercial contact fluid and/or gold-plated connectors to help prevent such problems.

Do not add unnecessary grounds. Additional grounding of equipment will generally increase circulating ground noise currents rather than reduce them. As emphasized earlier, never disconnect or lift a safety ground or lightning protection ground to solve a problem.

Use ground isolators at problem interfaces. Transformer-based isolators magnetically couple the signal while completely breaking input to output connections. This stops the noise current flow in the cable's shield, which eliminates common-impedance coupling. A variety of isolators are commercially available for audio, video and CATV signals.

Tips for Balanced Interfaces Be sure all balanced line pairs are twisted. Twisting is what makes a balanced line immune to magnetic fields that can cause interference. This applies especially to low-level mic cabling. Wiring at terminal or punchdown blocks is vulnerable because the twisting is opened up, effectively creating a magnetic pickup loop. In particularly hostile environments, consider star-quad cable; it has about 40 dB more immunity to magnetic pickup than standard twisted pair types.

Beware of the pin one problem. Lots of commercial equipment, some from respected manufacturers, has this designed-in problem. If disconnecting the cable shield at an input or output reduces a hum problem, the equipment at either end of that cable may be the culprit.

Use ground isolators to improve CMRR. A quality ground isolator can increase the noise rejection (CMRR) of a balanced interface by more than 50 dB and has other benefits. With a high-performance isolator in place, any balanced (or unbalanced) input becomes truly universal, accepting signals from balanced or unbalanced sources while maintaining more than 100 dB of ground noise rejection. Such isolators can also solve pin one problems, if they exist, and provide inherent suppression of ultrasonic and RF interference. The resulting reduction of spectral contamination is often described as a marvelous improvement insonic clarity. A poorly designed transformer or ground isolator, however, can cause signal degradations, such as loss of deep bass, poor transient response and increased distortion. As usual, be wary of cheap products with scanty or nonexistent specifications.

1. Pease, Robert A. Troubleshooting Analog Circuits, Butterworth-Heine-mann, 1991.

2. Neil Muncy, "Noise Susceptibility in Analog and Digital Signal Processing Systems," Journal of the Audio Engineering Society, June 1995, pp 435-453.

3. Jensen Transformers, Application Schematic AS032 (free on request or download from www.jensen-transformers.com).



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