Bill Whitlock, President of Jensen Transformers, gave a talk at the Dolby Theater in San Francisco for the February 2005, AES meeting. Whitlock covered a wide variety of topics related to noise in audio systems as well as pointers on electrical safety.

Signal interfaces are usually the source of audio noise; signals are fairly immune to noise inside equipment. Two factors contribute to noise problems: ground voltage differences always exist between properly wired outlets and leakage currents always flow in signal cables even if equipment has 2-prong plugs. Unbalanced systems are vulnerable to leakage current that flows in the grounded conductor that is usually the shield of A/V equipment cables. A high performance interconnect for unbalanced audio combines low capacitance per foot, very low shield resistance (e.g., the Belden 8241F cable), and reliable connectors.

Good noise rejection properties can be designed into balanced interfaces by relying on common-mode rejection properties. However, faulty equipment designs may allow shield current to flow through active signal ground traces which creates what is called the "pin 1 problem," common impedance coupling inside the box. Refer to the White Papers section of www.jensen-transformers.com for information on a DIY interface tester and system-troubleshooting guide.

Whitlock dispelled several myths surrounding cable noise. For example, extra shielding does not improve the noise performance of a cable and manufacturers’ claims involving subnanosecond response and transmission line theory are marketing hype.

Grounding is a safety feature to prevent shock and electrocution. It provides a path for the fault current to the circuit breaker. If equipment uses 3-prong ac plug, it is not safe to defeat the ground pin. For example, if a sub-woofer that has its ground pin lifted (usually done to eliminate a noise problem) becomes live at 120 V due to a problem, the current can flow through audio cables to return to ground. This will energize the rest of the audio/video system and cause potential electrocution. The only workaround is to use a GFCI receptacle, though this may introduce occasional nuisance trips.

Several veterans who attended shared comments with the audience of 50 people.

Thomas Merklein