Audio Engineering Society

Chicago Section

Meeting Review, July 21, 2005


other meeting reports

7/21/05 Meeting Highlights
by Jeff Segota

On July 21st, the Chicago Section concluded its season with the presentation “Real World Audio

Interfaces and Other-World Myths” by system interfacing expert Bill Whitlock. About 30

attendees took a big step toward reducing the noise in their understanding of hum and buzz in

their audio systems.

 

 

Because the regulations designed to protect against electrocution and fire play a big role in noise

problems, the presentation started with an overview of AC power with regard to safety,

equipment faults, and lightning. Because of the serious potential danger, these issues must be

well understood when acting to reduce system noise. It is a myth that most noise is caused by

improper AC wiring, as there will always be a small voltage between safety grounds in different

outlets. And leakage currents will always flow in signal cables, so it is the coupling of these into

the signal path, particularly at interfaces, that is the problem.

 

The presentation next moved to unbalanced and balanced interfaces. The former is extremely

susceptible to common-impedance coupling, where noise is induced by leakage currents flowing

in the grounded conductor of non-zero impedance. The balanced interface reduces noise

coupling by placing both signal conductors at the same impedance with respect to ground, so that

noise pickup is a common-mode signal on both conductors that can be canceled in the receiver.

Transformers were essential elements of balanced interfaces at the start, but began to be replaced

by cheaper differential amplifiers by 1970. It was soon discovered that transformers were more

effective than initially realized, because they maintain noise cancellation with “real world” driver

and line imbalances much better than differential amplifiers. This transformer advantage is a

much higher common-mode input impedance, but this can be imitated in circuitry with

bootstrapping, which has been implemented in the InGenius® IC.