Meeting Review, July 21, 2005
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.