Audio Engineering Society
Chicago
Meeting Review, July 21, 2005
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7/21/05
Meeting Highlights 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. |
