1198 Chapter 32
IEC had recognized that test is not an adequate assur-
ance of the performance of certain electronically
balanced amplifier input circuits. The old method
simply didn’t account for the fact that source imped-
ances are rarely perfectly balanced. To correct this, this
author was instrumental in revising IEC Standard:
60268-3 Sound System Equipment – Part 3: Amplifiers.
The new method, as shown in Fig. 32-31, uses typical
±10: source impedance imbalances and clearly reveals
the superiority of input transformers and some new
active input stages that imitate them. The new standard
was published August 30, 2000. The Audio Precision
APx520 and APx525, introduced in 2008, are the first
audio instruments to offer the new CMRR test.
32.5.2 The Pin 1 Problem
In his now famous paper in the 1995 AES Journal, Neil
Muncy says:
This paper specifically addresses the problem of
noise coupling into balanced line-level signalinterfaces used in many professional applica-
tions, due to the unappreciated consequences of
a popular and widespread audio equipment
design practice which is virtually without prece-
dent in any other field of electronic systems.^30Common impedance coupling occurs whenever two
currents flow in a shared or common impedance. A
noise coupling problem is created when one of the
currents is ground noise and the other is signal. The
common impedance is usually a wire or circuit board
trace having a very low impedance, usually well under
an ohm. Unfortunately, common impedance coupling
has been designed into audio equipment from many
manufacturers. The noise current enters the equipment
via a terminal, at a device input or output, to which the
cable shield is connected via a mating connector. For
XLR connectors, it’s pin 1 (hence the name); for ¼ inch
connectors, it’s the sleeve; and for RCA/IHF connec-
tors, it’s the shell.Figure 32-30. Common-mode impedances apply to a voltage applied to both inputs.Input X
Input Y
Common
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