3.8 - What is the "Pin 1 problem" and how do I avoid it?
Description
This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
3.8 - What is the "Pin 1 problem" and how do I avoid it?
This is a special case of "ground loop" or shared path coupling.
Recently this has been discussed in great detail and clarity by a
group led by the consultant Neil Muncy of Toronto. Suppose you
have a mixer, whose balanced output is connected to an
amplifier's balanced input through a correctly wired cable. Both
units are powered from the AC mains and one or both have some
small amount of AC leakage current that travels to ground through
all available ground paths -- including the shield of the cable
that connects the two units. So far so good, no harm done because
the circuit is balanced and any common mode voltage from current
flowing through the shield will be canceled by the amplifier
input. However... a small part of this leakage current also
travels through the shield of the wire going from the back panel
XLR connector to the PC board, through some "ground" traces on
the PC board, and back out through the power line ground cable.
No problem so far, except that some gain stage on that same PC
board also uses that piece of ground trace in its negative
feedback loop, and some part of that leakage signal will be added
to the signal in that gain stage; it might be video, or data, or
another audio signal, or (most commonly) power.
The solution to this variant of shared path coupling is the same
sort of approach that applies to other unbalanced signals: give
the leakage current a very low resistance path to follow, and
remove as many of the shared paths as possible. Within a unit of
equipment, all the XLR connectors' pin 1 terminals should be
connected to ground with very low resistance (big) wire or
traces, and preferably all of the ground connections should be
made at one point, the so-called "star ground" system. A brute
force approach is to assume that the back panel is the star
ground, and wire every connector's pin 1 solidly to the panel as
directly as possible, and lift all the ground wires but one that
go from the connectors to the circuitry. In this way, all the
external leakage currents (the "fox" to use Neil Muncy's term)
will be conducted through the back panel and out of the way,
rather than running them through the ground traces on the PC
board where they will mix with internal low level signals in high
gain stages (the "hen house"). Individual wires can be run from
points on the circuit board that need to be at "ground" potential
to a common point on the back panel, which is designated a "zero
signal reference point" (ZSRP). Equipment that has a reputation
for being "quiet" and easy to use in many different applications
is often found to be wired this way, while equipment that is
"temperamental" if often found to be wired in such a way that
leakage currents are easily coupled to internal signal lines.
There's a simple test that can be done to check equipment
susceptibility to this problem. Connect the output, preferably
balanced and floating, of an ordinary audio oscillator to the pin
1 of any two XLR connectors on the equipment. Now operate the
equipment through its various modes, gain settings, etc. You may
be surprised to find the audio oscillator's signal appearing in
many different places in the equipment. [David]
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