On 6/7/2013 12:54 PM, Roger Parsons wrote:
I think that a technically correct generic name for the 'Pin 1 Problem'
would be unwanted 'Common Impedance Coupling', and a bit of a search
shows that this term has been quite frequently used.
No, it is NOT the correct description.
** Yes it is and more aptly describes the much less known Pin 1 Problem
which is a connector specific subset of the overall descriptor.
Joe hit the nail on the head,
** Not even close unless he was using his head as the hammer (-;
and Paul got a lot of it right.
Old time broadcasters who "never saw these problems in big consoles" were
almost certainly working on gear built in the old days, when a jack was
screwed down to the chassis, and the shield went straight to the chassis.
Pin One Problems began to abound when mfg got too efficient for its own
good, with connectors that mounted to the circuit board, but forgot that
shields had to make contact with the chassis.
** Modern SS audio BC gear hasnt experienced those problems, they didnt read
your "tootertorial" either. Hams are using them on vintage AM BCB
transmitters converted to the ham bands and everything from Heathkits to the
latest SS ham gear made for use on AM and hi-fi SSB.
Now I'm going to really arouse folks -- there is no such thing as a
"ground loop!"
** Some simply dont understand history and try to reinvent the wheel or come
up with cute sounding names for an old problem.
.
When power line "buzz" couples into unbalanced wiring,
it is because the chassis of two pieces of equipment being connected are
at different potentials, and that potential difference is added to the
signal. The fundamental cause of the potential difference is nothing more
or less than LEAKAGE current from the AC power system. The path for this
current is capacitance between the "high" side of the AC power line and
the chassis, which must, by law, be connected to the "green wire" of the
AC line, that current returns to its source (the power system) via the
green wire, and the current produces IR drop in the green wire. That IR
drop is what we hear.
The leakage current consists primarily of the HARMONICS of 60 Hz, which is
why we hear "buzz" rather than "hum" (pure 60 Hz). Why harmonics? Because
nearly all equipment we connect to the power line as a load has a
capacitor-input power supply, which causes current to flow in relatively
short pulses at the top and bottom of each cycle, which in turn makes the
current rich in harmonics.
As if that weren't enough, in three-phase systems, "triplen" harmonics
(any harmonic number divisible by three) will ADD both in the neutral and
in the ground. Few of us have 3-phse in our homes, but the vast majority
of homes are fed by 3-phase distribution systems. As a result, almost all
of us will see a lot of triplen harmonics if we do an spectrum display of
the power line, and of the voltage between one chassis and another of
equipment plugged into different outlets.
So what we incorrectly call a "ground loop" and do stupid things to avoid,
is really AC leakage current, and the cure is VERY simple.
1) Minimize the voltage difference between one chassis and another by
getting power for everything that will be interconnected from the same AC
outlet, or from outlets that share the same green wire, or from outlets
that are bonded together by short fat copper.
2) Bond with short, fat copper from chassis to chassis of every piece of
gear that has an unbalanced connection.
#1 takes the IR drop of the long "green wire" from the outlet to the panel
out of the picture, so the remaining IR drop is only the short (typically
6 ft) line cord. #2 helps two ways. First, shorts out the remaining
potential difference by brute force. Second, it diverts that leakage
current (and most of the RF that might be picked up on a short cable) to
the chassis, so it also cures any Pin One Problems that might be present.
In the tutorials I do for pro audio/video trade shows, I call this
technique "local bonding," emphasizing the fact that it only works where
cable lengths are short enough that the resistance of the bonding
conductors is low.
As I see it, there is one, and only one, good time to use the word "loop"
in a discussion of noise coupling, and that is when the coupling mechanism
is the MAGNETIC field. With all magnetic coupling, the strength of both
the field and the sensitivity of a system to a magnetic field is the LOOP
AREA of the coupling path. In our stations, we commonly see magnetic
coupling in three situations.
1) Leakage flux from big power transformers close to our equipment and/or
its wiring.
2) Wiring errors in the power system that establish a magnetic field.
Perhaps a neutral is bonded to the green wire at more than one point, or a
load is connected between "hot" and "green," or an outlet is mis-wired.
3) We feed a long wire antenna that ends in the shack, and thanks to the
length of the antenna, there's a current peak near our equipment. That
current produces a strong magnetic field. As has been observed, a cable
shield provides NO magnetic shielding at power frequencies, and TWISTED
PAIR is VERY effective at rejecting magnetic fields.
73, Jim K9YC
** YAWN
There might actually be something useful in the last rant but I got bored
after a few paragraphs as it has nothing to do with common old fashion audio
hum. It is more drifting off subject as a method to confuse, confound and
put to sleep the readers when the author is obsessed with replying but has
nothing to say.
.
Im off to bed, I'll probably have to wear my hip waders tomorrow reading
thru replies from you and Joe.
An old axiom comes to mind:
"If you cant dazzle them with brilliance, baffle them with BS" W.C. Fields
sure got that right.
Carl
KM1H
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