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Re: [Amps] The Pin One Problem - Common Impedance Coupling

To: <jim@audiosystemsgroup.com>, <amps@contesting.com>
Subject: Re: [Amps] The Pin One Problem - Common Impedance Coupling
From: "Carl" <km1h@jeremy.mv.com>
Date: Fri, 7 Jun 2013 22:34:36 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>



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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