[TenTec] Heil Pro set 5 & Orion

[Robert & Linda McGraw K4TAX]

The pin 1 issue is more complex than one can imagine.

One must first understand that there is a difference between:
(1) Audio or analog ground
(2) Digital ground
(3) Chassis ground
(4) Safety ground

In many systems these are totally separate systems.  The key word here is
"separate".  Too often we think that ground is ground is ground is ground.
In low noise, high gain, hostile (RF) environments, they are different and
must be treated different as there are different currents flowing in the
ground circuits.  Ideally, ground is a 0 voltage point.  In reality if
current flows, and the conductor has resistance (and it does except at
absolute 0), then ground however you define it is not at 0 potential.
That's where the trouble starts.

Over the years, I've dealt with a lot of complex analog and digital systems
that must reside together and that must realize ground at some point.  Each
system is different thus requiring often radically different means of
achieving a 0 voltage point for a circuit.  I do not subscribe to the
statement that the "pin 1 problem" as being a design problem.  It is an
application problem.  If one believes that the only place to connect a
shield is the chassis, this is wrong because the chassis may not be at
analog or digital ground.  In RF applications, most likely the chassis is
not at RF ground.

Then of course comes the discussion of having the shield connected at both
ends.  Again, shield being correctly used as a shield, it should not be a
current carrying conductor.  If it doesn't carry current, regardless of the
resistance there is no difference in potential thus it can be at 0
potential.  Run a microampere of current through it and it is no longer at 0
potential.  If ground is needed for the circuit, then a separate conductor
must be used.  Shield is connected at one end only thus to prevent current
on the shield.  Current flowing results in thus nullifying the effect of the
shield.

73
Bob, K4TAX


----- Original Message ----- 
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: "tenteccontestingcom" <tentec at contesting.com>
Sent: Friday, May 14, 2004 7:44 AM
Subject: Re: [TenTec] Heil Pro set 5 & Orion


> On Tue, 11 May 2004 22:37:13 -0500, John Rader wrote:
>
> >grounding the shell of the 4 pin mic connector solved this.
>
> This is yet another example of what we in the pro audio world call "the
pin 1
> problem."  That is, to be effective, the shield of a cable must be
connected
> DIRECTLY to the shielding enclosure (chassis), NOT to the interior
circuitry or the
> circuit board. When it goes to the circuit board first and then to the
chassis, there
> will be a voltage drop across the inductance of that connection due to RF
current
> flow on the shield. That voltage drop will be added to the signal at
random points in
> the circuit depending on circuit board layout.
>
> The pin 1 problem is a MAJOR cause of RFI interference at audio
interfaces, and it
> is most certainly the cause of the one you have just fixed.
>
> Note that "the pin 1 problem" is an equipment design error! The radios
that connect
> the cable shield to any point other than their chassis are built WRONG!
The fact that
> MANY radios are designed this way does not change that fact -- the laws of
physics
> are not the result of a popular vote!
>
> BTW, it's called "the pin 1 problem" because pin 1 is the designated
shield contact
> of the XLR connector, and it that contact that must be connected to the
chassis
> within equipment.
>
> Jim Brown  K9YC
>
>
>
>
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