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RE: [TowerTalk] Station Ground

To: "Tower Talk List" <>
Subject: RE: [TowerTalk] Station Ground
From: "Jim Brown" <>
Reply-to: Jim Brown <>
Date: Sat, 15 Jan 2005 14:27:34 -0600
List-post: <>
On Sat, 15 Jan 2005 03:16:11 -0500, Pete Smith wrote:

>I had fairly 
>severe AC hum on the audio, apparently the result of a few millivolts 
>potential difference between chassis.  I finally cured it by experimentally 
>connecting the chassis of the computer and radios directly to one another 
>with heavy copper (not through the bus bar).

You were experiencing a "pin 1 problem," probably in both the computer and the 
radio. Nearly all computer sound cards are built with pin 1 problems, and so 
are nearly all ham rigs. The correct connection of a cable shield is to the 
CHASSIS of the equipment on either end, NOT to the printed circuit board (often 
called "audio ground" or "control ground" on the mic connectors and the rear 
panel jacks. One sure sign of a pin 1 problem is an RCA or 1/8" jack that is 
insulated from the chassis. The easy cure is to remove the shield from that 
insulated jack and tie the shield to the chassis. Simply shorting the jack to 
the chassis may establish ground loops interior to the equipment that are 



While these applications notes specifically address balanced audio circuits, 
the physics is the same whether the connection is balanced or unbalanced. The 
only difference is that with an unbalanced connection, the current also causes 
an IR (or IZ) drop in the interconnecting cable shield, which adds to the noise 
produced by the pin 1 problem. But in most real systems, the noise/hum/buzz/rf 
coupling produced by the pin 1 problem is greater than that caused by the IR/IZ 

When you bonded the equipment together with beefy copper, you reduced the 
voltage between them, which in turn reduced the current flowing on the cable 
shield, which reduced the drop on the shield and the current flowing into the 
improper shield connection to the printed circuit board. 

BTW -- that current results from the potential difference between the two ends 
of the shield, both of which are "grounded" to the power system's equipment 
ground. If the connected equipment is plugged into different outlets, there can 
be considerable noise (from hundreds of mV to a few volts) between the two ends 
of the shield. Where does it come from? One major source is the line bypass 
capacitors that are part of noise filters -- those caps couple noise to 
"ground" and there is IR (and IZ) drop in the ground conductor. Another source 
is leakage current from power transformers. In the pro audio world, two common 
techniques are used to minimize the potential difference between "grounds." The 
one that would apply to a ham shack is to power all interconnected equipment 
from the same outlet, or, if you need more than one circuit, from outlets that 
share the same equipment ground conductor (the green wire). Now, the only 
voltage between ends of the cable is what can be induced by antenna action or 
from magnetic coupling of a strong field. And you can minimize both by 
minimizing the loop area between the signal cable and "ground." That means 
running the signal cable tightly next to the big copper that bonds the 
equipment together. 

BTW -- one way to get that strong magnetic field is to tie neutral to ground at 
some point in addition to the power entrance. Double-bonded neutrals are BAD 
NEWS, and, as Tom corrected himself, are to be avoided at all costs. 

Jim Brown  K9YC


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