The reasoning behind grounding only one end of the shield is to prevent
"ground loops". In high impedance circuits ground loops will introduce
oscillation into the signal chain. In audio circuits this presents
itself as whistles, howls, and distortion.
For example: the shielded cable in an audio amplifier that connects the
input jack on the amplifier to the hi gain preamp circuit should be
grounded only on one end. Chances are the input jack is already
grounded to the chassis by its mechanical connection, and the input
preamp stage is also grounded to the same chassis by an electrical
connection. So the signal return path already exists without connecting
anything.
When you connect a shielded cable (inside the amplifier) from the jack
all the shield on the cable is for is to shield the inside signal wire
from external influence. If you connect both ends of the shield you now
have two return paths for the input signal (the chassis and the shielded
cable), and these two paths will be different in impedance, (capacitive
and inductive) This difference can (and usually does) make up a tuned
feedback circuit for the preamp - which turns it into an oscillator.
If you're connecting two things together that do not share a common
chassis (common ground) then both ends of the shield must be connected
to form a complete path for the signal. If you have items daisy chained
together and their cases are made of metal then if two, or more, is
allowed to come in contact (creating another return path for the signal)
then you just may find that you will have howls and or distortion.
The way to find out if you need to only connect one end is to connect
both ends and try it. It it works, then fine. If you have unwanted
distortion then disconnect one end. I don't think it really matters
which end the shield is grounded on, but I would ground it on the input
(jack) end.
By the way, the above also applies to rf circuits but in spades.
Hope this helps.
Jim, N5MSJ
On 9/10/2014 12:09 PM, Brian Carling wrote:
I still don't know what devices you're trying to connect together. However most
devices I have used do not have any difficulty with this arrangement and
grounding the Shields at both ends.
Best regards - Brian Carling
AF4K Crystals Co.
117 Sterling Pine St.
Sanford, FL 32773
Tel: +USA 321-262-5471
On Sep 10, 2014, at 12:15 PM, "Bob Jackson" <[email protected]> wrote:
Ah, Ha! You've hit on the issue!
I'm trying to convert the stereo audio output from a 1/8" jack (e.g. iPod) to
two RCA plugs to go into a switchbox. On the surface, it would seem that the shield
should be the common connection between the two RCA plugs. Using the shield and one
signal wire for one plug is OK but when I go to the second signal wire, I find that
it's actually common to the shield itself. It doesn't seem right to leave the shield
unconnected at this second plug but to connect it would seem to mean that now the
two signal wires are shorted together. What's the story here?
Bob AG5X
----- Original Message ----- From: "Brian Carling" <[email protected]>
To: "Bob Jackson" <[email protected]>
Cc: "porch.boat" <[email protected]>; "puck.boat" <[email protected]>;
"qth.boat" <[email protected]>
Sent: Wednesday, September 10, 2014 10:49 AM
Subject: Re: [Boatanchors] 2nd Newbie Q
What two devices are you connecting?
Best regards - Brian Carling
AF4K Crystals Co.
117 Sterling Pine St.
Sanford, FL 32773
Tel: +USA 321-262-5471
On Sep 10, 2014, at 11:29 AM, "Bob Jackson" <[email protected]> wrote:
I've noticed that some construction articles involving the use of shielded
cables advise that the shield be connected at only one end but they don't say
which end, i.e. near or far. Also, when is this practice most useful and why?
Thanks again,
Bob AG5X
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