[Boatanchors] 2nd Newbie Q Follow Up

[Jim Simmons]

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" <bob145 at suddenlink.net> 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" <bcarling at cfl.rr.com>
>> To: "Bob Jackson" <bob145 at suddenlink.net>
>> Cc: "porch.boat" <boatanchors at theporch.com>; "puck.boat" <boatanchors at puck.nether.net>; "qth.boat" <boatanchors at mailman.qth.net>
>> 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" <bob145 at suddenlink.net> 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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>>> Boatanchors mailing list
>>> Boatanchors at puck.nether.net
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