[Boatanchors] 2nd Newbie Q Follow Up

[Jim Simmons]

Situations will vary, depending on the equipment and what you want to 
do.  I was just answering Bob's question of why do some constructions 
articles tell you to ground only one end of the shield in a shielded cable.

Jim, N5MSJ

On 9/10/2014 10:22 PM, Brian Carling wrote:
> I mean the interface to the computer has a transformer. You don't connect straight to the I/o of the radio from a computer.
>
> Best regards - Brian Carling
> AF4K Crystals Co.
> 117 Sterling Pine St.
> Sanford, FL 32773
>
> Tel: +USA 321-262-5471
>
>
>
>
>> On Sep 10, 2014, at 10:03 PM, "Scott Johnson" <scottjohnson1 at cox.net> wrote:
>>
>> Checked three sound cards, and a motherboard with built in sound, not a
>> transformer in the group.  They all had a little stereo line driver IC.
>>
>> Scott W7SVJ
>>
>> -----Original Message-----
>> From: Boatanchors [mailto:boatanchors-bounces at puck.nether.net] On Behalf Of
>> Brian Carling
>> Sent: Wednesday, September 10, 2014 6:39 PM
>> To: Jim Simmons
>> Cc: boatanchors at puck.nether.net
>> Subject: Re: [Boatanchors] 2nd Newbie Q Follow Up
>>
>> Speaker output from a computer sound card is not high impedance nor low
>> level.
>>
>> Also most have transformer isolation so ground loops are not usually an
>> issue.
>>
>> Best regards - Brian Carling
>> AF4K Crystals Co.
>> 117 Sterling Pine St.
>> Sanford, FL 32773
>>
>> Tel: +USA 321-262-5471
>>
>>
>>
>>
>>> On Sep 10, 2014, at 5:26 PM, Jim Simmons <orion at datasync.com> wrote:
>>>
>>> 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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