[Boatanchors] 2nd Newbie Q Follow Up

[Scott Johnson]

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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>>>> Boatanchors at puck.nether.net
>>>> https://puck.nether.net/mailman/listinfo/boatanchors
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