[Boatanchors] 2nd Newbie Q Follow Up

[Brian Carling]

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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