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 <[email protected]> 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" <[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 >>>> _______________________________________________ >>>> Boatanchors mailing list >>>> [email protected] >>>> https://puck.nether.net/mailman/listinfo/boatanchors >> _______________________________________________ >> Boatanchors mailing list >> [email protected] >> https://puck.nether.net/mailman/listinfo/boatanchors > > _______________________________________________ > Boatanchors mailing list > [email protected] > https://puck.nether.net/mailman/listinfo/boatanchors _______________________________________________ Boatanchors mailing list [email protected] https://puck.nether.net/mailman/listinfo/boatanchors
