🚀 go-pugleaf

On Tue, 30 Jun 2009 17:27:21 GMT, Rich Grise <richgrise@example.net>
wrote:

>On Thu, 25 Jun 2009 12:39:08 -0700, John Larkin wrote:
>
>> On Thu, 25 Jun 2009 13:48:35 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>John Larkin wrote:
>>>> On Wed, 24 Jun 2009 21:42:07 -0700, <castlebravo242@att.net> wrote:
>>>> 
>>>>> ... was reading it would get data errors. The problem was that the
>>>>> shield on the cable from the heads to the preamp was grounded on both
>>>>> ends.
>>>> 
>>>> That's usually the best way to ground shields.
>>>> 
>>>Depends.  With floating transducers and shielded twisted pair, it's best
>>>to ground the shield only at the transducer end.  That way any
>>>capacitive imbalance doesn't produce any currents, because the shield is
>>>at the transducer's idea of ground.
>>>
>>>
>> At the receive (instrument) end, the ungrounded end of the coax shield
>> becomes the hot side of a resonant "vertical" antenna. Any voltage there
>> becomes 100% common-mode signal on the inner twisted pair.
>> 
>> What's wrong with a little current in the shield? The signal pair isn't
>> affected. But volts, or tens of volts, of common-mode RF can be nasty.
>> 
>I think half of us are talking about low-noise audio frequencies, and
>half of us are talking about RF. The way I heard it, you leave one end
>unshielded for audio preamps and stuff, but ground both ends for RF;
>I think it has to do with wavelength and, well, propagation
>characteristics. Of course, the thing that was drummed into me while
>they were teaching me this was "GROUND LOOPS!". :-)
>
>Cheers!
>Rich

Audio: if one end of a shield is disconnected, the delta-voltage (60
Hz, RF, esd spikes) between chassis is greater. If the signal is
single-ended, that increases noise directly. If differential, it
increases common-mode noise.

Why would you want to do that?

A 60-Hz (and RF, and ESD) "ground loop" can result from different
boxes being plugged into different outlets which have some 60 Hz
difference voltage (or form a relative antenna for RF.) Connecting the
shield on both ends shorts out some or most of this difference voltage
and REDUCES the effect of the "ground loop" on the signal.

Whoever did the drumming was passing on folklore.

John