🚀 go-pugleaf

Rich Grise 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!". :-)
> 

Trouble is, since we moved from Alexander Graham Bell's wall phone (with 
crank) to cell phones RF frequently turns into audio after getting into 
such single-end shielded cables. All it takes is an opamp with bipolar 
input structure.

Of course, with a good old tube amp you'll continue to be fine :-)

-- 
Regards, Joerg

http://www.analogconsultants.com/

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