Re: OK to use TV coax for microphone?

"Michael A. Terrell" <mike.terrell@[EMAIL PROTECTED]
> wrote in message 
news:_aadne8Vi703_oPVnZ2dnUVZ_vGdnZ2d@[EMAIL PROTECTED]
>
> Dead Paul wrote:
>>
>> On Sun, 04 May 2008 15:17:37 +0000, Don Pearce wrote:
>>
>> > On Sun, 04 May 2008 11:15:45 -0400, "Michael A. Terrell"
>> > <mike.terrell@[EMAIL PROTECTED]
> wrote:
>> >
>> >>Dead Paul wrote:
>> >>>
>> >>> no pickup inside coax.
>> >>
>> >>
>> >>   Really?  Then you don't know much about Coaxial cables, ****elding
>> >>effectiveness, signal ingression and a dozen other topics.  In other
>> >>words, no single ****eld is 100% effective.
>> >
>> > And of course unless it is made of soft iron or mumetal, it doesn't
****eld
>> > magnetically at all.
>>
>> coax doesn't ****eld in that way. It works by cancellation and eddy
current
>> effects duh!
>
>
>   DUH?  That's the only part that you got right. If you believe that
> crap, then explain how ****elded cable allows RF into a microphone input.
>
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It's usually rf currents conducted through the ****eld conductor, through
the guts of the 
device being fed that cause the problem.  It isn't rf across the input.  A
plain piece of 
wire connected to the ground of the input can often still cause rf pickup.
 The cable's just 
an antenna connected to the ground of the device.  ****elding doesn't
matter.  I've seen it 
in everything from little (amplified) computer speakers to pro audio
mixers.  The fix is 
almost always to not allow the rf currents to flow through ground (or any)
conductors in the 
audio circuitry before they get to other external cables and such  tied to
the device at 
other points in the circuitry.  Tying the input and output cables,
chassis, etc. to one 
point is usually effective.  Failing that, get each cable's ground
(balanced or not) at 
least straight to the chassis, if not one point -- but never to circuit
paths inside the 
unit where things like op-amps with huge open-loop gains can get all weird
and demodulate 
it.  There's "pro" audio gear around (mixers, etc) with susceptibility to
rf currents.

Earl