Re: OK to use TV coax for microphone?

On Tue 06 May 2008 03:23:10, Michael A. Terrell wrote:

> 
> Earl Kiosterud wrote:
>> 
>> 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. 
> 
> 
>    I was in the commercial sound business for over a decade, and
>    never 
> had any of my installations pick up a radio, CB, or public service
> radio. I am now disabled, but I was a broadcast engineer
> installing audio equipment in some of the worst environments
> possible  At one site I had to drive a ground rod over 80 feet,
> through Alaska's permafrost to get a better ground. I had the AM
> BCB transmitter's signal riding on every 'ground' in the complex.
> You could even see it in the video baseband of the TV station at
> that site.  I removed a couple miles of substandard, and excess
> cabling to clean up that mess.  Some 'engineer' had paralleled the
> two audio boards at one time, then left the cables hooked to one
> of the boards after some upgrade. It was really crappy two 
> conductor, tinned braid, no jacket garbage that was so corroded
> that the ****eld was useless.  If the radio station's audio console
> hadn't been the original 1948 Gates, with transformer input, the
> station would have been useless.
> 
>    Any 'pro' gear that is susceptible to RF problems is nothing
>    more 
> than over hyped consumer crap. A two hole ferrite bead, and a 100
> pF disk capacitor made a decent low pass filter for non
> transformer balanced inputs. Any equipment with unbalanced inputs
> is just consumer crap, no matter what you pay for it.
> 

Hey, steady on old chap!  I'm a consumer.  Equipment made to suit my 
consumer needs may not seem quit so crap to me or other consumers.  
Perhaps it's crap to a pro but not to me!

But I'm not a pro and can't afford pro gear and probably have no need 
for something very highly specified.