[BC] Audio and RF grounds...

[Burt I. Weiner]

While it would seem that they should, RF and Audio grounds do not 
behave the same.  Audio Shields should be grounded at one end only 
and preferably at the receive end.  That may not always be 
practical.  Sometimes in consoles the ground or shield is carried 
into the equipment and onto a board where the shield is tied as close 
to the audio input/common as close as possible.  The RF Fairy seems 
to have a good laugh over this.  In the case of shielded audio cables 
that are grounded only on one side is to ground it on the other end 
at the RF frequency.  That is, to ground the shield through a .001 or 
smaller depending on the RF frequency involved.  I've also done the 
same thing immediately where a shield or Pin 1 on a XLR enters or is 
mounted to the chassis.  Most of the time this has been successful 
and has avoided ground loops at audio frequencies.  Installing 
ferrite cores internally to equipment has produced good results also. 
Again, solving these problems is generally as much art as it is science.

Burt

At 06:48 PM 1/22/2006, you wrote:

>Jeff & others:
>
>The audio and sound guys have fought with the RFI problem for year blaming
>the broadcasters for excessive power etc.
>
>When the real problem has been poorly designed audio gear.
>
>Some years ago we had a LOT of problems with RFI getting into small 16
>channel Mackie Mixers, as well as some of their large sound 
>consoles. Ferrites  and
>all sorts of other tricks were used with some degree of success.
>
>Jim Brown of Audio Systems Group in Chicago _www.audiosystemsgroup.com_
>(http://www.audiosystemsgroup.com)  has a  great web site with 
>downloadable pdf
>files on hum and noise in audio systems.  One issue is the improper design of
>audio gear where the pin 1 inputs of audio  gear are not immediately 
>bonded to
>the chassis of the gear itself! Give the  ground a nice loop inside 
>of the box
>and at RF frequencies, you have a great RF  signal injector at AM or VHF
>frequencies!
>
>I suggest that those who encounter this type of problem with their gear
>check it out more closely.
>
>
>
>Russel L.  O'Toole, P.C.

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at earthlink.net
K6OQK