[BC] RF interference problem

[RON DOT'O SR.]

I used to be a commercial soundman back in the '60's  when solid state PA 
amps where first coming in.  We had problems with the speaker leads acting 
as antennas and sending the AM RF to the output stage of the PA amp where 
stray capacity would filter out the RF and the transistor junctions in the 
output stage would detect and amplify the audio envelope and send the audio 
out the long speaker leads to the speakers.  In other words a very powerful 
and expensive crystal set!

The cure was to put RF choke coils (these days you might try ferrite beads 
or terrodial (sp?) coils) in series with each side of the speaker leads and 
a small RF bypass capacitor from each side of the speaker leads to ground to 
bypass the RF which was a low resistance to RF but a high resistance 
(impedance) to audio.  I don't remember any of the component values but am 
sure the brain trust here can help out with those.  you might start with 250 
mH pie wound multi layer chokes and around .0001 uF caps.

One of my "favorites" was the Reform Baptist church that we had just put a 
new installation in that was close to a local AM station and getting the 
horse race results from the local track during their services.

FM stations by their nature won't cause these RF detection problems.


Ron Dot'o, Salem, OR.

----- Original Message ----- 
From: "RadioPower.org" <webmaster at radiopower.org>
To: <broadcast at radiolists.net>
Sent: Saturday, January 21, 2006 15:24
Subject: [BC] RF interference problem


> My recording studio has a serious problem with RF interference from a 
> local
> Christian station. The station is 10 miles away but there's a huge FOX TV
> and radio tower less than a mile away. Judging by their program content it
> sounds like a fly by night operation that probably doesn't have competent
> engineering, but I digress. I'm assuming the transmission is coming from 
> the
> FOX tower. Anyway, the radio signal bleeds into all of my high end pre 
> amps,
> power amps, and monitors. I've tried moving things around and even 
> covering
> everything in copper foil. Nothing stops the holy RF from bleeding 
> through.
> I'm trying to create music here and it's driving me crazy hearing 
> religious
> nonsense in my monitors and headphones, not to mention it's wreaking havoc
> on my productions, as the only way to stop the bleeding is to use a noise
> filter set very high which also harms the sound quality. I've considered
> contacting the FCC and reporting the radio station. But I'll present the
> problem to the engineers here first?
>
> Thanks in advance,
>
> Shelby
>
>
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