[Amps] RF on audio using 374A

Roger (sub1) sub1 at rogerhalstead.com
Tue Sep 13 22:58:49 PDT 2011

On 9/13/2011 5:39 PM, Jim Brown wrote:
> On 9/13/2011 7:16 AM, kc0hfl at cox.net wrote:
>> Current set up Radio Flex 5000 driving the 374A
> The problem is almost certainly at the Flex, and I'm guessing that the
> difference is the additional power of the larger amp.

I ran into something similar when pushing my DX-2sp to the legal limit 
on 75 with the Yaesu FTDX5000MP.
It turned out to be RF coming back down the feedline to the tower.  I 
put a 5 core, 5 turn choke right where the coax reached the tower.  
Absolutely no RFI since then.


Roger (K8RI)

> Assume a Pin One problem at the Flex, and add multi-turn ferrite chokes
> to audio lines (mic, processing) and any other lines that plug into the
> radio.  Study http://audiosystemsgroup.com/RFI-Ham.pdf
> There's another mechanism that SOUNDS like RFI, but is NOT.  W8JI
> alerted me to it, and I've duplicated it in my station.  It happens when
> you have a 12V rig with V- bonded to the chassis of a power supply, and
> audio accessories that feed the radio powered from that 12V supply.
> What happens is that varying DC current with modulation causes
> modulation of the V- line by the IR drop, and that modulation is added
> to the audio. It SOUNDS just like SSB detected by an AM detector, so you
> think it's RF feedback
> 73, Jim Brown K9YC
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