[Amps] RF on audio using 374A

[Jim Brown]

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.

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