[RFI] RFI Clean Audio Amplifiers For Amateur Use?

[Jim Brown]

On 10/5/2020 4:17 PM, Rob Atkinson wrote:
> I did not
> realize the problem is that the audio amp is the SOURCE of RFI.

So-called "Class D" amps are digital -- they're generating the audio 
waveform digitally, so they have all the possible mechanisms associated 
with trains of rectangular waves. That's one kind of amp that can 
produce RFI.

Several years ago, neighbor W6GJB had a pair of USB-powered speakers 
that were generating enough RF trash that he could see it with an MFJ 
current probe on the USB cable. Turned out to be DC-to-DC converters 
(similar concept as SMPS, so all the same problems) that made the USB 
power usable for the power amp circuitry. I added that noise source to 
my "improving contest scores by killing RFI" app note.

73, Jim K9YC