[Elecraft] Hum on KPA1500

[Jim Brown]

On 4/5/2020 5:16 PM, Frank O'Donnell wrote:
> I'm not the OP, but can you remind us what buzz (multiple harmonics of 
> 60 Hz) might signify?

60 Hz hum is nearly always magnetic coupling in some form; buzz is 
generated in the three phase power systems used to distribute power. 
While few of us have 3-phase power in our homes, the systems out on the 
street ARE 3-phase. These harmonics are present because a very large 
fraction of the loads on the power system are some form of 
rectifier-capacitor-input power supply; most of the AC load current 
flows at the positive and negative peaks of the 60 Hz sine wave to 
recharge the filter cap.  This causes the current waveform to be a 
series of positive and negative pulses, and the IR drop causes the top 
of the voltage waveform to be rounded off. And we know that any 
distorted signal is composed of harmonics.

In an ideally balanced three-phase system with only sine-wave loads, the 
neutral current goes to zero. But when harmonics are present (the 
distortion), harmonics whose number is divisible by 3 ADD in the neutral 
rather than cancel. They also add in the ground!

Buzz is generated by leakage current in the power system; it can couple 
into our equipment by a couple of mechanisms. It can be eliminated by 1) 
proper bonding within our home and station; and 2) getting power for all 
station equipment, including computers and other accessories, from the 
same AC outlet, or from outlets whose green wires are bonded together. 
All of this is addressed in the slide deck for tutorial talks I've done 
at Pacificon, Visalia, and to several ham clubs.

http://k9yc.com/GroundingAndAudio.pdf

73, Jim K9YC