[Elecraft] KPA500 hum

[Wayne Burdick]

A low level of AC hum is normal with products like the KPA500 that use  
a high-power linear supply.

Nearly all reports of KPA500 hum have come from kit builders. So we  
did some reverse engineering of our kit building process to see how to  
make the normal hum worse. In the process we found three ways to make  
it better:

1. Hum is proportional to supply voltage. Using the yellow (lowest  
voltage) tap, there's little or no hum. The red tap increases it a  
bit, and the green tap a bit more. Per the assembly manual, the  
rectifier block should use the lowest-voltage winding that produces  
60V key down at 500W of output.

2. Loose hardware definitely increases hum. To minimize it, make sure  
your transformer mounting bolts and chassis hardware are all tight.

3. Make sure the transformer wires and other wires inside the KPA500  
are not in close contact with the top, bottom and side covers.   
Vibration of these covers against the wires can cause louder hum,  
especially when in transmit.

Of course we'll work with any customer who still has an obvious hum  
after trying the above.

Other possible hum causes (or not):

We've tested the toroidal transformers extensively, and have confirmed  
that they do not generate hum internally. Replacing the transformer  
has no impact on how much hum an amplifier produces.

You could also have hum due to DC on the AC line or by bad AC line  
waveforms. For example, the KPA500 will operate with the "modified  
sine-wave" output of a low-cost DC to AC inverter. But like other  
transformer-operated devices it will hum loudly when presented with  
harmonic-rich input.

With reasonable AC input waveforms, the proper transformer tap  
selected, and all the mounting hardware tightened properly, the normal  
audible hum from the KPA500 is lower than the combined hum and blower  
noise from nearly any AC-powered amplifier.

The KPA500 is a lot quieter than most amplifiers, in general, thanks  
to its thermostatically-controlled multispeed fan and diode-switched  
keying. However, it is not always *completely* silent. It is not  
library-approved.

>

73,

Wayne
N6KR