[Elecraft] KPA500 hum

[Phil Hystad]

Wayne,

I have not purchased my KPA500 yet but I do want to understand anything you
say.  Now, I don't understand item (1) below about using the yellow (lowest
voltage) tap.

I think my problem is possibly perspective but in reading the manual, the yellow
tap is associated with the highest primary voltage.  Why is it that you are calling
it lowest voltage?

73, phil, K7PEH


On Sep 28, 2011, at 4:40 PM, Wayne Burdick wrote:

> 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
> 
> 
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