[Elecraft] Audio Thump

[Dale Boresz]

Hello Jimmy,

I know exactly what you're referring to, and it is not due to the KDSP2, 
or the KAF2. It is there on the bare-bones K2, although it is primarily 
audible when using headphones capable of very wide frequency response. I 
have several pairs of various models of Grado headphones, as well as a 
pair of Beyerdynamic DT-990's, and the thump is noticeable on all. In 
fact, I can also hear a 'click' as well as a thump.

I've been wanting to investigate this myself, but haven't found the time 
to do so yet. I've temporarily reduced it to the point where it's a lot 
less noticeable by placing the series combination of a 3.3 uF cap and 
220 ohm resistor in line with the headphones, and shunting across the 
headphones with a .47 uF cap. The series 3.3 uF will help to reduce the 
'thump' by reducing the low-frequency response, and the shut .47uF will 
roll off the high frequencies, thereby reducing the 'click'. It's still 
not perfect, but it's a lot better than it was. You may have to play 
with these values depending upon the characteristics, and impedance of 
your headphones.

Of course a much easier solution is to just use a pair of cheap 
headphones, similar to the ones that come with a portable cd player, 
(especially since you're ruining the response anyway by inserting the 
caps and resistor) but in my case the comfort level of the Beyerdynamics 
is enough of a reason to go through the little bit of extra trouble.

73, Dale / WA8SRA


Jimmy Lee wrote:

>When I use headphones I have a" thumping" sound everytime I key the rig.
>(seems worse on wide range headphones, lots of bass response really
>shows this "thumping" sound)  Is this common and what is the fix?
>Thanks,
>Jimmy, AE4DT
>K2  #2832
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