[Elecraft] Turn on turn off transients

Randy Farmer w8fn at tx.rr.com
Tue May 1 21:47:30 EDT 2012


My K3 also "thumps" with power on and off transitions. In my case, I use 
the SPKRS output to drive either a set of headphones (Yamaha CM-500 or 
Sony MDR-7650) or the inputs of an old Radio Shack SA-155 amplifier, 
depending on whether the headphones are plugged into my headphone box. 
Looking at the schematic, it's no mystery what causes the thump. It's 
the classic effect of having large value electrolytic coupling 
capacitors at the output of the audio amp. The increase in value of the 
coupling caps to extend LF response that was made fairly early in the 
product cycle would certainly exacerbate this problem. My K3 has the 
larger caps installed.

I wonder if the presence or lack of a transient pulse in individual 
situations could be a function of the DC resistance (or LF response) of 
the transducer connected to the audio output. In my case, the load 
impedance presented to the output of the amplifier is considerably 
higher than it would be if I used an 8 Ohm speaker. Higher load 
resistance gives a larger RC time constant, hence a longer and more 
objectionable pulse. Just a thought...

73...
Randy, W8FN

>>> Is there any way to stop the tooth rattling turn on and turn off thump
>>> from the K3? It can't be good to have a blast of DC go through my external
>>> speakers I jump every time this happens.
>>>
>>> K6CG


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