[Elecraft] K3 Psk audio connections

[Don Wilhelm]

Fred,

We *were* talking about the K3 LINE IN (or so I thought).  That uses a 
stereo jack, but is only connected to the tip.  For those who need to 
convince themselves, plug a bare 3.5mm stereo plug into the LINE IN jack 
and measure the resistance between the ring and ground - you will find 
it is open circuit (nothing connected, and not grounded).  The tip, 
however has about 640 ohms resistance to ground - that says that one can 
use an off-the-shelf stereo cable between the computer line out and the 
K3 line end without concern for either the computer nor the K3.

As for the K3 LINE OUT, that is stereo.  The main receiver connects to 
the tip (left channel) and the subRX connects to the ring (right 
channel).  If the sub is not installed, there may be some noise on the 
right channel, but no sound.  The transformer coupling is assurance that 
no damage will occur if either of the line out channels is shorted to 
ground.  So if one uses a mono cable here, it will cause no damage, but 
nothing will be heard in the right channel of the soundcard.

Most computer soundcards will not be damaged by plugging a mono jack or 
a stereo jack with the ring grounded into the mic connector.  Most 
soundcards use the ring to provide bias to the electret elements found 
in many computer mics, so they do apply a voltage to the ring, but it is 
through a resistor of a value high enough that shorting the ring of the 
soundcard mic connector should not cause any damage, but then I do not 
know the particulars of all soundcards - if you are concerned, consult 
the soundcard manufacturer (if you can!).

73,
Don W3FPR

Cady, Fred wrote:
> OK, guys, here is the real skinny, from the schematics:
> As shown on the K3 Block Diagram (6/6/07), The DSP RX section provides a
> two channel AUX AF Out to the KIO3 board. This stereo signal is
> transformer coupled (see KIO3 Main - Sheet 3) to the Audio IO Board. The
> KIO3 - Audio Board does indeed show a stereo LINE OUT with the
> LINEJACK_R on the ring and LINEJACK_L on the tip. The input side of the
> audio transformers on the KIO3 board have series 600 ohm resistors. Thus
> it is possible to short the ring to the ground (which worst case could
> happen even by using a stereo cable between the K3 and a mono Line In or
> Mic In on a computer, depending on the computer input circuitry and if
> the computer and K3 grounds are connected). But, due to the transformer
> coupling in the KIO3, you won't hurt the K3. (Probably) :-)
>
> Cheers,
> Fred
>
>
> Fred Cady
> fcady at ieee dot org  
>   
>