[Elecraft] K3 - Noise over Computer Audio

[Jim - W6VAR]

I recently tried monitoring the audio of the K3 via the soundcard (usually I
had this muted). I get one noise when only an audio cable is connected
between the K3 and my laptop. This noise is mostly attenuated when I connect
the KUSB cable. However, as soon as either N1MM or HRD connects to the K3, I
get another set of pulsed noises (I think this is the polling of the K3
data). Each of these noises is eliminated if I connect a ground wire between
the K3 chassis and a metal chasis component of my laptop. (Samples below).

There is no difference if I change to a different laptop, operate the laptop
on AC or battery, change to another USB/COM converter or another audio
cable. 

Since grounding fixes this, it appears to me to be a ground loop. My DVM
shows no appreciable impedance between the K3 RS-232 GRND and chassis or the
computer USB ground and chassis (I know it only takes a minimal resistance
to create the potential for a ground loop).

What doesn't make sense to me is why a ground loop would even exist when
ONLY the audio cable is connected as the K3 has isolation transformers.

Any thoughts as to what may be wrong or how I can remedy this? This makes it
rather hard to run my audio through my PC either as a stand alone or when
using a logging program linked to the VFO. I'm also affraid that the errant
current may cause some damage to the K3 or my laptop.


Sound when only the K3 Audio out is connected to laptop Line-in - no serial
connection:
http://elecraft.365791.n2.nabble.com/file/n5865416/K3_Soundcard_Noise.wma
K3_Soundcard_Noise.wma 

Sound when Ham Radio Deluxe is connected to radio:
http://elecraft.365791.n2.nabble.com/file/n5865416/HRD_Polling.wma
HRD_Polling.wma 

Sound when N1MM is connected to radio:
http://elecraft.365791.n2.nabble.com/file/n5865416/N1MM_Polling.wma
N1MM_Polling.wma 

Thanks,

Jim
W6VAR


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