[1000mp] Low audio to sound card

[Pete Smith]

Recently I set about preparing to record contests this season, and to clean 
up my configuration for using SBDVP with TRLog anfd My Mark 5.  I had some 
fairly serious ground loop hum last year, and so I wired a Radio Shack 
isolation transformer into the audio line between my TopTen DXD's headphone 
jack and the sound card.  I also put a couple of low value disc ceramic 
caps between the two audio lines and the common cable shield ground, which 
is grounded to the TopTen box chassis through the headphone plug.

To my surprise, when I finished and tried it out, the audio level through 
the transformer to the sound card input was so low that I was effectively 
unable to record.

I very much want to have the sound card hearing exactly what I am hearing 
from the SO2R box, which is why I do not use the 600 ohm external audio 
output from the Mark 5 to supply the audio for the sound card.  Without the 
iso transformer, the audio level is adequate but there is ground loop hum, 
particularly when I use the sound card to provide audio to my transceiver's 
mic jack.

Is the mismatch between the 4-8 ohm headphone output of the Mark 5 and the 
isolation transformer simply too large, and that is the source of the 
problem?  In that case, I could add a small 8:600 ohm transformer to 
provide a better match.  Or should I be looking somewhere else for the 
solution to the problem?

73, Pete N4ZR
Sometimes a tower is just a tower