[TenTec] Fw: question Digital matching-- rig to sound card

[Ken Brown]

Even when all systems are properly connected to the same ground buss, 
there can be slight potential differences caused by magnetic fields from 
transformers, and in RF transmitting systems by electromagnetic fields.

In the days of phonographs, there used to be a separate ground wire from 
the turntable to the preamp/amplifier/receiver (whatever it was that had 
the phono inputs). This was not because the turntable had a two prong AC 
power plug without a ground pin. The turntable was generally powered 
from a AC outlet on the preamp/amp/receiver which was polarized (one 
blade wider than the other so there is only one way to plug it in). The 
separate ground wire is there to let any 60 Hz currents caused by small 
potential differences to flow on that wire instead of on the phono 
signal wire shields. In addition, a few ohms of resistance (maybe 10 or 
100 ohms) was sometimes placed between the phono input shield and 
ground, so that the current division between the dedicated ground wire 
and the shield would be even greater. If the shield conductor resistance 
was equal to the ground wire resistance the current would divide 
equally. By adding resistance to the shield, the current would divide by 
a much higher factor leaving a much smaller fraction of the unwanted  
60  Hz current flowing on the shield, and therefore much less 60 Hz hum 
induced into the phono signal wire. The additional resistance has 
negligible effect on the desired signal level in a many k ohm impedance 
system, and made a big difference in the unwanted 60 Hz hum.

A transformer can accomplish the same, as well as giving some more 
versatility, allowing balanced and unbalanced sources and loads to be 
connected.

DE N6KB
> And why does one need a transformer for ground isolation?.  All of the 
> inputs and outputs, computer and radio, are unbalanced to ground.  Perhaps 
> it is due to the systems having two different grounds, thus grounds of 
> different potentials.   That being the case then one doesn't have a real 
> zero potential ground.   That is the real problem.  The addition of a 
> transformer is simply a poor bandage covering a wound that will never heal.
>
> Look at your power distribution and wiring.  Specifically the neutral and 
> 3rd pin ground.  Also, if you use a separate "station ground" is it bonded 
> back to the AC mains ground?  It should be, otherwise problems will abound.
>   
>