Ground Loops

[digitalmod]

   I recently tried to tie two mixing boards into a SSB transmitter. I needed to use at least 30 db attenuation before putting the baseband into the mike jack. Well, the mixing boards available Radio Shack and Yamaha, both have power supplies from line current through a sealed power box.
Using DLSR it creates a residual hum about 20 db below peak baseband. I tried to isolate the issue, and discovered something I don't quite grasp yet. Using audio directly from a CD player or a Gemini Card recorder, does the trick. The hum disappears.
I can only assume its caused by ground loops. two or more grounds.. So the short term solution be I suppose, using a isolation transformer for audio ( a rare bird to find these days).
Anyone else have this pesky issue?

Jay

[digitalmod]

  I find I can get 1 to 1 Transformers for audio from a number of electronics outlets. They are for most part dirt cheap, less than 5 dollars. I believe this will allow feeding through a mixer without hum and other noise disturbances.

And I actually got ten 1:1 transformers (very small like you may recall from a 1950's transistor radio)
Using it the loop was reduced and noise went down a great deal. This resulted in far less audio distortion than previously was obtainable.