Strange buzz issue!


Hello audio friends! I’ve got a weird problem. I have an old ARC D-100B power amp (love the sound of it) and bought an ARC LS9 pre amp to match it with. Nice enough. Connected the LS9. Buzz/hum from both speakers. Consistent (clearly audible from listening position) at all volume levels/sources on the LS9. Crap, the LS9 must be faulty, I thought. For fun I connected the LS9 to two other power amps (Yamaha and Vincent). Dead silent. I then connected an old Yamaha pre amp to my D-100B. Silence. Same place, same cables. So, neither the LS9 nor the D-100B are faulty, but together they, or one of them, cause a pretty loud buzz. I’m going mad here! Any ideas? Thanks! 
teppas
If you used a ground isolator, the PS ground should fix the problem. It is the voltage offset between the two PS on the two pieces. What is common without a common ground, the audio circuit ground. You may have ran a wire but it was not to the right place, I'm thinkin..

A ground isolator in the signal path is the fundamentally correct way to eliminate system ground loop problems. ... For an unbalanced interface, the transformer effectively stops any current flow in the cable caused by ground voltage differences, which stops the noise.

I would look to the VAC ground between the two, just by moving alligator clips to different locations.  Might surprise you.
I'm also thinking eliminate the ground on one unit (via cheater plug). Then run the ground wire between the two and only use the ground on the other unit.. I expect that will do it... A little cap trickle.. Common single wire ground should do the trick..

If all else fails a GL eliminator, but it can KILL the dynamics of some systems.. Sometimes the signal won't activate a Sub when there is a GL eliminator in the circuit.. Dayton plate amps.. I know what your talking about, "impaired bass response". I have to use one on a cable box power supply.. It is the actual CC fix.. 

Just an old mechanic thinkin' out loud..

Regards
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Tanks guys for all the great advice! I’ll go ahead and try all of you suggestions! Tomorrow, I’ll be off to buy some alligator clips:) I’ll keep you posted. The buzz aside, I feel this is a combo with great potential, especially  for a fan of a bright, fast and detailed presentation, like myself. If I get it right, I plan to settle with the ARCs for my Yamaha NS1000Ms:) Thanks again!