Is it save to convert XLR to RCA?

Yes Sam. Chasing a buzz.

The preamp need only be in the system and not turned on. Just the ic's hooked up to it.

I ran a wire from the chassis of the pre to the chassis of the amp and still a buzz.

The preamp need only be in the system and not turned on. Just the ic's hooked up to it.

What if the preamp is hooked up to the amp but not plugged into the AC outlet?

And what if the ic's are connected to the amp but left unconnected at the other end?

Sam -- good luck on your issue as well!

Best regards,
-- Al

With the pre's ic's hooked up and the unit unplugged the buzz still lives.

My experience with hooking up ic's to an amp and leaving them open on the other end is a very loud and seemingly dangerous noise through the speakers. I don't think I will try that.... Right Al?

My experience with hooking up ic's to an amp and leaving them open on the other end is a very loud and seemingly dangerous noise through the speakers.

Yes, if the RCA center pin comes in contact with, or perhaps even close to, a source of static electricity (such as your body), or anything that is at a different potential than the amp's ground. I should have thought of that when I posted.

In any event, you've established that the problem is not due to a ground loop, since it occurs when the amp is the only thing that is connected to AC power. So it would seem that the problem must be due either to EMI (electro-magnetic interference) pickup in the cabling and/or the preamp, from some nearby source of EMI, or to some defect in the input circuit of the amp. I found the Atlas manual online, and I see that it has a front panel switch to select among four possible inputs (balanced and unbalanced, direct and high-passed). I'm just speculating, but perhaps something is messed up in the switching circuit which causes the ground side of the RCA connectors to not be properly switched in.

If you haven't already, you might try switching among the various inputs, and/or unplugging the amp for a few minutes, to see if that clears out any improper states in the circuit that selects among the inputs. As the manual indicates, MAKE SURE THE AMP IS MUTED when the input selection is changed. Also, it might be worthwhile seeing if the same problem is present when you connect to the high-passed RCA input.

And if all of that leads nowhere, perhaps purchase an RCA-female to XLR-male adapter, that would allow you to try the XLR input on an experimental basis.

Beyond that, I'm pretty much out of ideas.

Best regards,
-- Al