Hum while running Bottlehead Crack + Schiit Lyr 3 off Bifrost 2

[wolfhill138]

I built a Bottlehead Crack w/Speedball and it has been awesome for two years. I have run it PC > Schiit Bifrost 2 XLR > Lokius RCA > BHC while simultaneously running another amp off the Bifrost RCA outputs. No problems with hum when running Schiit Asgard or Schiit Vali 2++ alongside the Crack.

I recently tried a Schiit Lyr 3 in this configuration, and the BHC hums loudly through the headphones even when the Lyr is switched off. Unplugging the Lyr power cable or RCA cables stops the hum immediately.

I have tried replacing power and RCA cables, physically moving the amps around, and it makes no difference.  I tried replacing the Crack in the chain with the Vali and Asgard and get no hum from them.

I'm only using one amp at a time of course, but would like to be able to keep them plugged in. Any thoughts as to the source of the hum? Seems weird that it happens even when the Lyr 3 is turned off (but plugged in) - Thanks!

[Doc B.]

That's a ground loop being created by adding the Lyr3. It sounds like a cheater plug on the Lyr power cord could solve the problem.