Loud Electrical Hum from Sub Preout when units are in Standby!

[hardcore]

I have the same experience but a little different, with a 250 Pro CI (and same after upgrading to a 1000 Pro CI).

I used to have my subwoofer connected via RCA directly. This worked fine and had no hum when the Devialet was on or off. But I always felt like the subwoofer's RCA connection was introducing noise back into the Devialet and affecting sound quality.

So I recently got a subwoofer RCA-RCA isolation transformer, and connected it between the sub and Devialet's RCA out. Now, when the Devialet powers off, the subwoofer will hum. My subwoofer has a 2-pin power input and is un-grounded.

BTW, the isolation transformer (a Jensen Iso-Max) does work - I think the Devialet sounds better with it. I find the sound quality of the Devialet improves noticeably with clean power and ground.

This indicates to me that the Devialet disconnects the RCA ground when powered off. To me this is actually somewhat of a good thing, as it indicates the Devialet's RCA outputs are internally not directly ground coupled, or 'differential' as the previous link mentions.