Hum on Uniti Gain input on NAC 552DR following power cut

I use my Hi-Fi for the Left and Right channels of my Home Cinema. We had a power cut this morning. Obviously the equipment was on when this happened but I did switch everything off before power was restored. On powering back on there is now a hum on both channels on the AV input on my NAC 552 which is new. The input is set with Uniti Gain. While not loud it is easily noticeable and annoying. Muting the AV amp and changing the AV amp volume do nothing, muting the 552 takes it away, volume on the 552 does nothing (might be because of Uniti Gain???) but perhaps most relevant disconnecting the AV interconnect makes it louder. I’ve tried powering the 552 and 500 down a couple of times with no joy. It looks like the problem is with the input socket currently mapped to the AV button on the 552. I don’t think it’s an Earth loop unless a power cut can create one after six and a half years! Plus I’m using one of those filter thingmys (I can’t remember the exact name).

Would unplugging (rather than just turning off) the 552 and NAP 500 be helpful? What about turning Uniti gain off and back on again? I think I’m right that there’s a second socket on the 552 which can have Uniti gain enabled. I’m a bit nervous about playing around too much with Uniti gain as I gather mistakes resulting in very high volumes can have expensive consequences but am I safe to assume that in the absence of any changes to settings on the AV amp this shouldn’t be an issue? I would obviously do the diagnostic work with the AV amp switched off.

Thanks for any advice.

Inputs 4 and 5 are both Unity Gain capable, so you could try switching input. You may need to reassign the socketry if you have ever changed it from the default setup.

It’s odd that a hum should only now appear. It would suggest that something has changed somewhere.

Update – both Unity Gain input sockets behave in the same way. With Unity Gain ENabled the hum is present and louder if the interconnect is disconnected. With Unity Gain DISabled the hum is volume control dependent with an interconnect connected and absent if disconnected.

Hmmm… I wonder whether @NeilS has any ideas here.

Further update - there is also a quieter volume control dependent hum on the CD/streamer input. This goes away if the turntable is disconnected. With Unity Gain ENabled (either of the two sockets) disconnecting the CDP/streamer makes the buzz louder on the AV input which becomes louder still with all interconnects (CDP/streamer, TT, AV) disconnected. With Unity Gain DISabled there is no buzz/hum.

I’m assuming with Unity Gain ENabled there should be silence in the absence of a source?

What sources do you have connected to the 552? What if you just have the CD player (assume it’s Naim?) connected?

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I have three sources - a dCS Rossini Apex Player (CDP & streamer) with a separate clock, a Turntable with a Moon Phono stage and the AV input. The hum is present with all three disconnected but gets quieter when the dCS and AV input are connected, but significantly louder with the turntable connected (I made an error in my earlier post).

OK thanks. I’m a bit stumped. I wonder whether @NeilS has any ideas here.

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Removing this from the equation would be the first thing I would suggest.

Do you have any idea as to the cause of the power cut? Any storms around at the time?


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Thanks Richard and Neil. The hum gets louder with the filter thingmy removed. The filter been there for several years.

Don’t know what caused the quite widespread power cut but not a storm.

My dealer is bringing another NAC 552 tomorrow morning to try.

Not the NAC 552 or 552PS to blame. With just AV (Unity Gain ENabled) and Turntable the noise is really loud on the AV input. Still audible with Unity Gain DISabled but quieter. Adding the dCS CDP/streamer makes it significantly quieter.

Do you have a sky box near your system?
I did & this is what I experienced.
I tried unplugging things,
Soon as I unplugged the sky box it stopped.
I plugged the sky box in to my Puritan & it stopped.
Hope this helps.



Further to Mark’s post above, do yo have any ethernet-over-mains devices anywhere at all?

Thanks to Mark and Richard. My dealer had heard about the Sky box issue but not part of my problem although I do have Sky Q. I don’t have any ethernet over mains. The hum is barely audible on the AV (Unity Gain ENabled) and CD (full volume) inputs after a further visit from my dealer. Both my dealer and his colleague suggested that a tiny bit of noise on Naim equipment was essentially to be expected.

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