Low frequency hum - what is 'normal'?

This is a follow up from a related previous post:

In trying to get to the bottom of this I’ve been experimenting to try and better understand the problem or even whether I actually have a problem. I was assuming I had a ground loop as my AV system is on a different power circuit to my Hi-Fi so I took out my Oppo 203 BD player and played it directly through my NAC 552DR / NAP 500DR (rather than pre-amplifier output from my Arcam AV Amplifier and Uniti Gain enabled on the NAC 552). I also have a dCS Rossini Apex Player. Neither input is silent with the volume on the NAC 552 turned up to maximum whether On, in Standby or Off although the Oppo is clearly louder than the dCS particularly if in Standby or Unplugged (there is no discrete Off for the Oppo). All equipment is earthed and plugged into the same power block and I have swapped interconnects, power leads and sockets, and have tried all combinations of RCA and DIN.

What seems stranger is that if I select the dCS input it is noisier if the Oppo is connected and the Oppo input is a lot noisier when the Rossini is connected. I have an old Denon Mini system with two RCA inputs and it behaves in essentially the same way. I also have a Uniti Atom with a single RCA input with the other input being an optical digital input from a TV which behaves the same except there is no difference when both inputs are connected compared to just RCA. In case there is a problem with either the dCS or the Oppo I have tried a third source individually and in combination with the dCS and in combination with the Oppo with the same results.

Is this all ‘normal’ or at least within normal limits?

:flushed: :astonished: :thinking:

Checking - Preamp volume set to maximum - ??

If true, its probably beyond most people’s experience. It certainly is mine

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Obviously without playing any music or other media!

In case this information is illuminating I should add that the low frequency noise when the Oppo input is selected with the player either in Standby or Off is initially barely audible and builds up over a couple of minutes. Using the third source (a Pioneer DVD player connected via RCA) the noise also builds up but much quicker, perhaps taking 20 seconds to reach maximum volume.

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