Quiet hum on switch on NAP 250.2

I’m new to Naim so apologies if this is a stupid question. Wondered if anyone could answer this for me?

I have an NAP 250.2.

When I switch it on I get the pop sound through my speakers, which I know is normal. Then there is then a fairly quiet hum which sounds through the speakers. It builds a bit, then immediately disappears after almost exactly 30 seconds. If the unit is turned off and I wait a few minutes, the duration of the hum is a lot shorter. The noise never returns once switched on.

Is that normal, sounds amazing otherwise? I’m guessing it is the sound of something charging up perhaps?

This may well indicate that a service is due. How old is the 250 and when was it last serviced?

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It’s only 5 years old.

Interestingly, I get a constant ground hum when the pre amp is not connected to the 250. This sounds remarkably similar to the hum that appears on switch on with the pre amp connected, which then disappears.

It’s almost like something kicks in after 30 seconds of it being switched on whereby it grounds itself to the pre amp to stop that hum.

Yes, so the pre-amp should always be switched on first before the power amp (unless the power amp is one that powers the pre-amp).

When did the issue start occurring? Has anything changed recently? Is the DIN-XLR lead OK?

FWIW, I have never had any hum through the speakers at switch on with either my NAP250.2 or my NAP250DR. Perhaps @NeilS has an idea what’s a play here?

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DIN-XLR seems fine. The pre amp is an Nac-n 272. I get the same issue when swapping to a NAC-N-172.

My 250.2 just has mild transformer hum constantly but only from that unit and not the speakers. I use a 172 pre

Could it be a ground issue? What are your sources, and which have ground switch set to chassis?

so at the moment the only thing that is connected to the NAP250.2 is the Nac-n 272, which is set to chassis.

Hi David,

Just to be clear - are you switching the 272 on followed immediately by the 250? Or is the 272 switched on all the time?
There is a switch on delay in the 272 before the output is enabled - you can probably hear the relay click in a few seconds after it has booted up.


Hi Neil, yeh so I’m giving the 272 time to pass any delay before turning on the 250. Same thing happens even if the 272 has been on for ages.

In case it’s also useful, music will play through the 250 as soon as I hit the power on button if a suitable source is selected on the 272, but the hum like noise will still be there as before, then vanish.

Ok, thanks for clarifying.

Sounds like the input might be oscillating slightly, I haven’t seen this on a 250 before though, only 300s.
I take it it’s on both channels?


Yeh the same on both channels.

It definitely lasts longer when the amp has been off for a while vs if I wait a few minutes after turning it off - the duration of the hum is shorter.

I think it needs looking at - the usual fix is to replace the input matched transistor pair.
As you say, from cold the duration of oscillation is longer, then as they warm up it goes away.
I don’t think it will cause any damage to anything, just undesirable.



Many thanks Neil and others for your words of wisdom:)

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