My NPX 300 is humming, also when it is in standby mode. I have tried to solve by adding a DC blocker (IFI DC blocker plus), but unfortunately this did not help.
Has anyone same experiences or does somebody know how this can be solved?
The humming noise is not audible at the listening positioning, but I am afraid that if I want to upgrade my system in the future with a second NPX 300 that the humming sound will be amplified such that it will be audible.
Its new… ask the Dealer who sold it to you. Please.
I did already ask my dealer. He lent me the IFI DC blocker plus, but this did not solve the issue. Other option could be to exchange my NPX 300 for another one, but I doubt if another NPX 300 will be completely silent.
That is what, IMO - your dealer should offer you… 
Why assume another NPX300 will be noisy…?
You may have read on here, about some Naim PS’s humming or buzzing.
But you won’t read about any of mine - because they don’t… 
Historically Naim units did have hum variation between them. I had a 250 with quite a loud hum replaced by my dealer with one with a less loud hum - in my house, the hum is very dependent on one’s electricity supply. Both 250s sounded the same in my dealers wrt hum.
I’m surprised it’s humming in standby mode though as my understanding was that the toroidal transformers that hum were not active in standby, but perhaps I’m mistaken in that.
.sjb
Have you been through the following suggestions:
My Supercap is more sensitive than the 300PS. At the start my dealer said exchanging the unit would be possible. Interestingly the neighbours upstairs are in the process of moving and not there at the moment. All my Naim PS boxes are silent now. Wonder what it was that was disturbing them. Also without the transformer going into saturation the system sounds quite a bit better as well
I’m wondering if the NPX300 is actually going into standby as the main linear power supply (toroidal transformer) will be off when it is. If the 350 is anything to by and it’s not documented in the manual, in Instant On mode, pressing the power button just turns off the logo - the amp is still on. Pressing the Power button for a couple of seconds puts the amp into standby (click of relay, main power supply turns off - no mechanical hum from unit). This doesn’t help the OP’s hum problem which sounds mains supply related, but may explain the ‘standby’ hum.
Interesting, I normally put the whole system (222/300.350’s) into standby by pressing the power button of the 222. With this all the logos are turned off.
As you suggest I will also press the power button of the 300 and see if the hum will disappear in standby mode.
Just tried, but if I press the power button of the 300 (when the whole system is in standby) the system is switched on again. So apparently my 300 hums also in standby position…
Anyway, I will check with my dealer again. He already suggested to lend his 300 demo version to me for a test.
Correct Thing To Try.
If its better, your NPX needs to be either fixed or replaced.
My mains supply causes my NPX300s and NAP350s to hum. Before using my NPX300 I got my dealer to replace the 350s. The replacements (brand new from Naim) also hummed just the same. My 333 and 332 also hummed. Adding the NPX300 to the 332 also hummed - obviously the 332 hum stopped due to no mains attached. When I bought my second NPX300 (for the 333) it also hummed.
I have been using my Puritan PSM156 since receiving it on Monday and all of the hum has gone! My conclusion is that the Naim products are very good but are obviously affected by the quality of the mains.
Not knowing what the actual problem was with my mains, I decided to not just go for a dc blocker but for a cleanser and better distribution block. The 156 is the one I chose and I am very impressed with it so far.
Your story worries me a little bit, because I recently ordered the 332 and 333 to replace my 222. I am not waiting for another humming box.
I use the Furutech e-TP609E power block. On my previous set, which included the NAP 300DR , I had no issues at all.
Fear not @Daan-1975 , if you have no issues now then the 300 series should not give any problems. Your mains obviously has less noise than mine. As another reassurance, I used to have an NDX2 and SN3, both of which hummed. The issue is with my mains supply and not the Naim equipment.
I recently added an ISOL-8 PowerLine Extreme to my system and the humming completely went away from all naim kit. SQ improved all across the board too.
So that makes two sensible options at reasonable (but by no means cheap) price, Puritan PSM156 and Isol-8 PowerLine extreme. The only reason I did not consider the Isol-8 is purely because of the length - too large to fit in my cupboard!
I’m sure the PSM156 is a fantastic choice, I’m thinking about moving the PowerLine Extreme to my secondary system later this year, and will strongly consider PSM156 as an upgrade for my main system, along with the ISOL-8 MiniSub Axis.
Recently fired up my 252(SC2) and 300(300PS). The boxes were as silent as could be. Perhaps a coincidence. The neighbours are back, but I switched on the Hicap2 which powers my headline2 amp and the power supplies are humming away again. Ok a few hours later they are quiet again.
Can another Naim PS affect the others at all?
It is a possibility. However, the excessive hum is usually caused by dc on the mains (perhaps other mains noise can also cause the hum) and it varies throughout the day. Not sure if it is possible for a Naim PS to cause this dc offset. Perhaps something of your neighbours is contaminating the mains, perhaps it is something in your home.
I was never able to determine what was causing the worst of the hum in my system but solved it with the PSM 156.
Perhaps they have just turned on their heated swimming pool, or Sauna. Worth making notes of times when it hums, and when it stops to look for patterns