The plugs are silver plated, and silver turns black when it oxidises, so I think that is what you are seeing here rather than ‘burn marks’. Compared to other metal oxides this has a minimal affect on conductivity so it tends to look worse than it really is, although I’m sure a clean would still be a good idea.
I am curious guys. Does the silver oxidises result in severe distortion where the volume becomes lower music and hissing. Is this one of the symptom?
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That seems very unlikely to me. I would just clean it off with some fine wire wool but I can’t see it being the cause of any major problems unless it’s a really thick crust of oxide, which doesn’t seem to be the case looking at your photo.
Still worth giving the sockets a clean though.
Tks! While I dun have the alcohol to clean for now, the NAC A5 cable arrived this week, so I will use that as a safe benchmark to determine everything is fine first.
I found a crochet hook quite helpful if this happens.
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Purely as a technicality, it is more likely to be sulphide rather than oxide.
Today the A5 cable came! However the distortion is still there.
Will ask dealer for a NAP 250 and WiiM streamer to test amp and source later the week.
The 500s are giving you quite a handful , that’s sad…perhaps you should think of alternatives. Gives you more issues than enjoyment . 
Quite interesting the distortion. Unable to express how perplexed I am. While I am still patience I will just let Naim debug it in the coming weeks. I do have lots of problem with Naim indeed haha…
History of problem:
- NDX2 faceplate QC problem (replaced)
- ND555 screen white blob (replaced) => See: Got my ND 555, but screen got blob - #3
- NAC 552 rear indicator stick out (replaced) => See: 552 indicator light can come out?
- NAC 552 Burndy Cable faulty make my woofer dance (replaced) => See: Hissing on my left speaker
Each time Naim replaced outright all my equipments and support is outstanding. Hopefully my luck can improved.
So total replacement units = 4 times (Wow)
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Is this the same pre-amp you had with your Dynaudio SP 40? You shared the short video on YT for the fault on the same channel?https://youtube.com/shorts/DbBsaJ4SwSc?feature=shared
I share your pain; the CB NAIT 2 that I use in my bedroom and had fully rebuilt by Naim about 12 years or so ago has developed a distortion on the left channel. Time for a return trip to the mother ship and a service I think…
Nope. That 552 preamp has been replaced by Naim with a brand new set.
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Indeed I can now only wait. Problem is there is a change of dealership so I am relying now on unfamiliar personnel and it depended on their service level now.
Also just for context the ND555 and 552 bought in 2021 and NAP500DR bought in Nov 2023 so fairly new and recent
Btw what is Naim warranty for amplifier? 1 year? I am in non-UK.
I see here Extended Warranty - Naim | Naim Audio
For UK it has +3 year extended and result in 5-year, so my guess is all 500-series is a 2 years warranty which made my 500DR still within (whew). I hope is not 1 year only.
It is likely 1 year if your country doesn’t have special warranty requirements.
The Warranty is made between you the customer and the dealer, and the terms and period are down to the distributor of your country. It’s underwritten by Naim though for at least a year, so in theory if anything goes wrong within that time and the distributor goes under then as long as you can get the unit to the Naim factory they will likely fix it at no charge, save the costs of getting it back to you. Unfortunately if bought in 2023 then it’s outside that timeframe.