After a lot of research, I ordered a pair of Dan Clark aura phones to replace my Focal Elegias.
Plugging them into my Supernait 3, the sound was really rather good. These are closed back, intended not to disturb others in the room. Imagine my surprise when my wife told me she could hear music from my speakers, not loud, but discernible
Tried 3 pairs of old phones, no problem, swapped the adapter for another, same effect. Wiggled the plug, same thing. Any ideas? Think they are going back though
Hi @Mikee
Something not right there the Jack should cut the circuit to the speakers.
Might be some strange wiring incompatibility.
Hope you get it sorted. 
Thanks, definitely a bit strange. Must be some interaction between the phones and the Supernait. Just the new ones though.
I agree with @Skeptikal. When I had a SN3, the speakers always cut out when I used the headphones. Is the 1/4 inch male connector ok?
Yes, it’s fine, have swapped it out for another but still sound comes from speakers
I was intrigued by this. I’m not sure how the speaker switching mechanism works to mute the speakers. If it’s an electrical type switching (as opposed to mechanical) then all I can think of is that the impedance of the 'phones is such that it’s failing to properly activate the switching mechanism and this not fully muting the output to the speakers. Or else despite the 'phones sounding good they are just faulty in some way. It’s bizzare - things like this can be a real pain. Send them back and either try another pair the same (assuming your pair was faulty) or go for an alternative.
Normally my experience is there is a physical mechanic action inside the headphone hack itself there are some springy contacts which disconnects the signal to the speakers when a jack is inserted and reconnects when removed it’s possibly there is a fault with these in the headphone socket. A bit of contact cleaner on the jack and some inserting and pulling out might sort it. If not having replacement headphone socket should not be expensive it’s likely a sub £5 component.
Google “6.35mm switched TRS socket pcb mount”
What adapter? (3.5 to 6.25mm jack, or something else? Is the shaft of the adapter fully the same diameter as the successfully operating plugs? Or are there other dimensional differences with the shafts?
It may be worth contacting Naim support, because, as has been mentioned , the cause may depend on the actual switching mechanism, commonly a physical contact within the jack socket on the amp, operated by the jack plug pushing in, but potentially there could be some other changeover mechanism within the amp.
Was the 3.5 to 1/4 inch adapter. I tried 3 in the end.
All with existing headphones (Focal elegia, Sennheiser HD 650 and Oppo PM3) work fine. All with the Dan Clark give the same problem.
I have sent them back now ( Amazon), lesson learned, try before I buy.
Although they sounded fine, they felt flimsy for the price and rather uncomfortable.
I wonder if the impedance at 13 ohm was the root cause?
Thanks for all the advice
@Mikee
Hi
I’m glad that’s passed for you I looked up to see if I could find anything but just this little online test.
Better late than never.
I’ve never swapped between phones and just stuck with one for consistency and always favoured Sennheiser.
Don’t use headphones now I’m frightened I miss something. 
That doesn’t sound an unusual impedance to me.
Edit: whilst I have come across headphones as low as 16Ω, and one I recently put on a shortlist to hear was 20Ω, making me think 13 was within normal range, apparently 16 is actually the bottom end of typical headphone impedances, so the above is wrong and it is an unusually low impedance, but whether significant is another question.
I”m assuming that as the SN3 headphone amp is a class A unit built into the amp, the headphone socket detects the presence of the headphone jack and uses a relay to mute the feed to the internal power amp for the speakers. Perhaps you are hearing a bit of capacitive coupling which is still allowing a small amount of signal to reach the power amp stage and come through, albeit faintly, through your speakers.
One for @NeilS
Correct - it actually uses the output from the preamp stage, together with some extra bits switched in to limit the available current & stop pops/bangs on headphone connection/disconnection.
13 ohms is a pretty difficult load for the circuit to drive & it probably won’t do it very well (the current is limited to 50mA).
Potentially, the input to the power amp is picking up the signal from the headphone output resistors when they are under heavy load.
I’ll have a look into this if/when I get the chance.
Regards
Neil.
Thanks Neil - interesting to know.
