I have an Uniti Atom, 230V.
I want to use it with a step-up transfomer that is 500watt.
When I measured the 220V output section of the step-up transformer with a multimeter, it says 250V.
Will Atom explode 
if I plug it into this 250V output?
I am in the US.
230 Volts is the nominal figure in the UK, but the legislation allows it to be up to 10% higher, which would be a little over 250 Volts. So that in itself isn’t a problem, although for optimum performance the lower end of the permitted range is said it be preferable.
Not too sure about a plug-in transformer. It may affect performance. I believe Naim can quite easily modify their equipment for use in US/UK mains.
To be more precise: it does start with 250V for like less than a second and drops to 246/247 and fluctuates there. But I still should think as 250V. I know there’s some tolerance but I don’t know if Naim Atom has 10%+ tolerance. It never went above 250V.
As ChrisSU indicated, in the EU, which included the UK, in 1995 single phase voltage levels were unified at 230V with a statutory tolerance of -6% to +10%, giving a voltage spread of 216 V to 253 V. Nothing has changed with Brexit. For any electrical equipment sold in UK/EU to be fit for purpose it has to be capable of operating satisfactorily and safely at any supply voltage in that range, and that of course includes all hifi equipment, Atom included.
My Mains is currently around 246V, but for 20 years it was over 255V due to a mistake by the electrical company having the wrong tapping back at the substation. I never had any issues back then though other than the infamous Toroidal transformer hum is now slightly quieter.
As been discussed many times before. Naim transformers have the ability to run both voltages. Its just a matter off removing one winding from the transformer.
Obviously this needs doing by someone that knows what they are doing, but its a very easy and quick thing to do.
This would be a much better solution.
Assuming by “both voltages” you mean changing between nominal 230V and nominal 110V (as opposed to, say, 230V and 250V), I don’t know about Naim transformers but the norm is two primary windings that are connected in series for 230V and changed to parallel for 110V. Yes, electrically a very simple change - some manufacturers even wire in a way that repositioning a jumper or a switch changes between, but possibly not ideal for current flow/sound quality.
Yes, 240 or 110.
As you say above, but naim doesn’t have a switch. You need to unsolder the 2 wires and make safe
The OP is in USA, USA is 60Hz
A 230/50 transformer running on 230/250 at 60Hz is not a good idea.
A 230/50 traffo can work on 60Hz but it needs 20% (230*50/60=191) lower voltage
If the step-up traffo has a setting close to 190 or 200v, go for that. If not, you need another plan
Second point, a 220v transform may well show 250v with an open circuit, it needs a small stablising load to show actual volts.
This should be undertaken by Naim or the Naim distributor, otherwise it may well not be safe and any warranty voided. It’s not just a re-wire either. It will need a replacement fuse of a different value, a replacement mains lead, and labelling will need to be changed to reflect the new input voltage.
Some can be re-wired, others not. I can’t recall re. the Atom, but I’m sure @NeilS can advise.
You got me confused when I decided to go ahead and plug it after reading the comments above.
But if you rewire the transformer, does it automatically detects the voltage and changes the Hz accordingly? Or regardless the voltage, transformer works both 50Hz/60Hz?
Yes, direct voltage like you say. Maybe it will drop to 240V with a load.
I will use it with a transformer temporarily.
Transformer changes voltage only, it may work with both 50Hz and 60Hz, but it cannot change it.
Best advice is from Richard, take it to a dealer who will be best equipped to advise what is possible.
All good. No issues whatsoever after plugging it. Thanks for the replies.
P.S. Although rewiring is fairly easy for people who know what they’re doing, it’s not easy for everyone. I know someone who did it in 30 minutes from start to finish. I wish Naim would make it easier to switch to 120V/230V like some manufacturers do.
Yes, the Atom has two primary windings - series for 230V & parallel for 115V. Happy with 50 or 60Hz.
Conversion between the two is simple enough hardware wise, but the unit will need reconfiguring in the firmware to suit.
I think I’m right in saying that currently, this can only be done at the factory. 
Regards
Neil.
I am curious. Why not make it easy to work with both voltages? Just a switch covered with a transparent plastic with screw so not easy to accidentally change the voltage.
Because a mechanical switch fitted directly in-line in a power supply or signal path will significantly degrade the sound.
Consider a NAC42 for example, with a single input hard wired to the input buffer (instead of through the select switch) it would sound far better. Hence why all Naim preamps now have relay controlled inputs. Mechanical switch contacts become tarnished & dirty over time. The cleanest signal path is obvious - the same can be said for the path of AC/DC power.
Regards
Neil.
Edit: This is also the reason that unswitched power outlets are so fondly spoken of.
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