From the Naim Amplifier manual:
A hi-fi system usually shares a mains circuit with other household equipment some of which can cause distortion of the mains waveform. This distortion can in turn lead to mechanical hum from mains transformers. Some Naim transformers are large in size, making them relatively sensitive to such distortion, and it may be necessary to take account of transformer hum when siting your equipment.
Transformer hum is not transmitted through the speakers and has no effect on the performance of the system; however, a separate mains circuit may reduce it. Such a circuit (ideally with a 30 or 45 Amp rating) will also generally improve system performance. Advice on the installation of a separate mains circuit should be sought from a qualified electrician.
On the question of Hum, Naim’s Electronic Design Director, Steve Sells commented as follows:
Hum is a caused by a phenomenon called magnetostriction and/or saturation.
(magnetostriction - As the iron core is magnetised it will physically change shape as the magnetic domains swap with the AC)
If the mains is excessively high the core will saturate and they can hum.
If the mains has a high ‘even-ordered distortion’ it essentially causes a DC off-set (e.g. a heater on half power - only uses half the mains). A DC offset causes saturation.
Using a lossy lower grade steel this can be minimised. We use a premium ‘gain orientated’ (magnetic domains) core. These are more efficient and give a lower impedance transformer, lowering conduction angle… which makes them sound so much better.
All our large toroidal transformers have their core go through a new annealing process. The core then has a primary winding wound. Then it is measured for noise before the secondary is wound and a further noise test.
It is possible to buy DC blocks for mains (big capacitors with diodes over them). I’ve not had too much success with these and find they slightly degrade SQ.
Isolation transformers can also work too.
There are a number of threads on the forum regarding this potential issue (“potential” because it will very much depend on the cleanliness of the mains where you connect the equipment). A summary of discussion of diagnosis and possible solutions from earlier forum threads has been put together by member GadgetMan and may provide useful further reading. It can be found here;