Generally, there are 2 types of stepdown transformers. Non-isolated, autotransformer and isolated like the EI or torrodial.
From what little I know, 2 of the short comings of an auto transformer are,
- failure mode. Generally they will fail open circuit, this will (can?) cause the mains voltage to be presented to your audio device, generally means toast.
fyi, an auto transformer is a bit like a trimmer or volume pot except the wiper is in a fixed position. Variacs are auto transformers with a variable wiper.
- noise is the second issue, keep in mind some people spend money on power conditioners to reduce noise.
I recently built my own step down, after all it's just a transformer with a primary winding that must have the same rating as your mains supply. The
secondary winding must match the voltage of your audio device, eg, Primary=220Vac, Secondary=110Vac. The third part of the equation is the power
rating of the transformer, the so called VA (Volt Amps) rating. This is APPROXIMATELY equal to the number of watts of your device. A better
approximation is WATTS = V x A x 0.8 The 0.8 being called a power factor (I think) since voltage and current aren't in exact phase. Add fuses, a switch
power cord and a box and job done. I would advise against building your own unless you have very high end needs. It's cheaper to buy ready made.
Also, transformers are rated for a particular VA, so if you buy a 220Vac->110Vac 1000VA transformer, the secondary voltage will be 110Vac when
you are drawing 1000VA, which will be never. An amp rated at 250W (consumption) will spend 99.9% of its time consuming less than 100W, unless
you dislike your neighbours, so don't over do the VA rating. Maybe someone can convince me why larger VA is required, "on-demand" power, what ever
that is...
Some equipment are switchable/configurable to different mains voltage (voltage selector at rear). Also some units can be easily rewired if they
have multi-voltage primarys. Suggest open the unit up and take 2-3 hi-res photos of the transformer and immediate surrounds, however most likely
that USA models are hard wired to 110Vac, so stepdown required.