Acupwr also makes step down transformers that only effect the voltage - frequency remains unchanged. I have confirmed that all of my gear will be fine on 50hz, so am only considering these units.Great! I see no reason to be concerned about functionality issues or damage occurring with the step-down transformers that don’t purport to convert frequency. I don’t have any feel, though, for whether or not they would compromise sonics to any degree. Although I’d expect the degree of any such compromise, if present at all, to be system dependent.
FWIW I once had occasion to use a much less expensive 120V to 100V step-down transformer I purchased from www.voltageconverters.com, in a relatively non-critical application in a second system. It worked fine in that application, with no perceivable sonic side-effects.
Inna, while as you say USA voltages can vary significantly, and are considered to be in spec as long as they are between 114 and 126 volts (i.e., 120V +/- 5%), my impression is that the 60 Hz frequency is very tightly controlled. And things I’ve read in the past have indicated that typically it is monitored such that if synchronous clocks accumulate very small errors over a period of say a few hours (perhaps even well under a second of error, although I’m not sure of the exact amount), the frequency is adjusted very slightly to gradually bring them back to the correct time.
Regarding turntables having synchronous motors, if a particular model does not provide a means by which it can be adjusted for 50 Hz operation, then, yes, I’d expect it to run slow.
Best regards,
-- Al