What is the best AC Voltage step down converter?

AC line voltage in Japan is 100V. Don't you mean the SACD player you're considering is 100V, not 220V ? The current requirement of an SACD player is not much, so any small 120V to 100V converter should work fine, without hum or buzz because of its diminutive size. Frequency would only be a problem with devices containing synchronous motors that depend on AC line frequency to maintain rotational speed.

Thanks Gbart..I misread the info on the site in Japan and was thinking European.

Japan uses 100V 60Hz/50Hz. All Amps/Cd players are designed for both 60Hz and 50Hz. You can change the voltage from 100V to USA 115V by moving the Jumper inside.
If you want use a transformer the best ones are
Nisso NDF-1500U or Sanyo both made in Japan.

Thanks Renjy651..checked out the Nisso and Sanyo online..very impressive..

Nissyo NDF-1500U It can be purchsed from a Japanese store in New York for $200. I recommend this one 1500 Watts. Sanyo is same in power but bigger in size.