I'm not sure they are even made anymore. They were tightly controlled from Japan and very pricey. I bought bootlegged versions in lower manhattan for around $300-400 as I recall including their special driving amplifier. They also had portable versions - I bought one for my son. This was all long ago. Say, 1970-ish. They are full range electrostatic earphones -= full meaning below 20 hz. (could be to DC, but I doubt it. ) - Dick