Neruda: I believe the impedance was 120 ohms. The seem to be about as efficient as the HD600, as far as driving them i.e. portables can drive them to decent levels but they are not nearly as efficient as a Grado SR60.
Although I owned several pairs of phones before I bought these Yamahas in 1990, these were the first phone I truly fell in love with. Compared to my SR325 and HD600, they are somewhat cluttered but they do things in the lower midrange and bass that no other phone I have heard can do. I rarely use them but I did pull them out last week and gave 'em a couple of hours of time.
There is a very strange phenomenon with these phones. At first listening, they do not sound very good, no highs and metallic in the upper mids. But after about 3 or 4 hours, your ears make an adjustment and it smooths out and the highs make their appearance. This is not break-in, this is your ears/brain adjusting, because if you use other phones and go back to the Yamaha after a week or so, you have to re-adjust again. But if you simply go back the next day and resume listening, the sound is still there.
One of the best things about this phone was the openness of the sound. The middle things (like vocals,kick drum etc) would be in your head and the left/right information would seem like it was outside your ears and over the top of your head.
The drivers were a unique design. Two flat magnets with holes which had a flat voice coil (made of aluminumized mylar) in between.