Yamaha Orthodynamic Line Overview
Based on extant Yamaha brochures, Yamaha's Orthodynamic line of headphones appeared in the US market in the fall of 1976, consisting of the HP-1, HP-2 and HP-3. The HP-1 at first sold for the then-breathtaking price of $200.
All -1 and -2 Orthodynamics have a Mario Bellini headset design, revolutionary for its day and very comfortable. All Orthodynamics are supra-aural with the exception of the extremely rare YH-5M, which is interaural.
It perhaps goes without saying that all Yamaha capital-O Orthodynamics have small-o orthodynamic drivers in them. Other common names for magnetic drive of a flat plastic-film diaphragm are isodynamic, planar-magnetic and magnetostatic.
On the left you can see a diagram of a YH-1 driver that helps explain how the orthodynamic principle works.
There have been two revisions of the Orthodynamic line, a minor one in 1978, when the HP series was replaced by the near-identical but cheaper YH series, and a major one in 1985 when the compact YHD series was introduced. With the discontinuance of the YHDs after the 1989 model year came the end of Yamaha's 14-year romance with planar-magnetics.
Since Yamaha's usually-complete online archives hold almost nothing about their most famous headphones (a few studio photos of the HP-1 is all you'll find), what follows is only an approximate chronology derived from ads, brochures, printed reviews and owner-remembered dates:
1975 - Yamaha HP-1, HP-2
1976-? - Yamaha HP-3
1978 - Yamaha YH-1000, HP-1000, YH-1, YH-2 , YH-3 and YH-5M (Japan only)(?)
1979 - Yamaha YH-100
1980-? - Yamaha HP-50, HP-50S, HP-50A, HP-50M
1985 - Yamaha YHD-1, YHD-2, YHD-3
1985-? - Yamaha YHE-50A, YHE-50S
Some flagship models existed outside any of the normal lines. An example of this is the YH-100, a cost-reduced replacement for the very expensive YH-1000 which, like the YH-1000, had unique styling and electromechanical improvements that set it apart from the other YH models. Other Orthodynamics were introduced outside the normal model lines from time to time but, barring the appearance of an old Yamaha employee to confirm, there isn't yet enough information to slot them in accurately on the Orthodynamic timeline: Yamaha YHE-50S, HP-50A, HP-50S and the mysterious HP-1a which appears in a brochure with an HP-2 but hasn't appeared in real life. These models are marked in gray in the chronology.
Below is a scan from 1989 end of year issue of Audio magazine with the pricing information for the YHD line highlighted, right before it was no more. Note that the impedance listed for the YHD-1 is incorrect. The YHD-1 owner's manual lists it as 125 ohms, not 45. The 125 ohms spec has been verified by direct measurement of a YHD-1 owned by HF member wualta.
Yamaha Orthodynamic Line Models of Interest
Brochures and Various Pictures
HP-1 and YH-1. Photo by Wualta
Yamaha YH-100. Photo by Wualta