So for those who don't know, the NAD RP18 is an OEM of Fostex's original large driver ortho made in 1975, the T50v1. Yes, this headphone was made in 1975!!! It has a large driver of similar size to the LCD-2, is open back. There are 2 versions that were made for NAD, one with a mylar diaphragm and one with a kapton diaphragm. This is the kapton version, which in stock form puts out more bass than the mylar version, but which is controlled through damping. Both versions can be very neutral from bass to midrange with the right damping. http://wiki.faust3d.com/wiki/index.php?title=NAD_RP18
Guru has some good links too which have some good info posted by Wualta.
I've been wanting to see measurements of these for a long time (as have many ortho-heads), and I hope this is a testament to how far we haven't
come with ortho technology. This is what Fostex was able to do nearly 40 years ago when they tried to make the best ortho they could, targeted at the audiophile market. Goes to show what should be possible with today's advancements in materials and manufacturing. Think about where electrodynamics were in 1975, and how much improvement we've seen over the last 40 years. Compare an HD800 to the 70's electrodynamics. Fostex never updated this design, it was (AFAIK) their first attempt, and their only attempt at an audiophile ortho. Ever since then they have focused on cheaper models, which were all products for a target price and target market with limited, specific design goals. Today's T50rp is not an "all out" design either, it's made specifically for the prosumer market and isn't intended to sound good. Let's compare to Sennheiser's products- if this was Fostex's "HD800" type product (statment, all out design) in 1975, today's T50rp would be like Sennheiser's HD280 or something. The other neo orthos are good, but they could be much better. I would love to see what Fostex could do today if they tried to make something that sounded good. I honestly think they could rival the best electrostats and electrodynamics (the "super" LCD-3's probably give us a taste of what is possible). Though I'm sure I couldn't afford it with today's pricing and the way Fostex prices their TH series.