NAD RP18: Mylar "Bass Light" Version This is one of the highly sought after 1970s Fostex T50 variants, with the big 70mm isodynamic drivers. This particular pair is the mylar or "bass light" version, with a much higher diaphragm tension than the kapton diaphragm variants. However, this pair is particularly special, in that it has been modified (by yours truly) with a headband and earpads from the much more recent Fostex TH500RP. The earcups of these two headphones are the exact same size and the newer headband is far lighter and more comfortable -- making the idea of a transplant far too tantalizing to resist. Sorry Hans! Measurements: Raw FR (no smoothing): Distortion (R/L): CSD (R/L): Slight bass imbalance -- don't read too far into this as the left earpad needs to be re-adhesed to the baffle and I'm out of double sided tape 5kHz null is actually a peak in disguise Slightly higher distortion on left channel Very linear down to 10Hz Subjective impressions: Neutral, maybe very slightly lush Wonderful treble extension Fantastic clarity and speed Airy and spacious Bass goes very deep with slight roll off at the absolute extremes Very tight and natural sounding The ridge at 5kHz appears to be a measurement artifact, likely caused by the high diaphragm tension. Marvey refers to this phenomenon as "ortho wall" ringing, and similar behavior is witnessed on his measurements of the LCD-3 and some other high-tension planars. While I cannot guarantee this is what is going on here, I can say I strongly believe this is the case as I do not hear anything of the sort on my subjective tests. My measurement process: https://dl.dropboxusercontent.com/u/34404748/noDel/BL_F/meas/PROCESS.txt A big thanks (and apology ) to Hans for the TH500RP! Better photos will be added when I get the earpads attached correctly. ~W.