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Strain reliefs are also different. Black strain reliefs were shortened when I recabled but you can still see the difference



The TDS-15 react very well to damping density changes. Their FR kind of reminds me of the Stax L500 (except for upper treble of course), here is the current lighter damping on the gray TDS-15 vs my L500. The Lambdas always have a bad seal on my rig, so the midbass bump and deep bass rolloff are exaggerated in the chart.

Spoiler: measurements vs L500

 

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I bought a lot of different models of headphones - ortho, izo, electrostats. of dynamic models, I was surprised only Sony R10. I noticed that many models had the same markings, but had differences because they were made in different factories. For example - English wharfedale ID2, Stax 007A>mk2, Japanese Yamaha HP-1 and YH-1. that's why I wrote that TДС- 7, 15, 16 were produced in different cities of the former USSR.
Аlas, I don’t do this now, you can only buy from good dealers who understand the difference between dynamic and ortho-iso

Caught another specimen, and it too has a difference.....
https://www.head-fi.org/classifieds/tds-15-isodinamics-rare.65771/