Two years ago at work, I saved those French brand LEM DR80CR headphones form the bin (I work at an airport).
After opening them up and checking the available space in the earcups, I decided to upgrade them for Hi-Fi. The original earcups contained crappy 8 ohm speakers, like the ones in cheap alarm clocks, impedance-balanced with a small transformer. (The primary purpose of the headphones was to listen to air traffic radio, not music).
Then, I bought Sennheiser HD205 headphones. They have a good quality sound with compact and comfortable earpads. (Unfortunately, I don’t have any photo of the teardown, but the build quality is good, lots of screws and superior moulded hard plastic). They’re more than enough for music listening. Any headphones above $200 are just a rip-off (unless they have other features than just music reproduction)
The LEM headphones have removable imitation leather earpads. Under them, I found an aluminium gird plate where the speakers were attached to. The gird was riveted to the earcups plastic.
First, I removed the speakers, the transformer and all the original parts, leaving only the wires running from one earcup to the other. I cut out an opening in the aluminium gird where I could fit the Sennheiser earcups. I had to cut off some plastic, but I could leave the screw wells and the leather/foam ear cushions plus the plastic assembly that is volume-matched with the speakers.
I made holes for screws in the aluminium gird, to be able to attach the Sennheiser speakers to. I had to make small pieces of soft plastic to link the speakers to the gird. They also act kind of springs and cancel vibrations.
I also left the Sennheiser wire and jack plug. I’m still looking for a nicer one, that would be spring-shaped with a fabric ext. insulation (if someone have brand names…).
Was it worth it?
Totally! The LEM headphones have a unique vintage look (I did the mod before the vintage-headphones-signed-by-famous-douchebags started to be sooo popular), combined with the great Sennheiser sound quality. Plus, they now have a greater noise isolation due to the double earpads (it’s almost dangerous to walk on the street, as I barely hear cars coming) and stay pretty comfortable to wear, even several hours in a row.
On a more electronics-focused side, there’s plenty of room in the earcups to fit batteries and circuits to make for example, a Bluetooth version.
In conclusion, except for the jack cable I still have to find, it’s a complete win!