The HD 5XX series of Sennheiser headphones has a cable lock mechanism which requires compatible cables. Here's how to remove the lock.
Sennheiser uses a "twist to lock" mechanism on their HD 5XX series of headphones. (This series includes the HD 559, HD 599, HD 560S, and other headphones with the same chassis.) This is a non-standard mechanism which requires compatible audio cables. These cables can be twisted when inserted to lock them into place, and twisted again to be unlocked for removal. In addition, these headphones have a 2.5 mm jack instead of the more common 3.5 mm jack. The jack is inside the left earcup.
Take a look at the 2.5 mm end (the end that plugs into your headphones) of your compatible cable. It has grooves cut into the end of its sheath which allows it to fit into the headphones and lock/unlock when twisted. The fact that the grooves are immediately below the plug means that there must be a complementary component inside the headphones, between the cable entry hole and the jack, whose protrustions precisely fit into the grooves. Luckily the cable lock component isn't built into the cup, which means it's removable.
I will update this post later to include pictures.
First, remove the left earpad. It has a ring on its underside which snaps in place beneath eight equally-spaced ridges on the earcup. More specifically, these ridges are on the outer edge of the cover that protects the driver. Simply pull the earpad away from the earcup, one part at a time, to release it from the ridges.
Next, remove the cover. It's held in place by three small screws around its perimeter. Be very careful while removing the cover! The driver is attached to the cover and there are wires connecting the driver to the earcup. I moved my headphones onto a cotton sheet for this step so that I could gently support the cover with a bundle of fabric.
The cable lock component is a shaped like a small plastic cylinder with two "wings" that are each held down by a single screw.
At least on my headphones, the cylinder had
< PCM > and
T1-4 written on it in raised text.
The component is placed between the cable entry hole and the jack.
The jack is hidden under a circuit board that transmits the left and right audio signals to the driver.
Aside: You might notice that there are four signal wires soldered to the circuit board: left channel audio (L+), inverted left channel audio (L-), right channel audio (R+), and inverted right channel audio (R-). This confirms the "rumour" that the headphones are capable of producing balanced output. It also explains why the Sennheiser locking connector uses a 2.5 mm TRRS plug.
On the HD 560S, which is the latest model in the series and the model that I own, the lock is glued in place. I used a flat screwdriver to pry it off, but I had to be careful because it was stuck very tightly. I started by lifting at the "wings" because they extend further than their threaded inserts. This released the bottom end of the component, so I started lifting from there until the top end also released.
This step is optional but recommended. Now that you've removed the cable lock component from the earcup, you will be able to insert any 2.5 mm plug into the jack that you want. However, the component does more than just lock cables. It also has a "ledge" at the bottom which reduces the size of the cable entry hole, helping to guide the plug and ensure that it fits snugly into the jack. I'm not sure, but it might also help support the jack and/or circuit board.
To disable the lock, you will have to remove the protrusions inside the cylinder. You can do this with a knife, metal file, or any other object that can smooth out the bumps. If you don't want to modify the original component, you can make a copy of it with a 3D printer. You can even try removing the protrusions from the scanned mesh before printing the copy. Afterwards, push the lock-less component back into place and screw it down.
Hooray, you've completed the mod! Test that the drivers work by plugging your audio cable into your headphones and your source. Try not to move the headphones if the cover is still open. Ensure that you can hear audio from both earcups. You can listen through the outsides of the earcups if your headphones are open-back.
If there are no issues, screw the cover back into place and reinstall the earpad. You have to press it into the cover until the ring has snapped into place under all the ridges. Finally, enjoy your modded headphones! If you still have the original cable lock mechanism, or a copy, you can reverse the mod whenever you'd like.