I believe that the 580 & 600 cables are terminated with a mini-plug [1/8"], while the 650 cable is terminated with the standard 1/4" plug.
Sennheiser replacement cables
It's interesting that the Sennheiser HD580 and 600 use the same replacement cable, h-81435, while the 650 uses the h-92885. The 650 replacement cable is actually a bit less expensive than the other cable, and I've been told it's a better replacement cord for all 3 models. Any thoughts? Thanks