I just received my Hart Audio balanced cable ($67 US) for my Audio Technica R50x. It has a 4.4 pentagon at one end and what appears to be a custom AT locking plug in at the other end. My cable is 3 feet long and I obviously got it in red. I had previously read that a M40X balanced cable would work, but all I got was monophonic out of it. Now that I have the balanced cable from Hart Audio the bass and overall definition is much improved over the stock 3.5 unbalance cable. Well worth the money to me.