There are a few other ones out there, but I'll just leave that open for you to explore.We're on a side topic, but for driving a balanced line with tubes there's really only two ways to do it. You either use a transformer or you use a Circlotron with a DC feedback servo (which the technique used in my preamp). Both float WRT to ground and both generate the pin 2 signal WRT pin 3 and vice versa.