Dean, that Tetra is a single ended circuit, because the input ground connects to the output ground and chassis. Balanced (differential) circuit has separate + leg and - leg that are both signal.
I was looking at this one:
http://www.tubecad.com/2007/09/blog0120.htmNo step up for MM, so cart floats with grids!
Only 1 cap in signal path, super simple. Is there really RIAA EQ in there?
Because a phono cart is a magnetic coil, AC voltage is seen at both ends, despite one of its pins traditionally called "ground." Normally one pin is attached to ground to make a single ended signal as in Tetra, but with a coil as signal source why not make a balanced preamp? I have balanced amps now so I am looking at converting my single ended sources to balanced.