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Original Message
There is a simpler way to do this.
Posted by Ralph on December 9, 2015 at 13:20:29:
You can can simply use a single-ended circuit as the input voltage amplifier.
Since you are are not going to be running loop feedback, you could drive the cathode of the tube in addition to driving its grid.
Its not balanced (the cathode input being fairly low impedance), but it is differential in every aspect of the word. So it will accept and process a balanced input.
In order to drive the cathode, you would use an electrolytic capacitor of exactly the value one would use for cathode bypass. You would tie it to pin 3 of the XLR, and the grid to pin 2 of the XLR. If you run single-ended, you would put a jumper between pin 3 and pin 1 of the XLR. Or just let it hang.
Don't laugh till you try it- this was how the original opamps designed by George Philbrick worked way back in the late 1940s. I've been using the technique for years.