Balanced in phono stages preamp?

That still doesn't explain how you can establish bias with 2 floating supplies like you claimed earlier. I say you can't.

I’ll say it again. The cathodes are tied to a common point; there must be a DC path from grid to cathode to establish bias so the grids must also be referenced to this point. The supplies can’t float.

As soon as you hook up these differential signals they are referenced to the common connection via the grid-cathode bias circuit.

I need to see a schematic of how you can accomplish what you claim before I believe it.

Don't get me wrong, I am intrigued by the idea of feeding my cartridge into a diff amp and have that on my list of things to try. I just don't think what you described is possible.

Hi Herman, the input of the preamp is differential so the cathodes are tied together. The cathodes are also tied to a 2 stage vacuum tube constant current source whose cathode is tied to a resistor that is tied to B- (-250V).

So my bias references are a set of resistors that go from B+ to B- and have a -1 volt output on each one. One side is adjustable to match to the other network. Although this is not how we normally do it, it works as well.

Interesting. So the voltage on the grids is slightly different and the coil of the cartridge is hooked across the grids? Doesn't this put some DC current through the coil?

Hi Herman, not if the voltages are kept the same. That's what the adjustment is for (in this demonstration case).

In our production preamps, the grid resistors are tied to ground and for the sake of simplicity, double as the input loading (however when other cartridge loading is used the single resistor for that purpose is tied directly across the output of the cartridge and has no connection to ground). We could have done a variety of scenarios here but I favor simplicity whenever possible.