How do the design a balanced phono stage without any solid state components in the signal path?
@jlbkmb1958 The tubes we use are 12AT7s, which have two triodes inside a single glass envelope.
The input signal is applied through an XLR connection.
The input 12AT7 tube is arranged as a differential amplifier. A differential amplifier gets its name from the fact that it only amplifies what is different between its two inputs. So we use the two sections of a 12AT7 as the differential amplifier; the input signal (+ and - of the cartridge) is applied to the two grids.
The output of the tube sections is a balanced signal like the input but larger. It directly drives the cathodes of another 12AT7. So the two 12AT7s together form both a differential amplifier as well as a cascode amplifier. Most of the gain we get occurs in this circuit.
The two plates of the 2nd 12AT7 then drive the passive equalization circuit; that in turn is applied via a pair of matched coupling caps to the grids of a 3rd 12AT7, also arranged as a differential amplifier. Its output is then connected to the output of the phono section thru a pair of matched coupling caps.
To make the circuit work we use a plus and minus high Voltage power supply.
No semiconductors in the signal path. You don't need semiconductors to be balanced, differential or both. That quality exists independently of tube or solid state. Our phono section was the first balanced differential phono section regardless of solid state or tube to be in regular production.