Differential Unity Gain Circuit
    With the addition of the negative power supply rail comes a new source of noise. Of course, a negative power supply is not strictly required; the long-tail phase splitter only requires a large valued cathode resistor. The negative power supply allows for a greater resistor value; but if the B+ voltage is sufficiently high, or if a constant current source used, then need for a negative power supply falls away. Or does it? If the output stage uses only a mono-polar power supply, then certainly the large valued resistor or current source will prove adequate.
    On the other hand, if output stage uses a bipolar power supply, then the task of noise elimination becomes more difficult. The problem lies with the bottom device being referenced to the negative power supply rail, not to ground. Thus if the bottom device's input is grounded, its cathode or source or emitter will see the full negative power supply rail noise and amplify it at its output. On the other hand, if the bottom device's input is referenced to the negative rail, then both its input and its cathode or source or emitter will see the same noise signal. But no amplification of the noise ensues.

    Reversing the connections to the phase splitter requires reversing our strategy. Now the phase splitter's cathode needs to see no noise and the plate needs to see the full negative power supply rail noise. The filter capacitor's connection to the negative power supply rail ensures the coupling of the rail's noise to the phase splitter's plate and, then via the coupling capacitor, to the bottom device's input.
     (The 4k resistor that spans the output to the negative power supply rail only serves to prevent the phase splitter's current from being drawn through the output stage. It could be eliminated, if desired. It is, however, useful for allowing testing of the driver stage in isolation of the output stage.)
     I can imagine that some readers are troubled by the task of superimposing the negative power supply rail noise on the phase splitter' input signal. Just how do you do that? Depending on the input circuit, several techniques are possible. For example, a common-cathode amplifier whose cathode resistor is connected to the negative power supply rail will find an easy path to passing the noise to its output, as the signal present on the cathode resistor will be relayed to the plate resistor.

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