Single Polarity Power Supplies
   We still need to use cathode followers to isolate the filter elements and to provide positive feedback. We cannot just ground referenced DC couple the input to the cathode follower, as the cathode follower's distortion increases with a decrease cathode load resistance. This means the cathode follower must find a DC bias voltage so that a large valued cathode resistor can be used. Two approaches present themselves: we can add a grounded-cathode amplifier to the input and then DC couple to its output or we can uses a coupling capacitor and a resistor voltage divider network to establish the required DC bias voltage. 

    Tube CAD does the hard math for you. This program covers 13 types of tube circuits, each one divided into four variations: 52 circuits in all. Tube CAD calculates the noteworthy results, such as gain, phase, output impedance, low frequency cutoff, PSRR, bias voltage, plate and load resistor heat dissipations. Which tube gives the most gain? Tube CAD's scenario comparison feature shows which tube wins.

     The circuit above has the advantage, or disadvantage, depending on your perspective, of providing some gain. The advantage of having gain is that it offers more flexibility. Passive attenuator boxes can be used with a active filter that provides gain, as gain overrides the insertion losses of the filter. And since different amplifiers differ in gain and since high-frequency drivers are usually much more efficient than low-frequency drivers, the active filter with gain allows us to use potentiometers to bring each driver up or down to a baseline.
    The disadvantage to gain is that the gain is not free. We had to pay for it with increased distortion and noise. Obtaining a cleaner gain requires more circuitry and a possibly a cleaner power supply. The simple cathode follower circuit, on the other hand, is clean and quiet.

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