But eight 5687s will require a truly robust power supply, as the heaters alone consume 45 watts of power and 20 watts of power will be needed to supply the rest of the circuit; truly extravagant. But maybe that is what is required to make the world's finest tube-based OTL single-ended headphone amplifier.

Push-Pull Headphone
Amplifier With Inverse
Pre-distortion Cancellation
     This circuit makes use of a specially designed split-load phase splitter that uses a variation on the inverse pre-distortion cancellation technique. In the circuit below, we find a grounded-cathode amplifier cascading into a 50% voltage divider. Yes, this means half of the first stage's gain is being thrown away, but it is purposely being thrown away. The voltage divider then cascades into a split-load phase splitter whose plate resistor and cathode resistor values combine to equal the grounded-cathode amplifier's plate resistor value.

       In addition, when the first stage sees its plate current drop by 5 mA, the phase splitter sees its plate current increase by 5 mA; in contrast, when the first stage sees its plate current rise by 5 mA, the phase splitter sees its plate current fall by 5 mA. Without the 50% voltage divider bridging the first triode to the second, no juggling of plate and cathode resistor values would yield this perfectly anti-phase cathode-to-plate voltage and plate current operation. (If you conceder that the split-load phase splitter effectively has a gain of 2 and not unity because of the two outputs, then the voltage divider function is more obvious.)
     The result is a phase splitter that with a final gain of 6 and very little distortion. Now all that is needed is a Class-A push-pull output stage that is strong enough to drive 30 ohm loads.

     The circuit above includes the Class-A push-pull output stage. Both output tubes function as cathode followers, as the capacitor that bridges the phase splitter and output serves to return all of the bottom triode's gain back to its grid. The output stage has an idle current of 20 mA and thus can deliver twice that amount into a 300 or 20 ohm load impedance. (One point to remember is that a 30 ohm load requires a 265 µF output coupling capacitor.)   

    In other words,  both the grounded-cathode amplifier and the split-load phase splitter undergo the same cathode-plate voltage swing and current swing, but in anti-phase. For example, if the first stage sees its plate voltage climb up by 50 volts, the phase splitter sees its cathode-to-plate fall by 50 volts.

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