Differential Input
Headphone Amplifier
   The problems of how avoid phase inversion and how to easily apply feedback can be overcome by switching to a differential input stage and a bipolar power supply. At first glance, the input stage looks as if it functions just a Differential amplifier, but closer inspection shows that it work more like a Common Cathode amplifier in that the plate of the first triode does not swing as great a voltage as the second triode. This is the result of feeding the output of the amplifier back into the first triode's plate resistor. It is this arrangement that both equalizes the voltage swings of the output tubes and allows both top and bottom output tubes function as Cathode Followers, i.e. unity gain and low Zo. 

Differential input stage for headphone amplifier

  To get the logic behind this circuit, working backwards may help. If a positive going pulse is applied to the output of the amplifier, this pulse will be transmitted to the top of the first triode's plate resistor. Since the very little voltage

division occurs at the plate, the pulse is then relayed via the coupling capacitor to the bottom pentode's grid 1, which will force greater conduction from this tube, which in turn will pull down the output voltage of the amplifier.

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