The circuit described in this article is a MOSFET follower for driving headphones. FET followers can supply high current, but have a voltage gain of slightly less than unity.
The class A headphone driver in figure 1 is ultra simple, except it does need a regulated supply and input and output capacitors. Most of the parts are not critical. I used a pot instead of the 100K bias resistors, so I could vary the output side of the MOSFET at 1/2 the supply. This maximizes the ouput potential of this driver. R1 limits the input current. The diodes are reversed-biased protection only. They short out spikes greater than 9 volts and negative 1 volt. In the original I used a 9 volt zener diode instead, but I figured it’s harder to get zeners. The circuit will work without the diodes.
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The volume control, Rp, is optional, because the driver is meant to be fed from a source with a volume control. If the volume control is not used, then it would be a good idea to put a discharging resistor in front of the input coupling capacitor C1 to ground – around 100K ohms. Sometimes the preamp is capacitor outputted, but may not have a discharging resistor. So, in the worst case, there would be capacitors back to back, and the voltage bias in between the capacitors would not be necessarily predictable (e.g., at or near zero volts to ground), or could cause some pops when connecting with the power on. A volume control accomplishes the same thing with its resistance to ground.
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