The Tube CAD Journal signature noise canceling technique of using a partially bypassed cathode resistor for the first stage is implemented. The potentiometer allows for idle current adjustment, which can be read from the voltage drop across the .22 ohm resistors. The LF411 Op-Amp works to reduce the DC offset at the output. All in all, we have moved quite a bit from the world's simplest hybrid amplifier. 

Simple Hybrid
Headphone Amplifier
    The following circuits do not really follow the previous thread other than illustrate simple designs. The first circuit was born out of a desire to come up with a portable tube headphone amplifier.

    Here is where the tube fans have it all over their solid-state brethren. Tube fanciers are happy to live with the 3 watts that a miserly 2A3 gives us in a single-ended amplifier...but what a nice 3 watts.
     This headphone amplifier does not use the 12AU7 to its best advantage, as it current starves the 12AU7. The second version uses a FET current source to both reduce the noise at the output and to increase the idle current. The cathode follower is eliminated and, like the previous circuit, it uses the 12AU7's heater element as the load for the output MOSFET. The potentiometer sets the idle current. If an auto-bias is desired, the last circuit should prove helpful. The base-to-emitter voltage drop is used as a voltage reference, which the transistor uses to compare against the voltage across 4.6 resistor. 

    The MOSFET chosen for this application is a true sleeper. The 2SK2013 is a very linear power MOSFET with a truly useable negative temperature coefficient. However, it is not destined to become popular because of its paltry 25 watt dissipation limit. Remember the food choice. Well, when choosing solid-state devices, twice the food of half the quality always wins. The 2SK1530 with its 150 watt dissipation limit, will find a thousandfold greater use. Yet it is a much poorer choice in terms of audio purity.

Improved simple hybrid headphone amplifier

Auto-bias hybrid headphone amplifier

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