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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