B² Spice

Case Study: Modeling Headphone Amplifiers in B² Spice

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Tube-Transistor Hybrid Headphone Amplifier

One last circuit to ponder, a vacuum tube driving cascading emitter followers. A feedback loop is used to lower the gain and the distortion of the amplifier. The power supply voltage is a mere 12.6 volts. A further trick here is the use of the tube’s heater as a load for the final transistor. This load gives us the ability to drive 32-ohm loads easily and it makes use of the current that would have to flow into the heater anyway, so we save some energy (battery time). (Download circuit: version 4.2 Tube Hybrid headphone amp 1.ckt ; version 4.0 Tube Hybrid headphone amp 1.ckt)

What happens when the tube is still cold and not conducting any current? How will the output stage handle a free-floating reference voltage? Or what will happen if the tube is not in its socket? To overcome these potential problems and other possible problems, it is best to give the first transistor’s base a fixed reference voltage. Adding two 100k resistors does the job nicely.

These resistors are large enough in value not to load-down the tube input stage and low enough in value to ensure that the output stage is biased correctly at turn on. The frequency response for this amplifier is shown below.