Of course, simplicity is in the eye of beholder. Few couldn’t build the amplifier shown to the right in less than a hour (only 4 resistors and 3 active devices) and even fewer wouldn’t describe the amplifier as simple; yet fewer still would realize that the amplifier is, in fact, amazingly complex, with the AD712 and LM317s each holding a huge array of transistors, diodes, and resistors, requiring pages to display accurately in a schematic. Just because the sub-circuits are not drawn does not mean that they are not there. (Do not get me wrong here: I am not saying that this amplifier is a bad design or that it shouldn’t be built; it is, after all, a single-ended class-A amplifier that might be perfect for a computer amplifier.)

Single-Ended Designs

      Single-ended amplifiers are in theory only half as complex as most push-pull amplifiers, as they hold only half as many tubes and require no phase splitter. If we limit ourselves to one active device and we expect some voltage gain, then we end up a grounded-cathode or grounded-source or grounded-emitter amplifier. If we use tubes, we end up with the “World’s simplest tube amplifier.” If we use MOSFETs, we end up with the Nelson Pass’s “Zen amplifier.”

Single-ended  class-A 4W amplifier

Pass ZEN Single-ended  Class-A amplifier

World’s Simplest SE Class-A amplifier

        Both amplifiers exploit their target technology. The simplest tube amplifier exploits the average tube linestage’s ability to swing easily ±30 volts; the Zen amplifier, the average solid-state linestage’s ability to work into a low load impedance (4300 ohms). Where these plans fail is when the technologies are swapped: the tube linestage cannot drive 4300 ohms; the solid-state linestage cannot swing ±30 volts.

        Given enough line-stage voltage swing, a feedback loop can even be added to the simple single-ended tube amplifier. In the amplifier shown to the right, the output is fed into the cathode in the phase as the input; which means that the gain, distortion, and the output impedance are reduced, but the drive requirement goes up. How much? Quite a bit actually, as the 16-ohm tap on the secondary offers 1.4 times more voltage than the 8-ohm tap, so 4 watts output would require 8 volts across the 8-ohm load and 11.3 volts at the cathode and 31 volts at the grid. (In this new topology, the tube is no longer triode-connected. In fact, it works in an ultra-linear fashion, as grid 2 effectively opposes the cathode’s movement.)

     One cheat might be to use the transformer’s own primary resistance to replace the cathode resistor. In the circuit at the right, we see ground referenced to the cathode and not the power supply’s negative terminal. This sort of topology is actually ancient, in electronic terms, but virtually unknown today. One reader wrote a scathing email to me the last time this journal displayed such a circuit. Unfortunately, I deleted the email long ago, but his contention was that such an amplifier could NOT amplify, as the tube was working as a cathode follower. If the ground connected to the bottom of the transformer’s primary, he would be right, but as it connects to the cathode, the tube actually functions as a grounded-cathode amplifier.

     If you learn only one thing about tube circuits from this journal, let it be this: just because the load is under the cathode, doesn’t mean the tube has to be functioning as a cathode follower; and conversely, just because the load attaches to the plate doesn’t mean the tube has to be functioning as a grounded-cathode amplifier. The placement of the reference point defines the circuit’s topology: if the reference point is the plate, then the tube is working as a cathode follower; the cathode, a grounded-cathode amplifier. 

 Zen Amplifiers

      Zen amplifiers are marked by their using a single output device to provide all the voltage and current gain. The top MOSFET is used as a blind constant current source, “blind” as it doesn't see any drive signal.

    The maximum theoretical efficiency of such an amplifier is only 25%. Replacing the constant current source with an inductor, however, increases the theoretical efficiency to 50% and allows us to halve the power supply voltage while still retaining the same power output. If we rework the Pass Zen amplifier to use an inductor, the circuit at the right develops. This amplifier functions close the original, but dissipates half the heat. Still, the same problem of too low an input impedance remains.

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