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February 22, 2003 Simple Amplifiers Few in parts, easy to understand, and quick to build, simple amplifiers have a lot going for them. Yet making a great simple amplifier is probably more difficult than making a great complex amplifier, as each part is asked to do so much. For example, in the Zen amplifiers we see a single MOSFET doing all the work of amplifying voltage, driving current into the load, and providing excess gain to power the feedback loop, which is needed to lower the output impedance and distortion— all of which is truly a great deal to ask of one MOSFET. A more complex amplifier might use twenty active devices to accomplish these same goals, with each function performed by a single or group of active devices. Still, an amplifier, which actually gets built is infinitely preferably to the amplifier that doesn’t get built. For every hobbyist-built amplifier in existence there are probably one thousand amplifiers which are planned on being built and maybe one hundred amplifiers that have all their parts collected in a box waiting to be built and ten amplifiers that are partially built and collecting dust in a closet. Thus, with its fewer parts and simpler topology, the tube amplifier stands a better chance of being built than a complex solid-state amplifier. (The same holds true for the complex-tube amplifier or the perfect amplifier; remember: the complex and the perfect are the enemy of the built). |
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One problem with simple amplifiers is that do not remain simple. For example, the amplifier shown to the right is about as simple as a hybrid push-pull amplifier gets; yet the amplifier as drawn would not work if actually built as shown; too much is missing. The input triode doesn’t have a consistent path to ground; nor does it have a grid-stopper resistor to prevent ultra-high frequency oscillations; nor does it have any biasing compensation to allow it and the second triode to share the same grid-to-cathode voltage in spite of so greatly dissimilar plate voltages; and the output MOSFETs lack protection circuitry. |
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While adding the missing parts undoes much of the original simplicity, it does make the amplifier usable. Below we see the amplifier fleshed out into a usable Class-A hybrid amplifier that might put out 10-very-listen-able watts, but whose increase complexity would scare most hobbyists away. |
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Still, compared to most solid-state amplifiers, the amplifier shown above is only half as complex and half as part bloated, yet most neophytes will stare at the schematic and sigh, knowing that they would never attempt to build such a complex amplifier. I remember an amplifier listening session where a few of us recommended to the amplifier’s owner that he add a cathode follower in between amplifier’s input tube and its phase splitter to extend its bandwidth; he turned to us and said, “Just add a cathode follower? You might as well ask me to go to the moon and bring back some rocks.” So what follows is my attempt to describe a few truly simple amplifier topologies that only require that you go to Antarctica and return with some ice. |
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More š |
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Class-A push-pull hybrid amplifier fleshed out |
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Class-A push-pull hybrid amplifier |
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readily swing +200 volts, but only -60 volts, and the output tubes needed more than that. By connecting the top triode's grid to the amp's output the clean negative swing greatly increased, as the top cathode now swung down with the plate. By the way, my circuit was never used as he claimed that Futterman's circuit was perfect and could not be replaced (odd how “perfect circuits” don't always work that well) and he went out of business shortly thereafter. The only feedback loop in place is the 200-ohm resistor that bridges the output to the top triode’s grid. An additional feedback loop could be added from the bottom triode’s grid to the output, but the amplifier’s gain would drop drastically. //JRB |
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All-Tube OTL After so many solid-state devices, I know many feel their skin crawling. So here is a token all-tube design. The story behind this schematic: about ten years ago, a fellow who was making and selling Futterman OTL copies contacted me. His amps didn't sound all that good and they were tweaky in setting up. I figured out that the problems started with the input tube, a cheap TV pentode that was being run in starvation mode: 0.3mA! I prototyped an alternative input circuit that used a 6922 in traditional cascode. One problem I discovered with the new circuit was that it could |