This Issue

Hybrid Amplifiers

      Another issue, and a fat one at that. Amazing. This journal's future may be uncertain, but this issue moves forward. Have you ever noticed how difficult it is to read a schematic with a white background, particularly on a computer screen? The contrast is too great. Hence the gray backgrounds for this issue's schematics. Feedback is encouraged. Is it more readable? Do you like it? Does it print as well?   
      Hybrid amplifiers, comprising vacuum tube and solid-state, are covered in this issue. (Even those readers who will not touch a solid-state diode should read this article, as the topologies are often directly transferable to pure tube OTL amplifiers.) We look at several topologies and try to weigh the advantages and liabilities of each. With the astronomical prices of new 300Bs and high quality output transformers, it behooves us to examine a cheaper means of delivering vacuum tube glory to a loudspeaker. But do not believe that miserliness is our only motive; potentially, the hybrid amplifier may prove the winner against the pure-tube or the unalloyed solid-state amplifier, just as the hybrid car may win against the electric car and the internal combustion car.
      Tube mixers get a treatment in this issue. I know from the many e-mails that building a tube microphone preamp / tube mixer is on many readers short list of desired topics. Here is some help.
      Remember, if you have a request or suggestion of your own for either an article topic or circuit explanation, please e-mail: 

      The promise made is that the palpability and sweetness of tubes can be added to the power and slam of solid-state. Or at least that is what the advertising copy claims. (Ever notice how often a solid-state amplifier is proudly described as sounding tube-like? Yet have you ever read of a tube amplifier being proudly described as sounding solid-state-like?) While some good hybrid amplifiers exist, too many sound like bad compromises, often laden with noise, dark tonalities, and poor bass definition, and even sometimes stricken with gritty highs. What went wrong? How was it possible to lose the attributes and retain the faults of both technologies?
      Sometimes the blame rests with the designer who is only fluent in one technology. Such a designer may competently assemble only half of the amplifier. Their designs are easy to spot, as circuits are only half understood. Half the circuit is original and the other half is lifted from old schematics. For example, the output stage from the schematic of a mediocre MOSFET amplifier is shotgun wedded to clever tube front-end; or an excessively retro tube input stage is lifted from a RCA tube manual design example or an old Western Electric schematic, which then cascades into a slick solid-state output stage that includes an opto-isolated auto-bias circuit.
       Most of the time, the blame rests in the real goal of the amplifier: a marketable dirt cheap amplifier with some of the tube's cachet. Tubes are added to promote sales, not sonics, as all the other design choices are subservient to cost. Here the tube is but an expensive LED replacement. Such amplifiers well deserve the "hybrid" name which shares its etymological root with the word "hubris;" when a wild boar mated with a domestic sow, the offspring was an
outrage,  a hybrid. On the other hand, if the entire design is given careful and inspired consideration, and if cost savings are not valued more than sonics, a good hybrid amplifier is certainly possible, as we have all heard pure tube and solid-state designs that sound good. Our task then is to make a happy wedding.

In This Issue


Hybrid Amplifiers
Vacuum Tube Mixers
Previous Issue Index
Publishing Information
Glossary of Audio Terms Articles
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