If only we could save our cake and eat it at the same time. The aim of a mixed class amplifier is to provide the high quality sound of Class-A operation with the greater efficiency and power output of Class-B operation. An additional aim might be to further the listener's ability to tune the amplifier's sound by means of a single potentiometer...but let's not get to far ahead of ourselves.
    At first, all new amplifier topologies are hard to understand. Imagine when all amplifiers were single-ended  how difficult it must have been to explain push-pull operation. What is "phase" and why does it need to be split? If the output transformer isn't partially magnetized and doesn't have an air gap, how can it work? These and other questions would require careful answering, as push-pull operation also brought the possibility and the complication of Class-AB and Class-B operation of the output tubes, which were not possible in the strictly Class-A world of single-ended operation. More questions and more answers would be needed.
   Well, now I am asking you to imagine a mixed-class amplifier, one that is at once both Class-A and Class-B. No, this is not the same as the marketing of OTL Class-AB amplifier as Class-A nor is this along the lines of those pseudo-Class-A amplifiers from the 70s that never let the output devices cutoff even though they made no real contribution to the amplifier's output. (Class-A operation is only valuable when both devices equal work into a load; when one devices gives up its grip, it may as well not be in the circuit.)
    In other words, like good cop and bad cop, good-but-weak amplifier is partnered with bad-but-powerful amplifier. (A better analogy might be the teaming of Hercules and Iolaus.)  If this arrangement sounds something like the Quad current dumping amplifier, it should, as the principles behind both amplifiers are roughly the same. In Quad's design, a small Class-A amplifier was assisted by a Class-B amplifier.

    Just as push-pull operation doubled the single-ended solo output tube, mixed class operation doubles push-pull's double output tubes; thus at least four output tubes are needed. One pair runs in Class-A push-pull and the second pair runs in Class-AB or Class-B (or even Class-C) push-pull. Thus, the first pair are always conducting, while the second pair can be completely turned off (or run at a much lower current) at idle.
    As the signal level increases, the second pair is activated, unburdening the first pair and greatly increasing the output power. In fact, we can just as easily mix Class-AB with Class-B or Class-C, either mix would give even greater power output.       
     In fact, we could easily create a three-way mix of operating classes, say Class-A, Class-AB, and Class-B, or Class-AB, Class-B, and Class-C. Of course, at least six output tubes would be needed. In all the mixes, the goal would be the same: to create a simple, seamless sounding  push-pull amplifier that uses only a single input and phase splitting (and driver) stage per amplifier. This goal can met with an near infinite mix of output tube types and bias points. One example is using a pair of EL34s for the Class-A grouping and KT88s for the Class-B pairing. Another example might be using a pair of 300Bs for the Class-AB grouping and 211s for the Class-C pairing. 

How to proceed?
Two separate amplifiers on two separate chassis with two separate power supplies could be used; but what a hassle. Using one chassis with one power supply and one input stage is preferable. 

< PREVIOUS

NEXT >