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If you chose the former, do not be surprised to find that the regulator is much larger and more complex than the amplifier itself. Furthermore, the series-type regulator is better suited to the Class AB than the shunt-type voltage regulator, as the shunt regulator needs an idle current at least equal to the peak expected current demand through the amplifier, which once again begs the question, why not use Class A instead? One possible answer might be that pentodes yield a more linear output in a Class AB amplifier than they do in a Class A amplifier; at least that was the conclusion I once read in an RCA technical brief from the 40's. The writer had found that the more linear triode worked best in the push-pull Class A amplifier and that the less linear pentode worked better in a Class AB setup. Apparently, the gooey turnoff characteristics of the pentode helped eliminate the crossover-notch distortion of the Class AB amplifier, whereas the triode's sharper cutoff characteristics contributed to the notch distortion.
Having said how important the power supply is to a Class AB amplifier, I must point out that often using a separate high quality power supply (or regulation) just for the front-end of the Class AB amplifier results in a fine sounding amplifier. My guess is that in both cases the power supply noise present on the signal fed to the power tube's grids was much lower than otherwise. And this reduction in noise made the difference. Remember, the noise from the front-end of the amplifier only cancels in the output transformer when both output tubes are conducting. This explains, I think, the haze that often accompanies an increase in volume.
Editor
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Subject: design of AB1/2 push-pull amps?
Do you plan to do an article on the design of Class-AB1 and AB2 push-pull vacuum tube amplifiers using beam-tetrodes and/or power pentodes?
Any recommendation(s) for book(s) presenting rigorous design information on push-pull and parallel/push-pull (not OTL) amplifiers? Thanks,
Earles
Yes is the quick answer to your first question. And as for the book recommendations, the best known book is also the best book on the topic: F. Langford Smith's The Radiotron Handbook. While the fourth edition commands the most respect and money, the third edition is probably more readily obtainable on the used market and in many ways a better book to start reading. Then reread the same topic in the fourth edition.
A third recommendation is Frederick E. Terman's Electronic and Radio Engineering, 4th edition, McGraw-Hill, 1955. This covers many aspects of a push-pull amplifier design and is well written.
The key to a good sounding Class B, AB1, AB2 amplifier is a good power supply. These amplifiers place little demand on a power supply at idle, but in use heavily tax the power supply's resources, as the current demand varies wildly with the output signal. On the other hand, the Class A presents a severe, but constant load on the power supply.
Two solutions are available: regulation or an over-built power supply. The first is complex and expensive; the second, simple and expensive. If you chose the latter, ask yourself why not use Class A instead, as the power supply can handle it?
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