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In Reply to: Holy Grail of Push Pull ? posted by WayneV on April 9, 2004 at 13:10:37:
... Lynn is probably already working on a merging of his and Allen's designs. Which of them will get their circuit posted to the internet is kind of a moot point. I doubt very much whether either of them would make any claim about it being the final last design. Where's the fun in that?The idea of using a large shared cathode impedance to force differential currents is very old; I don't know when the long-tailed pair was first described but I'd expect it to be in the twenties. It can't have been very long before someone used it in an output stage, combining phase splitting and P-P power amplification.
The debate about shared cathode impedance in push-pull output stages has also been going on for many many years. I first heard it in the context of the original Williamson amp (the one Williamson designed, not the later Hafler/Keroes ultralinear version!). At the time, the debate was in the letters column of the late lamented Wireless World magazine - this before the internet of course :^) The original amp had (IIRC) unbypassed cathode resistances, on the grounds that it improved AC balance - a theoretical argument. The subjective counter-argument was that it sounded better with a big bypass cap. I don't recall that any real consensus emerged. But after all, the Williamson amp is derived from an older P-P triode amp, and I imagine that the debate started as soon as there were capacitors large enough to bypass the cathode resistors.
This is just an extension of the Class A vs Class B arguments. Today, with very high impedance current sources, it is possible to force differential behavior to great precision, probably 0.1% or better. Whether you call it super-deep Class A or differential or whatever, the current claim is that it is audibly a different animal that "ordinary" class A, with or without bypasses. The other end of the argument is that by using Class AB you can get twice the power from the same tubes and power supply, and four times if you go to Class B. Of course, in many systems, more power sounds better. Hard to tell the apples from the oranges here.
As a perceptive friend of mine often says, "Ho hum, another major breakthrough."
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- Unless he's nuts ... - Paul Joppa 17:44:52 04/09/04 (0)
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