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Walt

With all to discussion over PP and SE attributes, no one mentioned the output transformer, the real heart of the amps discussed. The SE transformer is a gapped device, where as the P-P transformer is generally not gapped. This is really the underlying reasons for the differences in performance both measured and audible characteristics.

The SE transformer has to store energy so that it can supply speaker current for half of the waveform cycle where as the tube supplies it for the other half in addition to recharging the inductive component of the transformer. Because the tube is controlling the energy in the inductive portion of the transformer as well as the coupling component for the entire cycle there is NO CROSSOVER DISTORTION issue. The main sonic and measurement limitation with the SE has to do with the finite inductance available for making it work at low frequency. The air gap means more turns are needed to get a reasonable inductance for halfway decent bass, and the additional turns requirement means inter-winding capacitance that kills the highs. This is why SE amps are noted for having beautiful midrange but compromised highs and lows.

On the other hand, P-P transformers don't have to store energy as either one tube or the other, or a combination of both depending on the amount of class A vs B you have is supplying energy to the speaker for each half of the waveform cycle. But there is a cost. Iron-core, and many other magnetic cores as well, have hysteresis issues when the direction of magnetization is reversed as it nears the crossover point. In iron cores the inductance undergoes rather dramatic changes, especially in the ungapped core. Some of these wild gyrations are being passed on to the speaker at the crossover point which is unfortunately near the lowest instantaneous power output point where your ears are the most sensitive. Think of this as a slight nonlinearity as, say, a violin string is midway through its movement from one extreme to the other. On the plus side, fewer turns are needed on the P-P transformer because the inductance is higher, magnetization current parasitics are lower and fewer turns means less high-frequency losses because of winding capacitance. This is why P-P amps are noted for having more extended frequency response, but at the cost of compromised midrange compared to SE.

What a fabulous explanation Dave, thanks.
There are problems everywhere. How do we get such fine music out of our systems?

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Walt