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Re: Hiraga on Feedback

There is another inconsistency in Hiraga's article. His Fig 13A shows the spectrum of a single PX4 triode, for which Marconi gives a 4.5W rating at a max rail of 300V. Hiraga's chart shows the spectrum of the PX4 putting out 5W (!) so this is evidently the clipping spectrum. If not, it is certainly at the threshold of clipping.

Now, in his Fig 14 Hiraga states that "the majority of negative feedback loops has (sic) an unfortunate tendency to absorb the high harmonic orders, with a low level of applied audio signal." But the Fig 13A spectrum of the PX4 is clearly *not* operating at a low level so how are we to interpret Hiraga's statement? It causes me to question whether the signal level in his corresponding Fig 13B (his paradigm feedback case) is low or high?! Hiraga states that Fig 13B is for "the same amplifier" but he does not state whether it is for the same *level*.

It is well known that if you operate a triode at clipping with feedback you are going to get a spectrum that extends far beyond that shown in Fig 13A. This regardless of how many input signals you feed it in tandem. Although we can cancel harmonics we cannot prevent clipping, and the flattening of a waveform at clipping is *caused* by high-order harmonics.

My conclusion is that Hiraga fails in this particular article to account for the effect of negative feedback on the harmonic spectrum of amplifiers. OTOH, his thesis that the ear does not notice certain patterns of distortion that correspond to our innate hearing distortion is something I'm still investigating. If it's true that's a powerful argument in favor of SET amplifiers.

Hiraga also states (p. 50) that Wegel and Lane's measuring equipment does not extend beyond 2000Hz. This is incorrect, as can be verified in E.C. Wente, “A Condenser Transmitter as a Uniformly Sensitive Instrument for the Absolute Measurement of Sound Intensity,” Phys. Rev., 10:1, Jul 1917. But this is a minor historical point.

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