To A First Approximation

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In Reply to: Additional comment posted by mikeyb on November 4, 2006 at 10:45:13:

If - and only if - the tubes were distortionless, yes, the current draw of a Class A PP stage would be constant. In the world we live in, tubes have distortion. A perfectly matched pair of tubes with *only* 2nd-harmonic distortion will create a 3rd-harmonic current ripple on the power supply. (Replace the slightly nonlinear tubes with grossly nonlinear rectifiers and you get a balanced modulator, used in AM transmitters. This balanced-modulator effect is also present to a small degree in all Class A PP circuits.)
Real tubes create higher-order harmonics, 3rd, 5th, 7th, and so on. Minor tube imbalances (3 to 5%) will also create even-order harmonics, 2nd, 4th, 6th, and so on. So if you examine the spectra of the variations of the current drawn from the supply, you see a full harmonic spectrum, with the 2nd diminished, 3rd dominant, and a fairly rapid falloff after that.
So with a well-balanced Class A PP stage, the demand on the supply isn't completely steady, but reflects all of the distortion terms of the amplifying stage. This is different than a SE stage, where the current demand on the power supply is a direct mirror image of the audio signal (including distortion).
Although the Class A PP stage rejects most power-supply noise (by about 30dB), the issue of power-supply coloration remains, and in a way is worse, since the artifacts of the power supply are almost nothing but distortion terms, heavily weighted towards dissonant odd-order harmonics.
This is why a low source impedance that is constant with frequency is very desirable for a Class A PP stage. Odd-harmonic distortion is bad enough, but a funhouse-mirror reflection of it that is colored by reactive elements in the power supply is worse.

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Follow Ups Full Thread

Topic - SE vs PP; what are the differences? - jarthel 01:47:23 11/04/06 (7)

Page 2 of the linked URL gives the key... - Allen Wright 10:45:16 11/04/06 (0)

Re: SE vs PP; what are the differences? - danlaudionut 05:34:54 11/04/06 (0)

Re: SE vs PP; what are the differences? - Ray Moth 05:33:30 11/04/06 (2)

Additional comment - mikeyb 10:45:13 11/04/06 (1)

To A First Approximation - Lynn Olson 22:22:16 11/04/06 (0)

Re: SE vs PP; what are the differences? - NoWay 05:10:51 11/04/06 (0)

Re: SE vs PP; what are the differences? - lovetube 03:38:17 11/04/06 (0)

Follow Ups

To A First Approximation - Lynn Olson 22:22:16 11/04/06 (0)

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