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The 3rd harmonic

Posted by Ralph (M) on January 4, 2021 at 10:59:36

In Reply to: RE: I got a question posted by Paul Joppa on December 31, 2020 at 15:56:05:

I'm not following your logic here - specifically, what exactly do you mean by "The ear treats this [3rd harmonic] much the same way as the 2nd ..." ?

What I mean by that is exactly as you quoted me (so the logic should be easy to follow): The ear treats this [3rd harmonic] much the same way as the 2nd. The 3rd is as innocuous and audible/inaudible as the 2nd (however you want to put it) in that it adds 'warmth' and 'bloom'; both audiophile terms for the presence of either of these harmonics. To this end, you cannot hear the difference between the two if that is all that is present. IOW the 3rd is not any more obvious than the second. The audibility of the higher orders begins, it seems, with the 5th.

But in any system both harmonics are always present to some degree. Perfect cancellation does not occur in push-pull circuits and single-ended circuits will always produce a 3rd harmonic.

But since the presence of the 2nd and 3rd can mask higher ordered harmonics, what happens is that a circuit that expresses a 3rd harmonic as its main distortion product will be perceived as being more neutral than that which expresses a 2nd as the main product. This is because is a circuit that expresses the 3rd, the actual product is often a lot less than it is with one that produces a 2nd, and the succeeding harmonics are lower in amplitude as well.

I know that people really like SETs for what they do, but to be clear a lot of that is simply how they make distortion. What people almost never do is to compare apples to apples: when comparing an SET to a push-pull amp, the latter is usually some multiple of power of the former. To add to that, usually the push-pull amp has a pentode output circuit. What happens if the output circuit is the some DHT that the SET in comparison uses? What if the P-P amp has the same full power as the SET? These issues seem unexplored but most SET aficionados.

But I have explored this a bit; I've compared a type 45 SET to a push-pull SET, using the same input tube (6SN7) and also compared several SETs to a P-P amp using EL95s that only makes 5 watts. In all cases the sonic merits of the P-P amp over the SET were obvious. Now my results are anecdotal; I encourage others to make similar comparisons, doing as much as possible to eliminate the variables. When you think about it, you really don't see that happening; nearly all comments made by people that prefer SETs are anecdotal as well.

This is why I brought up the issue of the 3rd harmonic. If people understand how the ear perceives the various harmonics, then they can build circuits that suit without any loss of smoothness or detail. To this end, this is part of why I think when people compare SETs to P-P, its apples and oranges, since if you have a P-P output but a single-ended input, you'll get a bit more of the 5th, more than you would get with an SET. But if you use a differential input this doesn't happen so you get a smoother sound, not unlike an SET, even if the amp employs pentode outputs.

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