What is the ideal THD vs Frequency plot for the SET amp?
I checked Stereophile reviews and I find some amps are very flat, but some are not, which is basically no relationship to the price, so I guess it is a designer's intention. I wonder why the goal is so different.
Flat
Lamm ML2.2 monoblock power amplifier Measurements | Stereophile.com
Non Flat
Wavac SH-833 monoblock power amplifier Measurements | Stereophile.com
I checked Stereophile reviews and I find some amps are very flat, but some are not, which is basically no relationship to the price, so I guess it is a designer's intention. I wonder why the goal is so different.
Flat
Lamm ML2.2 monoblock power amplifier Measurements | Stereophile.com
Non Flat
Wavac SH-833 monoblock power amplifier Measurements | Stereophile.com
Do you mean some people find a lot of THD in the bass frequency pleasant? Is it some kind of common sense in the SET world? If so, why more THD in the bass makes people feel pleasant? Sorry for too many questions...
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No, and it's not really that simple either. Tubes with high plate resistance generally challenge transformer design in ways that lower rp tubes don't, why one might choose one over the other is another question, but transformer design is usually an artful trade off of a variety of parameters.
Don't mind Dave, that's one of his mantras. 
He likes to tease SET fans.
Don't think so much about THD as harmonic content. Lower is better, of course, but an even drop off of harmonics is best. There is a masking effect with harmonics that means what what you hear does not necessarily follow the THD levels. Small amounts of higher harmonics can be more noticeable and annoying than larger amounts of lower harmonics in the right proportions.
Look for a design that doesn't favor low total harmonics over a regular ratio that your ear will ignore.
He likes to tease SET fans.Don't think so much about THD as harmonic content. Lower is better, of course, but an even drop off of harmonics is best. There is a masking effect with harmonics that means what what you hear does not necessarily follow the THD levels. Small amounts of higher harmonics can be more noticeable and annoying than larger amounts of lower harmonics in the right proportions.
Look for a design that doesn't favor low total harmonics over a regular ratio that your ear will ignore.
If it is due to trade off, I wonder what is the reward or compromise when highish bass THD is chosen. (I don't know if high bass THD is a bad thing...).
I'm guessing these are expensive. The second amp is an effects box. Both the freq. response and the distortion are terrible.(the square wave response is laughable). How do you make your amp stand out in a listening test for people with a lot of Money but no ears? You make it an effects box. And the reviewer for stereophile magazine loved it. Lol. Sure reinforces the uselessness of those reviews. Is he deaf, or on the payola? Probably both. So what's the ideal thd of a set? Zero.
The test results of Wavac show clearly that it has low quality OPT.
The most visible indication about this is the high THD below 100 Hz,
but also poor 10 kHz square wave response.
This amplifier is a good example of how the measured performance, price and listening results correlate
The most visible indication about this is the high THD below 100 Hz,
but also poor 10 kHz square wave response.
This amplifier is a good example of how the measured performance, price and listening results correlate
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Yes, those amps are both extremely expensive, one is $37000, the other is $35000, but I find much less expensive SETs also can be either flat THD or non flat THD.
It is clear that both of those companies' goal is zero THD, and my question is, if the lowest THD is not their goal, how they (and all SET tube designer including DIYer) would determine their goal.
Flat THD response is considered to be better in general? Or if there is no consensus, what is the advantage (subjectively) choosing non flat THD.
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Pano, I think you mean there is little relationship between subjective experience and THD-FREQ plot, but then, I still wonder why stereophile and manufactures including Nelson Pass always publish those plots.
HeyBill, I think Pano is talking about total THD is not as important as the ingredient of the THD. As he explained before, lower harmonics is OK, especially 2nd harmonics.
How is that clear? I believe the bad performance is intentional. How do you make your amp stand out amongst a crowd of high performance amps, when they all have low distortion and flat freq. response? You add eq and "euphonic" distortions to make it an effects box and you pay off an audio rag to sing its praises.
It is like an average temperature in the clinic where some are already dead, others have a fever.
What is also significant, how distortions decay with decaying sounds. But it is common for any SE amp.
When distortions decay with sound and their order getting lower it may not sound distorted subjectively. Our perception is subjective, it does not follow rules of electric measurements.
"The client is always right"
If the client wants geographic orientation of wires in the amp according to magnetic lines of the Earth, they will provide it. It is a business!
Also, an amp can have a slew rate problem, for example, or open loop gain on high frequencies can be pretty low, but I doubt that properly designed tube SE amp can have such problems.
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Did you try to "add euphonic distortions"? Try it... It is a myth that never dies.
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