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Old 19th November 2016, 05:31 PM   #1
plasnu is offline plasnu  United States
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Default Ideal THD vs Frequency for SET

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
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Old 19th November 2016, 05:51 PM   #2
DF96 is offline DF96  England
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Some people aim for high quality sound reproduction. Some people aim for a sound which they find pleasant. Two quite different aims, so two quite different solutions.
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Old 19th November 2016, 07:05 PM   #3
plasnu is offline plasnu  United States
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Originally Posted by DF96 View Post
Some people aim for high quality sound reproduction. Some people aim for a sound which they find pleasant. Two quite different aims, so two quite different solutions.
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...

Last edited by plasnu; 19th November 2016 at 07:12 PM.
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Old 19th November 2016, 07:12 PM   #4
kevinkr is offline kevinkr  United States
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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.
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Old 19th November 2016, 07:20 PM   #5
Pano is offline Pano  United States
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Don't mind Dave, that's one of his mantras. :P 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.
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Old 19th November 2016, 07:22 PM   #6
plasnu is offline plasnu  United States
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Quote:
Originally Posted by kevinkr View Post
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.
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...).
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Old 19th November 2016, 07:33 PM   #7
cbdb is offline cbdb  Canada
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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.
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Old 19th November 2016, 07:47 PM   #8
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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

Last edited by artosalo; 19th November 2016 at 07:50 PM.
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Old 19th November 2016, 08:31 PM   #9
plasnu is offline plasnu  United States
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Originally Posted by cbdb View Post
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.
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.

Last edited by plasnu; 19th November 2016 at 08:45 PM.
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Old 19th November 2016, 09:08 PM   #10
HeyBill is offline HeyBill  United States
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I think what sounds good to you is best. Distortion cancellation in a SET might be a way to turn the dial until you like what you here. I'm putting together a circuit to try that technique. Regards, HeyBill
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