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Old 30th August 2009, 12:49 PM   #1
Borat is offline Borat  United States
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Default What causes high THD in tube amps ?

if tubes are more linear than SS then from where does the high THD come from ?

is it the low gain ? the output transformer ? unwillingness to design a proper circuit because it would be too expensive to implement ? an actual craving to have an amp with tons of THD ? something else ?
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Old 30th August 2009, 08:44 PM   #2
jlsem is offline jlsem  United States
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It depends on what your definition of high is. Most solid state designers think they can hear distortion less than 1%. Some think even less. There are plenty of tube amps out there that do less than 1% at normal listening levels.

John
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Old 30th August 2009, 08:46 PM   #3
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Originally Posted by Borat View Post
unwillingness to design a proper circuit because it would be too expensive to implement ?
Usually, this. Plus outright laziness or lack of ability. Some of the best tube amps, or at least those held in high regard, have distortion rivaling solid state. A Citation II is in the 0.008% range at 1 watt / 1 kHz.

As an addendum, some amps are designed with relatively high distortion in mind, because it allows discernibility when compared with normal amplifiers. Some $$$ customers would rather something sound "different", than actually sound "better" - hence a slight subjective preference for added distortion. This I don't get, but people are welcome to do as they please.

Last edited by jon_010101; 30th August 2009 at 08:52 PM.
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Old 30th August 2009, 09:10 PM   #4
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1% distortion is "peanuts" !
Do you have distortion values for your loudspeakers ?
If so, wich manufacturer does dare to publish ?
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Old 30th August 2009, 09:20 PM   #5
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The answer is simple: THD don't reflect directly cleanness of sound reproduction. It is well known fact repeated again and again, but still needs to be repeated, because some people keep thinking that THD directly reflects the sound quality...
In order to sound clean usual tube amp don't need so deep feedback as typical class AB opamp requires, that's why an opamp to sound well needs much lower THD.

However, the myth exists that the people who call reproduced by tubes sound clean and accurate are schizophrenics; this myth helps those who can't afford a properly built tube amp to feel better, and proudly use modern mass-production.
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Old 30th August 2009, 10:10 PM   #6
jlsem is offline jlsem  United States
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Quote:
1% distortion is "peanuts" !
Do you have distortion values for your loudspeakers ?
If so, wich manufacturer does dare to publish ?
That goes without saying. I was referring to amplifiers, not systems.

John
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Old 30th August 2009, 10:16 PM   #7
Borat is offline Borat  United States
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Originally Posted by Yvesm View Post
1% distortion is "peanuts" !
Do you have distortion values for your loudspeakers ?
If so, wich manufacturer does dare to publish ?
there are many ways to build speakers.

in most cases distortion could be slashed by an order of magnitude but the designers don't bother.

if i had unlimited resources i could design a speaker that would have no more than 0.1% THD. by unlimited resources i mean design every transducer from scratch. it would probably end up costing about $100,000 though.

most $100,000 speakers though will probably have more than 1% THD though.
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Old 30th August 2009, 10:20 PM   #8
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Where can I find info on speaker design vs. distortion?
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Old 30th August 2009, 10:20 PM   #9
Gordy is offline Gordy  United Kingdom
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Originally Posted by Borat View Post

if tubes are more linear than SS then from where does the high THD come from ?
What high THD? Are you comparing like-with-like?

I would guess that a line-level signal amplified x 15 by a single triode such as a 6H30 would have less distortion than the same signal amplified by a single transistor. It may be that only by using multiple transistors and negative feedback would you reduce the distortion. But then you could add feedback to the triode circuit and reduce the distortion also.
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Old 30th August 2009, 10:22 PM   #10
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THD (the single number version) is totally useless as a specification so why care?

You need to look at the spectrum to get any clue... 2nd & 3rd (particularily if 2nd is higher) are quite benign. Higher order harmonics are very unnatural and stand out like crazy...

And then you have to consider the HD content at different power levels and frequencies and how it changes as a function of these.

Your question could be classified either as trolling or a significant lack of understanding of the basics.

Tube amps do do have inherently higher distortion, and even those with high single figure numbers often have lower distortion where it counts.

dave
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