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What causes BJT based amplifier distortion?
What causes BJT based amplifier distortion?
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Old 6th February 2021, 07:10 PM   #21
Cortez is offline Cortez  Hungary
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Maybe a better question would be: what does not cause distortion in a BJT based amplifier?
As the device itself is far from ideal (almost everything depends from something other), unfortunately we are forced to adapt...
And the only chance is to embed the devices to a "protected/relaxed"
(electrical+thermal) environment where their imperfections does not have such a role.
Also at first we have to forget about the GNFB as a holy grail as it does not solve all of these problems.

My algorythm would be something like this:
  1. collect all known BJT errors/imperfections together in a list
  2. prioritize (but I'm afraid that step in itself could lead to a religion war )
  3. collect all known solutions for each problem + a collective brainstorming could be interesting
  4. try to integrate as much as possible based on the prioritized list in one system
  5. or have multiple paralell versions crosschecked with eachother
  6. simplify (occam)
  7. goto 6;

Last edited by Cortez; 6th February 2021 at 07:17 PM.
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Old 6th February 2021, 07:43 PM   #22
jstott is offline jstott  United States
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Douglas Self did that kind of analysis more than 20 years ago. Buy his book or read the article on the web.

Short version: power output stages suffer from crossover-related distortion, but are usually low-distortion otherwise. Most of the distortion occur in the voltage gain stage(s).

-JS
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Old 6th February 2021, 11:59 PM   #23
PRR is online now PRR  United States
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What causes BJT based amplifier distortion?
Quote:
Originally Posted by edbarx View Post
The first step to understand a philosophical statement is to search for and understand what assumptions/axioms it is based on. .....
Do you think distortion must be ZERO or else why bother?
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Old 7th February 2021, 02:23 AM   #24
RussellKinder is offline RussellKinder  United States
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Current SOTA of solid state amplifiers is pushing the magnitude of all harmonics below the noise floor of a very low noise amplifier. Most designers doing this have applied Doug Self's techniques of linearizing each stage of the amplifier, plus a few tricks of their own, while maintaining a very large open-loop gain. This means inaudible. Really.

I contend that the proponents of zero global NFB have no idea what they are talking about. They think if a little feedback is bad, then more is worse. Incorrect. Feedback compensation is a noise/distortion shaping method, that translates nonlinearity and noise to higher frequencies. A small amount of feedback only manages to move the problems to a higher location in the audible band.

A large amount of feedback moves the problems out of band, especially if a high loop gain can be maintained to at least 20kHz.
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Old 7th February 2021, 08:21 AM   #25
edbarx is offline edbarx  Europe
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Here are some quotes from Douglas Self's book.

Quote:
Originally Posted by D. Self
You will also see, in Chapter 1, that in the last few years I have found no reason to alter my views on the pernicious irrationality of subjectivism. In that period I have repeatedly been involved in double-blind listening tests using experienced subjects and proper statistical analysis, which confirmed every time that if you cannot measure it you cannot hear it.
Quote:
The principles of negative feedback as applied to power amplifiers are explained in detail, as there is still widespread confusion as to exactly how it works.
Quote:
You may have two units placed side by side, one giving 2% THD and the other 0.0005% at full power, and both claiming to provide the ultimate audio experience. The ratio between the two figures is a staggering 4000:1, and this is clearly a remarkable state of affairs. One might be forgiven for concluding that distortion was not a very important parameter. What is even more surprising to those who have not followed the evolution of audio over the last two decades is that the more distortive amplifier will almost certainly be the more expensive.
These quotes show that Douglas Self is not himself an audio subjectivist and considers subjectivism as irrational. They also show, he regards negative feedback as a great way of realising highly performant, and at the same time, cheaply produced amplifiers.

Sincerily, I am perplexed reading posts in these fora very strongly defending subjectivist views with some even going to the extreme of entirely denying objective measurement.

May I ask, is Negative Global Feedback INHERENTLY BAD, or is it only, a delusion? The application of negative feedback in control systems is good as it is a simple way of achieving stability. However, when audio is concerned we have to deal with the perception of hearing, and perceptions are purely subjective as they cannot be shared directly with third parties.

I have seen the amplifier using no global feedback by, if I rememeber well, Valerie. My impression is that it still using feedback from the output, but instead of feeding it into the inverting input, it uses an intermediate opamp to compare the signal differentially with the actual signal at the non-inverting input. It then feeds the opamp output to feed the VAS.

Please, don't tell me it is all opinions, especially, making the claim that if it is my opinion that GNFB is bad, nothing can be done. I do not hold mere opinions highly, I make an effort to be open to facts. This is why I am asking. Douglas Self is a researching amplifier engineer who definitely knows his area of expertise.
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Last edited by edbarx; 7th February 2021 at 08:25 AM.
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Old 7th February 2021, 09:22 AM   #26
indra1 is offline indra1  Indonesia
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D. Self only measure output voltages because 0.0005% acoustic distortion at full power is hard to believe. Normal humans can not perceive audio directly from output voltage of amplifiers but hear the acoustic output, so relevant distortion measurement should be obtained from the acoustic output. THD of amplifier output voltage can only show part of the picture but generates a lot of unnecessary confusion from incomplete interpretation on significance of the measured quantity.
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Old 7th February 2021, 09:40 AM   #27
JMFahey is offline JMFahey  Argentina
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Amplifiers are designed by and for humans, so if accurate reproduction is desired, itīs enough to make distortion undetectable by humans, period.

Which does not mean Mathematical zero by any means.
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Old 7th February 2021, 10:27 AM   #28
edbarx is offline edbarx  Europe
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I am not of the opinion zero distortion is a necessity, but I am curious to understand, why complicated workarounds succeed to reduce distortion to impressive values.
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Old 7th February 2021, 11:16 AM   #29
Charles Darwin is offline Charles Darwin  United Kingdom
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Quote:
Originally Posted by N101N View Post
They don`t.
This one looks half decent independently measured:

MC2 Audio MC650 Amplifier Test Results :: ABELtronics
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Old 7th February 2021, 11:44 AM   #30
RussellKinder is offline RussellKinder  United States
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Quote:
Originally Posted by indra1 View Post
D. Normal humans can not perceive audio directly from output voltage of amplifiers but hear the acoustic output, so relevant distortion measurement should be obtained from the acoustic output.
It's true. Amplifiers at this point are contributing almost nothing to the total distortion. I have some former co-workers at Purifi Audio in Denmark that have done a great job in reducing loudspeaker distortion. Someone here on diyaudio interviewed Lars Risbo on YouTube. He did a great job of explaining how they have lowered loudspeaker distortion.
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