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Why Let an Amplifier Sound Good when You can Force it to?
Why Let an Amplifier Sound Good when You can Force it to?
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Old 27th January 2018, 12:14 AM   #11
scottjoplin is offline scottjoplin  Wales
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If you want to hear something really scary, I ran my Maplin MOSFET amps with 100% negative feedback for years, Mooly even did a simulation because I think he didn't believe me
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Old 27th January 2018, 12:15 AM   #12
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by Fast Eddie D View Post
I haven't seen any amplifiers that have no feedback.
Me neither. Those who say that 'feedback is bad in audio' normally have to make a distinction between 'global negative feedback' and 'local negative feedback' and then exclude the latter from their strictures. Local negative feedback can't be avoided when using common-drain and common-gate configurations so ISTM a no-feedback approach would mean only pure common-source use of amplifying devices which would mean lots of gain and lashings of non-linearity.
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Old 27th January 2018, 12:34 AM   #13
globalplayer is offline globalplayer  Germany
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Quote:
Originally Posted by scottjoplin View Post
Often people say random stuff to confuse, it's a way to control you
Control is just an illusion.
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Old 27th January 2018, 12:42 AM   #14
bwaslo is offline bwaslo  United States
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Why Let an Amplifier Sound Good when You can Force it to?
Common source has some feedback too, there's built-in source resistance to any transistor. Even triodes in common cathode have feedback, from their internal space charge (a triode modified to avoid that feedback is called a "pentode").
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Old 27th January 2018, 03:03 AM   #15
Eva is offline Eva  Spain
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- Feedback is an ideal universal mechanism, only imperfections in the elements comprising the feedback loops contribute to imperfections in the output of the system.
- Feedback is everywhere, even inside electronic gain devices such as transistors, at electron and molecular level.
- Feedback requires some element outputting a magnitude which is the sum or difference (and/or other math operations) between other 2 or more magnitudes: The error amplifier. Feedback requires the output of the system to be fed back to input of error amplifier.
- Feedback happens inherently in every amplification circuit, even in follower configurations (the simplest case). The conditions for a follower to be unstable do exist, and can be created in practice.
- In linear electronic circuits: there are lower and upper practical limits for feedback. Lower NFB loops have to work with higher error magnitudes, requiring wider linear range of operation of error amplifier (and earlier amplification stages). Higher NFB loops encounter the opposite problem, error magnitude becomes low enough to be disturbed by other magnitudes, as by: parasitic inductive and capacitive coupling between circuit elements, noise floor of components, drifts in error amplifier characteristics due to temperature.
- In switched-mode amplifier circuits: feedback is employed too. "It does not need to be stable in order to be linear". The interest for switched-mode amplification arises when the practical limits of feedback are explored (the raw conditions where linearity improvement takes place).

The following simulation pictures (based in a real circuit) show a class-D amplifier (modulator type: self-oscillating post-filter single-integrator) doing <0.007% 3rd harmonic at 5khz @ 50W @ 8ohm. The circuit is sized for up to 800W/2ohm output, using two N-ch TO-220 power MOSFET. This matches the performance of finest class AB projects shown in best solid state threads, while not being a stable NFB loop, but it is an unstable NFB loop constructed to be highly linear (this proves the 1st statement in this post).
Attached Images
File Type: png class_D_50W_THD_1.png (33.0 KB, 533 views)
File Type: png class_D_50W_THD_2.png (34.9 KB, 530 views)
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Old 27th January 2018, 04:36 AM   #16
Wavewhipper is offline Wavewhipper  United States
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I was just going to say using negative feedback for linearization requires a precision comparator stage for the magic to work. Differential amp pairs on one die are a good idea for that. Sort of funny how the errors wiggle their way through the system until they stabilize at the comparator. There has to be some sort of high speed hunting and settling time, well above audio range.

I could not find but a few transistors even somewhat able to produce linear gain over the functional range without feedback. And they probably deteriorate with time and heat.

Last edited by Wavewhipper; 27th January 2018 at 04:40 AM.
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Old 27th January 2018, 05:52 AM   #17
MarcelvdG is offline MarcelvdG  Netherlands
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I think the whole low-overall-feedback and no-feedback movements can be traced back to 1966, when Daugherty and Greiner published "Some design objectives for audio power amplifiers", IEEE Transactions on Audio and Electroacoustics, vol. AU-14, nr. 1, March 1966, pages 43...48. Their conclusions on error signal overshoot were later repeated in a series of articles of the research group of Otala, with some refinements to the amplifier model.

In the appendix of their article, Daugherty and Greiner prove for a single-pole feedback amplifier driven by a first-order low-pass filtered step function that the error signal exhibits no overshoot when the open-loop bandwidth is greater than or equal to the bandwidth of the low-pass filter that preceeds the amplifier. Error signal means the difference between the input and feedback signals of the amplifier, which is the signal that drives the input stage.

That is, they have proven that a sufficient condition to prevent slewing is to make the open-loop bandwidth greater than the bandwidth of the filter in front of the amplifier. Given the fact that the achievable gain-bandwidth product in a practical amplifier is limited by non-dominant poles, a large open-loop bandwidth implies a relatively small low-frequency loop gain.

Unfortunately, in the article they present their sufficient condition as if it were a necessary and sufficient condition. It is by no means necessary. The fact that the error signal overshoots in an amplifier with a small open-loop bandwidth is no problem as long as the amplifier's input stage is designed to handle the overshoot.

When you increase the open-loop bandwidth and reduce the low-frequency loop gain of an amplifier by connecting a resistor in parallel with the capacitor that sets the dominant pole, all that happens is that the final value of the error signal gets larger. The overshoot, which is by definition the ratio of the initial peak to the final value, gets smaller but the required linear range of the input stage is not reduced at all. When you also take square wave input signals into account, the required linear range is reduced, but only by a factor of two.

Peter Garde pointed out these facts in his articles "Transient distortion in feedback amplifiers", Journal of the Audio Engineering Society, vol. 26, nr. 5, May 1978, pages 314...321 (reprinted from the Proceedings of the IREE Australia, October 1977) and "Slope distortion and amplifier design", Journal of the Audio Engineering Society, vol. 26, nr. 9, September 1978, pages 602...608 (reprinted from the Proceedings of the IREE Australia, December 1977). He also explained how the linear range of an input stage can be increased by local feedback.

Apparently the damage was already done by then, because 40 years later the idea that a feedback amplifier with a small open-loop bandwidth necessarily suffers from transient problems still exists in high-end audio circles. In fact, it has only become more extreme over the years, changing from 'use as much feedback as you can without making the open-loop bandwidth smaller than 20 kHz' to 'feedback is evil, avoid it at all costs'.
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Old 27th January 2018, 07:32 AM   #18
suzyj is offline suzyj  Australia
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Quote:
Originally Posted by DF96 View Post
There are two approaches to feedback:
1. talk about it
2. learn about it
Sadly, many people choose option 1 and carefully avoid option 2. Doing it the other way round (2, then 1) means you have something useful to say.
You win the internets today.
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Old 27th January 2018, 11:16 AM   #19
mchambin is offline mchambin  France
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Quote:
Originally Posted by DF96 View Post
There are two approaches to feedback:
1. talk about it
2. learn about it
Sadly, many people choose option 1 and carefully avoid option 2. Doing it the other way round (2, then 1) means you have something useful to say.
So true.
There is such a gap between the "1 then 2" people versus the "2 then 1" people that dicussions are of no use.
All we can do is keep aware of their arguments just in case something new would make sense, otherwise, just ignore them.
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Old 27th January 2018, 11:54 AM   #20
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by globalplayer View Post
Control is just an illusion.
True, I used to wonder why disillusion is considered a negative
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