[AllanS]

Maybe I misread it, but a take away from reading one of Nelson Pass’s tech papers is no amplifier feedback.  I thought the reason was feedback eliminates natural harmonics.
Regardless of my understanding, I’m still confused about how feedback would differentiate between one fundamental frequency and the harmonics of another.

[FullRangeMan]

My 2 cents are:
NFB dont differentiate freq or harmonics but its bad effect may affect freq and harmonics, its used when the output circuit are instable or non linear, it send an small negative Tension to the audio circuit to correct the output signal, usually its specified in dB.

Tech people can post a much better and detailed explanation than it.

[Speedskater]

All amplifiers have negative feedback!
If they don't they quickly turn in to oscillators.
Low distortion, high accuracy amplifiers have more types of feedback.
* * * * * * * * *
Bruno Putzeys to the rescue:
Negative feedback in audio amplifiers: Why there is no such thing as too much (Part 2)
https://www.edn.com/negative-feedback-in-audio-amplifiers-why-there-is-no-such-thing-as-too-much-part-2/

[FullRangeMan]

All amplifiers have negative feedback!
If they don't they quickly turn in to oscillators.
Low distortion, high accuracy amplifiers have more types of feedback.
* * * * * * * * *
Bruno Putzeys to the rescue:
Negative feedback in audio amplifiers: Why there is no such thing as too much (Part 2)
https://www.edn.com/negative-feedback-in-audio-amplifiers-why-there-is-no-such-thing-as-too-much-part-2/

Engineer speech, remind me to never buy an amp from this guy.
Nelson have various amps which claims to be zero NFB.

[AllanS]

Thanks much.  I’d love have Bruno and Nelson is a room discussing the topic. 
Schiit released their newest amplifier, Wotan, a few months ago.  It features “switchable no overall feedback” which I would love to listen to.  It’s a first that I know of but do remember coming across a tube amp a while back that i think had a knob you could use to dial in desired feedback.

[newzooreview]

I asked Perplexity AI to search the web for white papers and forum discussions regarding why negative feedback in amplifiers is disfavored by some. I had run across parts of this before, but the AI output captured a couple of things I had not previously run across:

"Why Avoid Feedback?
Preservation of Harmonic Structure: Nelson Pass often designs amplifiers without global negative feedback (open-loop designs) to preserve the natural harmonic structure of the audio signal. Feedback can reduce overall distortion but tends to introduce higher-order harmonics, which are less musically pleasing to the human ear compared to lower-order harmonics like second and third harmonics.

Natural Sound: Without feedback, amplifiers often produce a more "organic" and "musical" sound that some listeners prefer. This is because the distortion pattern remains monotonic (gradually declining higher-order harmonics), which aligns with how humans perceive sound.

Feedback and Harmonics: Feedback doesn't inherently differentiate between a fundamental frequency and the harmonics of another frequency. Instead, it reduces all distortion components but can create additional higher-order harmonics as a byproduct. This can mask the natural harmonic content of the original signal, leading to a less natural sound.

How Feedback Affects Harmonics
Feedback operates by correcting errors in real-time, effectively reducing distortion across the spectrum. However:

It reduces lower-order harmonics (e.g., second and third) while introducing higher-order harmonics (e.g., fifth, seventh), which are less consonant and more audible as distortion artifacts.

The process does not distinguish between fundamentals or harmonic components of different frequencies; it treats all deviations from the input signal as errors to be corrected.

Summary
Feedback doesn't differentiate between a fundamental frequency and the harmonics of another because it operates on the total error signal (the difference between input and output). This includes all components—fundamental frequencies, harmonics, and any distortions. As such, feedback applies its corrective mechanism uniformly, which can inadvertently alter the harmonic balance in ways that some audiophiles find undesirable.

In summary, Nelson Pass's preference for minimal or no feedback stems from his goal to maintain a natural harmonic structure and avoid introducing higher-order distortions that feedback mechanisms may generate. This design philosophy prioritizes subjective musicality over purely technical measurements like total harmonic distortion (THD)."

[newzooreview]

I just followed up with a question regarding Purifi amps, because Bruno Putzey clearly doesn't see feedback as a problem. It seems to be a case of traditional vs. more refined ways of using feedback:

"Purifi amplifiers, specifically the 1ET400A modules designed by Bruno Putzeys, do employ negative feedback—but they do so in a sophisticated manner that addresses and largely avoids the sonic drawbacks traditionally associated with feedback.

How Purifi's Feedback Approach Differs

1. High Levels of Negative Feedback
Purifi amps utilize exceptionally high amounts of negative feedback, but crucially, they implement it in a carefully engineered "nested" or multi-loop arrangement. This advanced technique allows for extremely low distortion without introducing the higher-order harmonic distortions that typically plague amplifiers using conventional feedback methods.

2. Nested Feedback Loops
The Purifi Eigentakt amplifier design incorporates nested feedback loops—multiple layers of feedback loops within each other. This approach allows the amplifier to correct distortion more precisely and at multiple stages, significantly reducing higher-order harmonic distortion that conventional single-loop feedback may inadvertently introduce.

3. Feedforward and Dual-Feedback Systems
Purifi's Eigentakt technology also employs feedforward mechanisms alongside feedback loops. This combination helps the amplifier anticipate and correct errors proactively rather than merely reacting after distortion occurs. Such a dual-feedback system contributes to exceptionally low distortion levels without generating the unwanted higher-order harmonics typically associated with simpler feedback implementations.

4. Transparency and Low Distortion
Measurements confirm that Purifi amplifiers achieve extremely low total harmonic distortion (THD) across the audible spectrum (e.g., 0.00017% THD at 100W output), demonstrating transparency and accuracy comparable to or surpassing traditional Class AB amplifiers.

5. Avoiding Typical Feedback Pitfalls
Bruno Putzeys, the designer behind Purifi technology, emphasizes that while historically high-feedback amplifiers sometimes sounded poor due to implementation difficulties, modern engineering techniques—such as those used in Purifi designs—overcome these issues by ensuring stable, linear operation even under very high-feedback conditions."

[AllanS]

Finally.  A use for AI!
Thanks much for the query and posting the results.
It is interesting that the benefit of Bruno’s brainchild is avoiding unwanted higher order distortions but there’s no mention of preserving the desirable lower order harmonics.

[newzooreview]

If I understand correctly, the focus of the Purifi amps is not adding any harmonics at all, leaving the harmonics in the recording undisturbed. A piano key or guitar string will generate harmonics that are recorded, and if the amp stays out of the way then the sound made by the instrument will be reproduced more accurately.

There's an interesting Youtube listening session (almost two hours long) with very good studio mics comparing the newest Hypex amp (the Nilai) with a Purifi amp. Listening with headphones, it seemed to me that the Nilai was portraying the harmonics in acoustic instruments more naturally than the Purifi. On the test bench neither of them should be adding any audible harmonics, but they do sound different in that regard (at least in my perception): https://www.youtube.com/watch?v=NdZFAWe1Nxo

Regarding the AI, Grok 3 and Perplexity (which allows a choice among several LLMs) are both efficient at compiling and summarizing information from online sources. It's like having a smart high school senior as a research assistant. Perplexity automatically lists sources and can search YouTube transcripts as well. Grok 3 can pickup discussions on X in addition to the web (not sure that it searches YouTube).

[FullRangeMan]

4. Transparency and Low Distortion
Measurements confirm that Purifi amplifiers achieve extremely low total harmonic distortion (THD) across the audible spectrum (e.g., 0.00017% THD at 100W output), demonstrating transparency and accuracy comparable to or surpassing traditional Class AB amplifiers.

5. Avoiding Typical Feedback Pitfalls
Bruno Putzeys, the designer behind Purifi technology, emphasizes that while historically high-feedback amplifiers sometimes sounded poor due to implementation difficulties, modern engineering techniques—such as those used in Purifi designs—overcome these issues by ensuring stable, linear operation even under very high-feedback conditions."

This are a very interesting article, thanks for posting.
IMO low DHT, stable and linear operation dont grant great sound quality,
they are just fancy numbers that the marketing manager will frantic shake.

Iam in audio since the 1970s, all the Class AB amps I had heard had poor sound quality, cheap and expensive models. But now with Class D and GaN-FETs amps the situation becomes dramatic and difficult for Transistor Class A amps, imagine for Class AB.

[AllanS]

There's an interesting Youtube listening session (almost two hours long) with very good studio mics comparing the newest Hypex amp (the Nilai) with a Purifi amp. Listening with headphones, it seemed to me that the Nilai was portraying the harmonics in acoustic instruments more naturally than the Purifi.

Thanks for sharing the listening session.  After taxes and other equally mundane stuff I’m looking forward to setting aside time to have a good listen to the comparison.  I’ve always been intrigued by Purify (in particular NAD M23/C298 and more recently March Audio P482) but haven’t seen much of any talk of the Hypex Nilai.  I’m considering the next amp so am curious if I’ll pickup on any differences, especially harmonics. Am I remembering correctly that you have a Nilai implementation in your system?
Maybe I’ll poke at AI after all that.

IMO low DHT, stable and linear operation dont grant great sound quality…

Iam in audio since the 1970s, all the Class AB amps I had heard had poor sound quality, cheap and expensive models. But now with Class D and GaN-FETs amps the situation becomes dramatic and difficult for Transistor Class A amps, imagine for Class AB.

Your experience is interesting.  I cannot explain why, but I boxed up a class D amp to sell and left a class A-ish (Schiit Aegir) in the system - for now.  I really could not define any specific reason. I typically do not hear many of the differences that others describe but it seems my ears are better tuned to the Aegir - at least with my speakers (Spatial Sapphire M4).  I have read plenty of comments from others who now prefer some class D amp after many years of tube and SS amps.