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Hanson on Amplifier Negative Feedback


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From Update 33 Pono / Kickstarter

 

A Message from Charles Hansen

Charles Hansen will be offering some of his comments in these updates to help explain more about the player’s technology. This month he discusses negative feedback, something unique to a portable device and something being used in our player.

99.999% of all amplifiers made in the last 70 years use negative feedback. There is only one thing you have to remember about negative feedback: Negative feedback is a very easy way to improve the measured performance of an amplifier. The problem is that it makes it sound worse....

Specifically, adding negative feedback will lower the steady-state distortion (test equipment measures steady-state signals, but music is anything but steady-state). It will also extend the bandwidth (frequency response). Finally, it will lower the output impedance. All of these things seem like good things, and they are very easy to measure, so everybody uses it.

The problem is that negative feedback makes the circuit sound worse. Nobody really knows why. My best guess is that it messes up the time response, but nobody has been able to measure it, and there are all kinds of mathematical "proofs" that say this is not the case. Negative feedback compares a scaled-down version of the output signal to the input signal. Since no amplifier is perfect, there will be differences. The negative feedback pre-distorts the input signal opposite to the distortion that the amplifier creates so that the final output signal is closer to the input signal.

It is very clever. The problem is that feedback can only attempt to correct a problem after the damage has been done. One cannot go back in time and fix the signal before it is distorted by the imperfect circuit.

Here is the best proof that negative feedback does not work:

IF NEGATIVE FEEDBACK ACTUALLY WORKED, THEN ALL AMPLIFIERS WOULD BE PERFECT, AND ALL AMPLIFIERS WOULD SOUND EXACTLY THE SAME.

Anybody that has ever listened to amplifiers knows that no two amplifiers sound the same. Therefore anybody that has ever listened to amplifiers knows that negative feedback does not work as advertised.

With feedback, it is trivially easy to build an amplifier with any arbitrarily low output impedance that you want. It won't sound very good, but it will have very low output impedance.

That is why no Ayre product ever made has ever used negative feedback. When we started Ayre, there were a few companies that made zero-feedback tube preamplifiers and that was about it.

For more information on this subject and others check out ayre.com.

In any dispute the intensity of feeling is inversely proportional to the value of the issues at stake ~ Sayre's Law

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First read the Bruno Putzeys paper (skip the parts that are over your head):

[h=1]Negative feedback in audio amplifiers: Why there is no such thing as too much

Bruno Putzeys -July 24, 2013 [/h]http://www.edn.com/design/consumer/4418798/Negative-feedback-in-audio-amplifiers--Why-there-is-no-such-thing-as-too-much

Negative feedback in audio amplifiers: Why there is no such thing as too much (Part 2) | EDN

 

Then move on to the Hanson paper.

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IF NEGATIVE FEEDBACK ACTUALLY WORKED, THEN ALL AMPLIFIERS WOULD BE PERFECT, AND ALL AMPLIFIERS WOULD SOUND EXACTLY THE SAME. ~ Hanson.

 

 

 

I thought the article was interesting because I have seen various amplifiers, which cost more than a small car, emphasizing negative feedback!

In any dispute the intensity of feeling is inversely proportional to the value of the issues at stake ~ Sayre's Law

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It's one of those disputes with a lot of sound reasoning on both sides. Great fun to listen to! :)

-Paul

 

 

IF NEGATIVE FEEDBACK ACTUALLY WORKED, THEN ALL AMPLIFIERS WOULD BE PERFECT, AND ALL AMPLIFIERS WOULD SOUND EXACTLY THE SAME. ~ Hanson.

 

 

 

I thought the article was interesting because I have seen various amplifiers, which cost more than a small car, emphasizing negative feedback!

Anyone who considers protocol unimportant has never dealt with a cat DAC.

Robert A. Heinlein

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First read the Bruno Putzeys paper (skip the parts that are over your head):

Negative feedback in audio amplifiers: Why there is no such thing as too much

Bruno Putzeys -July 24, 2013

 

Negative feedback in audio amplifiers: Why there is no such thing as too much | EDN

Negative feedback in audio amplifiers: Why there is no such thing as too much (Part 2) | EDN

 

Then move on to the Hanson paper.

 

 

 

If I skipped the parts that were over my head, I would not get past the title!! :)

In any dispute the intensity of feeling is inversely proportional to the value of the issues at stake ~ Sayre's Law

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I'm hoping this part is a misunderstanding? If negative feedback is so evil, why would they use it in the Pono?

 

I think what they intended to say is that negative feedback was *not* being used in the Pono circuitry, making it unique among portable devices in this regard.

One never knows, do one? - Fats Waller

The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. - Einstein

Computer, Audirvana -> optical to EtherREGEN -> microRendu -> ISO Regen -> Pro-Ject Pre Box S2 DAC -> Spectral DMC-12 & DMA-150 -> Vandersteen 3A Signature.

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It's one of those disputes with a lot of sound reasoning on both sides. Great fun to listen to! :)

 

I wonder if that quote means that all amplifiers without negative feedback sound exactly the same... Which is hardly the case.

 

Personally, I'm yet to see any low distortion amplifier without negative feedback. I've heard many fine sounding amplifiers that have plenty of negative feedback. For example from Accuphase and Musical Fidelity.

Signalyst - Developer of HQPlayer

Pulse & Fidelity - Software Defined Amplifiers

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Personally, I'm yet to see any low distortion amplifier without negative feedback. I've heard many fine sounding amplifiers that have plenty of negative feedback. For example from Accuphase and Musical Fidelity.

Hi Miska

Personally, I believe that for negative feedback to work well, you also need wide bandwidth devices in the output stages, not just the small signal stages, in order to avoid a minute amount of smearing of the waveform.

I realise that most EEs are unlikely to agree with me on this issue though.

It seems highly likely though that Charles Hansen's design does use local negative feedback in the way of perhaps emitter degeneration, if the design is discrete. Properly applied emitter degeneration can improve all aspects of an amplifier's performance, although there may be small trade offs in the area of S/N ratio.

 

Regards

Alex

 

How a Digital Audio file sounds, or a Digital Video file looks, is governed to a large extent by the Power Supply area. All that Identical Checksums gives is the possibility of REGENERATING the file to close to that of the original file.

PROFILE UPDATED 13-11-2020

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Query:

 

Doesn't this technical issue, like so many in audio, depend on implementation? There are many outstanding amplifiers that employ negative feedback. And outstanding ones that do not. It is a design choice. I am very skeptical about blanket statements such as: The problem is that negative feedback makes the circuit sound worse.

 

Of course, the unanswered question in such an assertion is, "Compared to what?"

"Relax, it's only hi-fi. There's never been a hi-fi emergency." - Roy Hall

"Not everything that can be counted counts, and not everything that counts can be counted." - William Bruce Cameron

 

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I am very skeptical about blanket statements such as: The problem is that negative feedback makes the circuit sound worse.

 

DITTO ! It all depends on the way it is implemented. Nevertheless, even with negative feedback applied, it pays to make the circuit as linear as possible before introducing negative feedback.

 

How a Digital Audio file sounds, or a Digital Video file looks, is governed to a large extent by the Power Supply area. All that Identical Checksums gives is the possibility of REGENERATING the file to close to that of the original file.

PROFILE UPDATED 13-11-2020

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Personally, I'm yet to see any low distortion amplifier without negative feedback. I've heard many fine sounding amplifiers that have plenty of negative feedback. For example from Accuphase and Musical Fidelity.

 

Two highly regarded amplifier manufacturers that do not use negative feedback are Ayre and ModWright.

 

Another consideration when weighing negative feedback vs no negative feedback designs is that amplifiers that do not employ negative feedback typically have lower damping factors. This may have consequences in the form of poorer bass control with some speakers.

"Relax, it's only hi-fi. There's never been a hi-fi emergency." - Roy Hall

"Not everything that can be counted counts, and not everything that counts can be counted." - William Bruce Cameron

 

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Personally, I believe that for negative feedback to work well, you also need wide bandwidth devices in the output stages, not just the small signal stages, in order to avoid a minute amount of smearing of the waveform.

 

My own designs have always used MOSFETs throughout and I've got great results that way. It is pretty easy to make these amplifiers work fine far to MHz range with low THD. It is important also when used with digital sources, because many DACs spit out ultrasonics to 5 MHz range or higher.

 

 

P.S. A place where you have plenty of negative feedback is DSD or SDM in general... For D/A side it's in digital domain and for A/D side it is mixed domain.

Signalyst - Developer of HQPlayer

Pulse & Fidelity - Software Defined Amplifiers

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My own designs have always used MOSFETs throughout and I've got great results that way. It is pretty easy to make these amplifiers work fine far to MHz range with low THD. It is important also when used with digital sources, because many DACs spit out ultrasonics to 5 MHz range or higher.

 

 

P.S. A place where you have plenty of negative feedback is DSD or SDM in general... For D/A side it's in digital domain and for A/D side it is mixed domain.

 

Lyngdorf says their TDAI amps do not use negative feedback. Their both amplifiers (TDAI 2200 and the new one) sound very good...

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Lyngdorf says their TDAI amps do not use negative feedback. Their both amplifiers (TDAI 2200 and the new one) sound very good...

 

Isn't it a class-D amp? There is no absolute need to use feedback when you are just driving a switch.

 

But if you have any delta-sigma modulator there, then it has digital domain negative feedback.

 

IIRC, NAD and Devialet digital amps use mixed domain negative feedback, where the feedback goes through A/D converter back to digital domain for applying correction.

Signalyst - Developer of HQPlayer

Pulse & Fidelity - Software Defined Amplifiers

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All analog amplifiers use Negative Feedback.

 

Do my Bottlehead Paramount 300B's use negative feedback?

 

In 300B configuration the two amplification stages are R-C coupled. In 2A3 configuration the additional voltage headroom is used to employ direct coupling between the stages. Both configurations use zero feedback.

Positive emotions enhance our musical experiences.

 

Synology DS213+ NAS -> Auralic Vega w/Linear Power Supply -> Auralic Vega DAC (Symposium Jr rollerball isolation) -> XLR -> Auralic Taurus Pre -> XLR -> Pass Labs XA-30.5 power amplifier (on 4" maple and 4 Stillpoints) -> Hawthorne Audio Reference K2 Speakers in MTM configuration (Symposium Jr HD rollerball isolation) and Hawthorne Audio Bass Augmentation Baffles (Symposium Jr rollerball isolation) -> Bi-amped w/ two Rythmic OB plate amps) -> Extensive Room Treatments (x2 SRL Acoustics Prime 37 diffusion plus key absorption and extensive bass trapping) and Pi Audio Uberbuss' for the front end and amplification

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All analog amplifiers use Negative Feedback. Some designers just redefine what the phrase 'Negative Feedback' means. Then they give their circuit a special or limited name.

 

And your authority for this statement is?

"Relax, it's only hi-fi. There's never been a hi-fi emergency." - Roy Hall

"Not everything that can be counted counts, and not everything that counts can be counted." - William Bruce Cameron

 

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Isn't it a class-D amp? There is no absolute need to use feedback when you are just driving a switch.

 

But if you have any delta-sigma modulator there, then it has digital domain negative feedback.

 

IIRC, NAD and Devialet digital amps use mixed domain negative feedback, where the feedback goes through A/D converter back to digital domain for applying correction.

 

This stuff is hard for me to understand, but they mention a conversion between PCM and PWM (pulse widht modulated)...

 

"This is why the EquibitTM modulator can accurately be described as the heart of the system

since this technology facilitates linear conversion from PCM to PWM in a pure non-feedback

design like the TDAI-2200."

 

http://lyngdorf.com/downloads/product_descriptions/TDAI-2200_and_roomperfect_product_description.pdf

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This stuff is hard for me to understand, but they mention a conversion between PCM and PWM (pulse widht modulated)...

 

At one point they say

"The D/A converter used for the analog output is the high performance Wolfson 8740 DAC, highly acclaimed for its pure, rich and natural sound. Just as importantly, the critical re-construction section after the DAC is based on high-end Texas Instruments OPA 227 op-amps and is designed, as are all other signal paths in TDAI-2200, to be fully balanced foroptimum purity and the best S/N ratio."

so this section certainly has plenty of both digital and analog negative feedback.

 

"This is why the EquibitTM modulator can accurately be described as the heart of the system

since this technology facilitates linear conversion from PCM to PWM in a pure non-feedback

design like the TDAI-2200."

 

The description sounded very similar to NAD's "Direct Digital" amps. I would say the digital domain modulator likely has negative feedback. But as it just drives the output switches it doesn't necessarily require analog feedback.

 

These kind of designs tend to have varying amounts of treble boost depending on connected speaker. NAD has adjustable digital EQ for compensating it.

Signalyst - Developer of HQPlayer

Pulse & Fidelity - Software Defined Amplifiers

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i will curiously wait if ayre will have dac without negative feedback. So far the ti and ess dac and arda adc chips they've used have tons of negative feedback in their delta-sigma modulators...

 

Means going for r2r multi-bit ladders!

ayre amps still have single stage local negative feedback.

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