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Hanson on Amplifier Negative Feedback


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Reading Nelson Pass might be a good start. This and other posts & pages (do a search Nelson Pass negative feedback)

It answers the question as to why you do need negative feedback.

 

https://passlabs.com/articles/audio-distortion-and-feedback

 

Those no feedback people are just playing word games.

 

Perhaps I read the whole article and came to a different understanding of what Mr. Pass is saying. I read his comments to be that a well designed amplifier WITHOUT feedback will sound better than one with feedback.

 

From the article you posted:

 

Negative feedback can reduce the total quantity of distortion, but it adds new components on its own, and tempts the designer to use more cascaded gain stages in search of better numbers, accompanied by greater feedback frequency stability issues.

 

The resulting complexity creates distortion which is unlike the simple harmonics associated with musical instruments, and we see that these complex waves can gather to create the occasional tsunami of distortion, peaking at values far above those imagined by the distortion specifications.

 

If you want the peak distortion of the circuit of figure 13 to remain below .1% with a complex signal, then you need to reduce it by a factor of about 3000. 70 dB of feedback would do it, but that does seems like a lot.

 

By contrast, it appears that if you can make a single stage operate at .01% 2nd harmonic with a single tone without feedback, you could also achieve the .1% peak in the complex IM test.

 

I like to think the latter would sound better.

[my bold added to Mr. Pass' statement]

 

© Nelson Pass, 2008

 

In fact, Mr. Pass designs many amplifiers without feedback (and many with feedback). Here is a simple example:

 

FIRST WATT S1

 

SIT-1 Power Amplifier

 

There are other amplifiers which operate without negative feedback loops for the purpose of improving performance, but it is a poorly understood fact that the bulk of them use degeneration on the gain devices to improve the performance, which is widely regarded as just another form of feedback.

 

The SIT-1 uses neither loop negative feedback nor degeneration. As a result, the sound of the amplifier entirely reflects the character and chosen operating point of this single device. It is no exaggeration to say that the amplifier speaks with a single voice. Nevertheless the measured performance is quite good compared to the single-ended tube circuits (SETs) operated with multiple stages and feedback.

 

© Nelson Pass

 

Best,

John

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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Those no feedback people are just playing word games.

 

Bullfeathers! See above post.

"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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Perhaps I read the whole article and came to a different understanding of what Mr. Pass is saying. I read his comments to be that a well designed amplifier WITHOUT feedback will sound better than one with feedback.

 

The spectral distribution will be different. Amps with lots of loop feedback will have overall lower levels of distortion, but those nasty higher order distortion components pop up. Open loop (so-called "no feedback" ones) tend to have lots more 2nd and 3rd.

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The spectral distribution will be different. Amps with lots of loop feedback will have overall lower levels of distortion, but those nasty higher order distortion components pop up. Open loop (so-called "no feedback" ones) tend to have lots more 2nd and 3rd.

 

With well implemented NFB, provided that care is taken to reduce distortion BEFORE NFB is applied, those nasty higher order distortion components should according to presently accepted theory at least, be well below audibility thresholds.

Perhaps those nasty higher order distortion components don't play by the rules, and can still sound quite objectionable despite being at minute levels ?

 

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The spectral distribution will be different. Amps with lots of loop feedback will have overall lower levels of distortion, but those nasty higher order distortion components pop up. Open loop (so-called "no feedback" ones) tend to have lots more 2nd and 3rd.

 

So you are saying you agree with Mr. Pass and that you attribute the poorer sounding amplification to higher order harmonics?

 

One thing I like about my amps is, for all their "old fashioned parts", is the distortion they do have is primarily 2nd order harmonics, they soft clip, and they have low SID. I was reading about the topic of slew induced distortion (SID) on another site. It seems very important (kind of like jitter is important). I then read some on the Lavardin Model IT amp...amazing design. So there is so much that comes into play...but often it is a lot of little issues like that play a big part in things.

 

I'm always learning...

 

John

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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Not sure if anybody else ever mentioned this other factor, but yet another reason for the "nasty higher harmonics" in feedback systems is that they have decreasing loop gain at high frequencies (i.e. less feedback). The Hypex amps keep a constant amount of feedback all throughout the audible band so this problem is avoided. To say (in the original post) that "The problem is that negative feedback makes the circuit sound worse. Nobody really knows why." is disingenuous.

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  • 1 year later...
From Update 33 Pono / Kickstarter

 

A Message from Charles Hansen

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.

 

 

This kind of description about negative feedback and how it works is a classic example of thinking over too long a timescale. The comparing of the input with the output and error correction happens continuously at very high speed, from microsecond to microsecond. It is true that negative feedback cannot somehow go back in time and undo something that has already happened, but because the error correction is happening continuously at high speed, the error (or deviation from the true signal) is never allowed to get very large before it is corrected. The more powerful the feedback, and the faster it is, the smaller the deviations from the signal are, and hence the lower the distortion. The small deviations or wobbles of the output signal caused by the feedback applying continuous high speed minor corrections to bring the signal back on track form a high frequency harmonic superimposed on the signal - e.g. added harmonic distortion. However, this very low level distortion is traded off against the much larger harmonic distortion, caused by the inherent non linearity of the active devices, that would have existed had negative feedback not been in operation.

(A good analogy would be steering a car down a straight road. The car will continually move slightly off course, but the driver will notice very early any deviation and apply correction via the steering wheel. The car will not follow a perfect straight line, but if the drive responds rapidly and accurately, deviations are kept to a minimum. The faster the driver responds, the smaller are the actual deviations). Negative feedback can never bring distortion down to zero, but if it is fast enough and powerful enough, it can get very close.

 

I have yet to come across any other technique that can get so close, e.g. distortion levels of 0.000015% typical of high end op amps, or 0.001% and better achieved by Douglas Self's "Blameless" power amp designs.

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From Update 33 Pono / Kickstarter

 

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.

.

 

Of course negative feedback cannot "pre-distort" the signal, since negative feedback occurs after the signal has passed through the amp, not before (which would imply some kind of time travel).

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