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Negative feedback bad for audio?

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Dec 27, 2018
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#1
Hello,

I bought 2 monoblocks NC400 DIY kits from hypex directly, because of the measurements here in the forum. They sound amazing and the assemly was very easy. Thanks for the great reviews!

Last week I had a listening session with a guy who build high end OTL tube amps and there were many discussions. The biggest point from him was the statement that negative feedback especially in class D amps ensure that detail in the highs and mids get lost and overall dynamics are much worse. Are there graphs in the standard "armir reviews" where I can see the impact of nevative feedback?

Is here anyone who can say something about it. I cant believe that it makes a difference like night and day like he wanted to describe it. Technical details are welcome.

Thanks

Quo
 

Fred Jacquot

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#2
Hello,

I bought 2 monoblocks NC400 DIY kits from hypex directly, because of the measurements here in the forum. They sound amazing and the assemly was very easy. Thanks for the great reviews!

Last week I had a listening session with a guy who build high end OTL tube amps and there were many discussions. The biggest point from him was the statement that negative feedback especially in class D amps ensure that detail in the highs and mids get lost and overall dynamics are much worse. Are there graphs in the standard "armir reviews" where I can see the impact of nevative feedback?

Is here anyone who can say something about it. I cant believe that it makes a difference like night and day like he wanted to describe it. Technical details are welcome.

Thanks

Quo
https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf
Is pretty didactic.
 

ahofer

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#3
He is BSing to sell you an expensive and defective amplifier (inviting you into confusion..) Ask him to demonstrate his ability to hear different levels of feedback in solid state class A vs D amps , level-matched, without knowing which amp is playing. (The OTL will sound different, see below)

He will get mad, but won’t accept the challenge. That’s how you know he’s full of it.

Do the same with cables. Those are easy to compare, but if he does it he will “prime” you like crazy-tell you what you are going to hear in the hope you are suggestible.

remember: level-matched with same equipment. Generally, you won’t hear much or any difference between adequately powered amps, and you won’t hear any difference between cables larger than 12G

one caution-his OTL amp *will* sound different. It will alter the frequency response of the speakers through impedance interaction with them. This is a DESIGN FLAW of tube amps. It is often corrected or ameliorated with an output transformer, but OTL means it lacks an output transformer.
 
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Wes

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#4
Global vs. Local, and how much are questions.

I'd say ignore your friend - or ask him where he got his EE degree...
 

SIY

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#7
Hello,

I bought 2 monoblocks NC400 DIY kits from hypex directly, because of the measurements here in the forum. They sound amazing and the assemly was very easy. Thanks for the great reviews!

Last week I had a listening session with a guy who build high end OTL tube amps and there were many discussions. The biggest point from him was the statement that negative feedback especially in class D amps ensure that detail in the highs and mids get lost and overall dynamics are much worse. Are there graphs in the standard "armir reviews" where I can see the impact of nevative feedback?

Is here anyone who can say something about it. I cant believe that it makes a difference like night and day like he wanted to describe it. Technical details are welcome.

Thanks

Quo
It's complete nonsense that's been around for decades and won't die. Here's a simple to read explanation from someone who is actually a skilled engineer and not a hack.

edit: Fred got there first.
 
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mansr

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#8
Negative feedback turns a high-gain amplifier with poor linearity into a lower-gain amplifier with much better linearity. This works very well as long as the delay in the feedback loop is short compared to the signal frequency. If the input frequency is too high, strange things can happen. For audio applications, ensuring good behaviour is not a problem. The audiophile myth that negative feedback is bad for the sound is nothing but nonsense.
 

anmpr1

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#10
Last week I had a listening session with a guy who build high end OTL tube amps... the biggest point from him was the statement that negative feedback... ensure that detail in the highs and mids get lost and overall dynamics are much worse.
Julius Futterman, a man who channeled the god of OTL, always used a lot of NF in his amps. No one complained about their sound. They complained about other things. But not about the sound.
 

cjfrbw

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#11
Small amounts of feedback can improve frequency response, prevent high frequency oscillation, improve measured performance via distortion figures, reduce output impedance etc., at a small cost in gain, all nominally good things. Feedback loops can be local or global i.e. routed back from voltage gain stages or routed back from the end amplification stage.

Here we get into belief systems. Some listeners believe that the multiplying IM products from time delay caused by the feedback are audible and undesirable at any level. They also believe that the inherent higher distortion of the output devices is 'personality' that can more easily be processed by the brain and add flavor over the less pleasant time modulated products of feedback.

Many output devices have trouble performing without 'degeneration' which is a form of local feedback at the device level.

Whether feedback is ALWAYS a devil's bargain is one of those objective vs. subjective batting cages that will never die. Pick your poison according to preference. I find it hard to believe that small amounts of feedback are audible.

Nelson Pass' various First Watt designs play around with feedback schemes, from using none (or just degeneration on output transistors) to one that actually uses positive feedback (First Watt F7). The Pass XA25 has large hockey puck output devices that use no degeneration. There are fans of all these variants.
 

TomB19

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#12
Last week I had a listening session with a guy who build high end OTL tube amps and there were many discussions. The biggest point from him was the statement that negative feedback especially in class D amps ensure that detail in the highs and mids get lost and overall dynamics are much worse. Are there graphs in the standard "armir reviews" where I can see the impact of nevative feedback?
How does he control tracking error? How much time does he spend matching transistors and testing for linearity? It's bad enough doing it for the output stage.

I'll bet his amps are colourful with that "live" sound.

On the other hand, his negative feedback to you wasn't all that helpful so his point isn't entirely off the mark.
 
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DonH56

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#14
The type of guy that says such things is hardly worth debating. Such a fundamental misunderstanding (or lack of understanding) of basic concepts is difficult to surmount. Most everything about an amplifier, such as sound, stability, sensitivity to device/thermal/voltage variations, etc. is improved with feedback. And of course it can be overdone, like anything else, but such an "all or nothing" attitude is pretty misguided IME/IMO.

All negative feedback is bad.
Man never landed on the moon.
The earth revolves about the sun.
The earth is flat.
<Insert various audiophile marketing "truths" here.>

Some beliefs just don't seem worth challenging, too much effort for so little reward... Smile, nod, and move on.
 

LTig

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#15
The type of guy that says such things is hardly worth debating. Such a fundamental misunderstanding (or lack of understanding) of basic concepts is difficult to surmount. Most everything about an amplifier, such as sound, stability, sensitivity to device/thermal/voltage variations, etc. is improved with feedback.
It's almost impossible to build an amplifier without any negative feedback. Almost all amplifying devices have too much gain - those infamous vintage triode tubes being the only exception I know of. When someone claims his amp uses no or very low negative feedback he actually means overall feedback (from output to input), and his amp uses lots of local negative feedback around the amplifying device.
 
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#17
Is here anyone who can say something about it. I cant believe that it makes a difference like night and day like he wanted to describe it. Technical details are welcome.
Bruno Putzeys is utterly clear about this. Some quotes:

Hi-fi review is a complete shambles. The few magazines that do measure are capable of reprinting the most frightening distortion spectra from amplifiers and actually call them good. “Objectivity” got downgraded from “independent of who’s doing the observing” to “not favouring particular brands”. For me personally the affair hit rock bottom when in 2009 two reviewers, one Dutch, one British, independently remarked of the same amplifier (a reasonably priced product with exemplary performance) that it sounded surprisingly musical for an amp with such low distortion. In the 21st century audio engineers build equipment while actively avoiding two of the most powerful tools available to the whole of science and engineering: measurement and error control. The damage to the audio industry and its reputation in the wider engineering world will remain immeasurable until we decide to take control.

There are only advantages and no disadvantages to applying stratospheric amounts of negative feedback in an amplifier. The only hard part is figuring out how to do it.

From: “The F-word, or why there is no such thing as too much feedback”
https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf
 

veeceem

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#20
The more I listen to my NAD M22 v2 (Hypex Nc400 modules), the more I see nothing to talk about Hypex or Class D amp/module. It just does what it does, effortlessly, without adding anything to complain, be it on my Revel M16s or Salon2s :)
P/s: ah there is something to complain, it makes me want to listen to music everyday =))
 
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