Negative feedback (NFB) vs zero negative feedback (ZNFB). There seems to be unsubstantiated contention that ZNFB amps sound more realistic. I know this is an age old debate favoring the ZNFB design, but I think most audiophiles have never bothered to look into this matter and believe the advertisements and proponents of zero NFB design. I have been in that camp until recently. My own experience and research into articles on this matter leads to me believe NFB is needed for faithful reproduction of music. I'm not saying NFB design is more "musical", which is a highly subjective term and usually means more euphonic or colored. I've posted a similar question awhile back, but I was hoping we can have a more evidence based discussion on this matter. Perhaps, we need clarification of descriptive terms we use to describe sound. My contention is, in general, NFB designs produces a more accurate or faithful reproduction of music than ZNFB designs. Here is a very good article on feedback and distortion:
Atmasphere, could you please clarify a statement you made on another related thread?
"Marqmike, unfortunately all amplifiers make odd ordered harmonics. It is the 5th, 7th and 9th that we are concerned about (the 3rd is considered musical to the human ear). When you add negative feedback to any amplifier design, the result will always be an enhancement of odd orders, even if the feedback is successful in removing most of the open loop distortions ('open loop' means the amp operating without feedback).
The reason this is so has to do with the fixed amount of time it takes a signal to move through a circuit. This time is called 'propagation delay'. The issue is that a low frequencies feedback works pretty good, but as frequency is increased, the propagation delay means that the feedback signal is arriving later and later to correct the circuit. In fact a designer has to be careful with feedback because this effect means that at some frequency the feedback as actually positive rather than negative. This is entirely due to the propagation delay in the amp."
I find your statement misleading. You state that NFB "enhances" odd order harmonics. According to the article I cited, NFB does not enhance (ie, increase) or add odd order harmonics. NFB reveals the odd order harmonics already produced by the amplifier before NFB is applied; this is a result of NFB greatly reducing other distortion components that mask the odd order harmonics. And NFB can reduce those higher order odd harmonics to below 110 dB from the fundamental. Can one hear distortion that low? You've claimed that we are very sensitive to odd order harmonic distortion, but could you provide a study that shows we can detect distortion this low?
As for propagation delay, an amplifier operated in its linear range will have delay in nanosecond range as far as audio frequency is concerned, including frequencies at the limit of human hearing. This is also explained in the link. Nano second range delay seems pretty negligible to me.
I, obviously, do not have more technical knowledge in amplifier design than you, but your claims seem to be directly at odds with the article I cited.
Atmasphere, I've seen you write about how bad odd order harmonics can sound (ie, subjectively bright and hard) even at very low levels. Can you please specify at what level are odd order harmonics perceived as unpleasant? I don't recall you ever giving an actual number.
I have owned your M60 amp which I think uses a 2 dB of NFB and compared to a pentode, class AB, tube amp with 20 dB of NFB. Everything else in my system was constant. The volume level was as close to even as I can get them with SPL meter, and the volume ranged between 85 to 90 dB max during the comparison. The speaker had minimum narrow impedance band of 5.8 ohms and was 90 dB efficient. By your own words, 20 dB of NFB is considered excessive. However, I did not hear any excessive hardness or brightness in either amp. Both were very smooth in the midrange and treble regions. How do you explain that?
Atmasphere, It make little sense to seek very high DF for better damping (assuming that speaker needs it) when inductor in series with the woofer has most likely resistance of 0.1 ohm limiting DF to 80 even with perfect amplifier.
Mapman, Woofers have only one invention - lack of suspension (spiderweb) compensated by large diameter voice coil mounted to flat disc in place of dustcap. It makes membranes lighter, faster and stiffer. Midrange (6.5") uses, in addition ferrofluid as a suspension. Midrange is absolutely breath taking. Tweeter is so well integrated that I cannot really tell transition from midrange.
They are not bad looking. What I mean by very nice is that they appear to be well designed to really deliver the music well at lifelike volumes and with meat on the bones for very reasonable cost compared to many tower or monitor type designs common today in the same price range.
Its been said that teh main reason to have an expensive stereo is the ability to go lifelike loud and clear as needed. I agree with that! THose speakers look up to the task!
Mapman, That's the first thing I hated - The looks of two small coffins in glossy black (I hate glossy black). Overtime they grew-up on me and now I love them.
(It is a little bit like Eiffel Tower or glass Pyramid if front of the Louvre Museum, once called atrocity but loved today).
There seem to be several things not yet discussed on this thread that seem IMO to be important.
The first is that the ear interprets harmonic distortion as tonality. For example, the 2nd harmonic of some tube amps is why they sound 'warm' or 'lush'. Another example is the trace amount of odd ordered harmonics typical of a solid state amp with lots (+20db) of feedback; they will sound 'hard' or 'bright'.
(BTW, the lower orders, 2nd, 3rd and 4th, are proven to not be irritating to the human ear. But not irritating simply means they are musical; IOW that is not to say that they are inaudible!)
The second is that negative feedback is used to control the speaker in two ways: by altering the power output of the amp depending on the speaker impedance at that frequency, and by providing increased damping.
So here are the problems with adding feedback to any amplifier. I did note BTW that this seems to be ignored in the article in the OP. In fact I've seen a lot of engineers hide behind theorems (Shannon, Nyquist, et. al.) to explain why this is not a problem, and that problem is the delay time between input and output of the amps.
The reason the theorems don't work is they don't apply. They apply in a theoretical world where propagation delay in the amplifier circuit does not exist. But almost any oscilloscope can measure this delay time in almost any amplifier made. If we could build amps that had no delay (propagation) delay, then feedback would not cause the problems it does.
However we have to live in the real world, and experience has shown that being pragmatic about that fact reaps big rewards.
In this case all amps have propagation delay so the application of loop negative feedback will have the benefits that Kijanki has described, except for a tiny bit of odd-ordered harmonic that is considered 'negligible' by degreed engineers. This 'tiny bit' of distortion can be modelled as a 'ringing' effect in the amplifier. Chaos Theory has something to say about this, I will go into it if people are interested.
It is the mark of a good engineer to know when to use the word 'negligible' correctly! In the case of 'negligible odd ordered harmonics added' the word is misapplied. The reason is the human ear is more sensitive to the presence of odd ordered harmonics than it is to almost anything else. This is because the ear uses odd ordered harmonics (present in any sound) to figure out how loud that sound is. If the odd orders are slightly louder due to electronics, the resulting sound will not only sound louder than it really is, it will also sound brighter. Its funny that an engineer might know how to design the feedback loop but be completely uninformed about how the ear will perceive the result!
Really the only way to get around this is to get rid of the distortion. But as Nelson Pass points out in a distortion article on his site https://passlabs.com/articles/audio-distortion-and-feedback you can add more feedback to reduce that distortion but you need to add more gain (adding to the distortion) to do it. You are chasing a carrot on a stick.
With regards to damping factor: Some speakers require high damping factor (up to 40:1) and some speakers want very little (0.1:1 and yes you read that right). No speaker made needs in excess of 40:1. The chief engineer of Electro Voice wrote a two-part article on this some decades ago and the facts of that article have not changed which is how it usually works with facts. I will try to dig up the article (its online) if anyone is interested.
For me there has always been a bottom line, which is that if your speaker requires that the amp have feedback you won't be able to get it to sound like real music because the amp is violating one of the most fundamental rules of human hearing.
How do you get around this if you don't want the system to have colorations (audiophiles talk about these colorations all the time; warm, rich, soft highs, wooly bass, etc)?? The fact of the matter is you have to do everything in the book that you can to reduce distortion without using feedback, and stick to speakers that don't expect a low output impedance from the amp (BTW the crossover design rules can be quite different!). A fully symmetrical design will, for example, cancel the even ordered harmonics (otherwise the 2nd will be predominant in any design, tube or transistor). The even orders will thus cancel not only in the output of the amp but in every proceeding stage. That leaves the odd orders, and if you are careful they will not be generated.
There are plenty of other techniques BTW. In addition any ZNFB amp, IMO should have a very low IM distortion figure if its done right, because IM is very audible. That is very possible in a ZNFB design- we get numbers that are as low as many amps with feedback. IM distortion (or lack thereof) is a function of linearity.
Kinjanki, I think the Hyperion 938 is truly one of the great speakers in high end. I regret selling them to a friend. You can get 938s at bargain prices now. I am thinking of getting another pair.
KJ: Never said that anything is wrong with Class D. My powered sub is a Class D amp. I mentioned that it is interesting because it has a different way of using NFB.
Edwyun, this particular design (Icepower) uses Nielsen Karsten multiple feedback. One feedback comes from modulator while the other is from the speaker output. Class D is relatively linear to start with, while output impedance is inherently low hence it doesn't require a lot of feedback. In addition it is only one stage (modulator) making for small signal delay - less chances of TIM. Early class D consisted of primitive sawtooth modulator while this resembles sigma-delta A/D converter's modulator. There is nothing wrong with class D (SACD,DSD are class D) but carrier frequencies are still too low (Mosfets are getting faster every year). For the money it is bargain IMHO.
Edwyun, this particular design (Icepower) uses Nielsen Karsten multiple feedback. One feedback comes from modulator while the other is from the speaker output. Class D is relatively linear to start with, while output impedance is inherently low hence it doesn't require a lot of feedback. In addition it is only one stage (modulator) making for small signal delay - less chances of TIM. Early class D consisted of primitive sawtooth modulator while this resembles sigma-delta A/D converter's modulator. There is nothing wrong with class D (SACD,DSD are class D) but carrier frequencies are still too low (Mosfets are getting faster every year). For the money it is bargain IMHO.
Dracule1, What I'm getting from this is that some amount of imperfection can sound to many people more natural. Difference between natural, pleasing and faithful is fluid. Article was very interesting and thank you for posting it. I would only disagree with notion that deep feedback has no negative effects if amplifier is stable and input limited to slew rates equivalent to 20kHz. In ideal world it might be true but bandwidth never ends at perfectly 20kHz (TT often go to 50kHz) while high gain amplifiers, including op-amps have, (in spite of compensation) tendency to ring when presented with capacitive loads (speaker). It is big temptation for designer when he can improve everything 10-fold just by introducing more gain. I know that top designers like Nelson Pass or Jeff Rowland don't take shortcuts and that's why I bought Rowland class D amp. I know that modules were designed and made by B&O but Jeff Rowland name is enough for me to get interested. Some amplifiers are very expensive but you get what you pay for (Brand name). My amazing Hyperion HPS-938 speakers were a bargain but company doesn't exist anymore (so it seems). Now I worry what to do if something happens to these exotic drivers since nobody else makes them. Next time I'll pay more.
"Mapman, the question here is if NFB makes reproduction MORE faithful. Some answered that there is no faithful reproduction, which doesn't answer this question, while others stated they prefer sound of ZNFB which doesn't answer this question either. The truth is, that there is no SS amp (and very few tube amps) with zero feedback. SS amp by nature is a voltage source, and as such needs some form of negative feedback, at least for the output stage. The question is how much and that might depend on the load, your preference etc. It is undisputed fact, that even small amount of NFB widens bandwidth, lowers output impedance and reduces THD/IMD distortions. Does it make reproduction more faithful on average? I think it might. What sound you like has nothing to do with original question."
Kijanki, you seem to be the only one who understand the intent of my original post. Thank you for putting into words better than I.
I find it amusing some found my OP to be an attack on their equipment and taste in music, which was not my intent at all. If some of you were offended, sorry.
NFB is a subtractive process, although as proved by GE long ago, it increases the perceived odd order harmonic distortion. NFB removes some of the signal, and not in real time, either,due to propagation delay within the amp. Although I prefer to trust my ears, and could not care less about the theories involved, theory does match what I hear: with NFB it sounds less "alive". Deadened. Something essential has been deleted from the signal along with the improvement in distortion specs. There does sound like slightly less distortion, but the baby is thrown out with the bathwater. I personally think those who cite the "improved" specs from NFB are missing the point, but to each his own. Not so different from those who prefer SS over tubes, who often cite the "less distortion" specs. No, I don't enjoy listening to distortion, just unwilling to sacrifice aural immediacy to lower it.
Edwyun, Yes, we have to realize that when they talk about zero feedback they mean global feedback. Amplifier can have many local feedbacks and they might still call it ZNFB. Why? Because ZNFB became catch phrase and sells amplifiers.
You stated the you would buy amplifier with zero output impedance, infinite bandwidth zero distortions etc. but I'm not sure everybody would. Some people would complain that it doesn't have good bass with their speakers (zero output impedance), others would call sound analytical and sterile (lack of distortion and zero noise) while some would call it bright (infinite bandwidth) since their system has tendency for it. In addition many people like warm sound (enhanced even harmonics) and would not be happy with this amplifier.
Shallow global feedback, properly used, can reduce distortions, widen bandwidth and lower output impedance WITHOUT introducing Transient Intermodulation.
As for getting rid of the equipment, I think you're right. For the money some people constantly spend on audio they could hire Symphony Orchestra to play for them.
"Mapman: The answer to your question is to do away with reproduction altogether. Sell all your equipment and go to stadiums, concert halls, jazz clubs, auditoriums, amphitheaters, subways, bathrooms, etc. Go live!"
Agreed! Live is the reference standard! Everything else is a reproduction all of which are flawed in some way. Choose your poison!
Abstract art is regarded by many as the most enjoyable! Its also one of the most highly distorted forms of art compared to other styles. The art is represented by the distortions. A photo would work best to reduce the distortions. A three dimensional hi res CGI model perhaps could do the best! Which of these would you prefer? Does it matter? As long as it affects you, I'd say it realy does not matter. I like variety! I'll take one of each!
Still the hi res CGI model is probably the most faithful technically, though still surely noticably imperfect.
Give it time. Technology will continue to advance to the point where the remaining distortions are below the threshold of human perception.
Sound reproduction is not as data intensive and will probably get there faster. I tend to think it may already be getting there in some cases these days.
Unfortunately, audiophiles may always still be futilely sweating the details that really no longer matter, though I suspect the #s that obsess on doing such things will continue to dwindle significantly.
OK, so I will go out on a limb and agree with Kijanki that educated application of NF = more faithful reproduction, in a pure technical sense, though as we all know, no solution is perfect and there are always potential drawbacks to deal with. In the case of NF, it would seem to be potentially higher IM distortion that just so happens to come into play in the most sensitive region of the human hearing spectrum where it matters most. That would up the ante to get it all done right!
Now I will go back and do more listening to determine further for myself who has done it right and who has not.
Mapman: The answer to your question is to do away with reproduction altogether. Sell all your equipment and go to stadiums, concert halls, jazz clubs, auditoriums, amphitheaters, subways, bathrooms, etc. Go live!
"What sound you like has nothing to do with original question. "
It does in that there is no concrete objective answer to the question possible.
It is a subjective question not an objective one to start with, so an objective/quantitative answer cannot be had.
Therefore one must practically rely on what they hear and prefer as data points towards obtaining a meaningful answer. All else is just a particular spin on the truth, and I do not mean that in a derogatory sense, only stating the facts.
Kijanki, I can think of nobody I would rather have solve that equation! Take your time! I can survive in the interim. :^)
"Only by testing, working with the devices and taking careful measurements will you find out what really happens. Relying on maths formulae (regurgitated ad nauseam) or 'common wisdom' is not always the best way to get to the truth."
So what did the author use to test his theories?
"For the majority of the tests described, the effects were simulated rather than measured. There are some very good reasons for this, with the primary reason being that the simulator has access to 'ideal' amplifiers. These have infinite bandwidth, infinite input impedance, zero distortion and zero output impedance. Being perfect, they also contribute zero noise."
I know of no amplifier that has infinite bandwidth, infinite input impedance, zero distortion, zero output impedance, and contributes zero noise. If anyone has one, let me know, I would like to purchase it.
Also, you did read his qualifications, right?
"PROPERLY IMPLEMENTED, negative feedback will invariably reduce distortion to levels that are well below audibility. Not just harmonic distortion, but the much more intrusive intermodulation distortion. IF DONE INCORRECTLY the results can be awful."
I agree with this statement. But therein lies the rub. And you do realize that in the 70's NFB was used to get distortion below audibility. But most agree that, as implemented, it sounded pretty bad.
Mapman, the question here is if NFB makes reproduction MORE faithful. Some answered that there is no faithful reproduction, which doesn't answer this question, while others stated they prefer sound of ZNFB which doesn't answer this question either. The truth is, that there is no SS amp (and very few tube amps) with zero feedback. SS amp by nature is a voltage source, and as such needs some form of negative feedback, at least for the output stage. The question is how much and that might depend on the load, your preference etc. It is undisputed fact, that even small amount of NFB widens bandwidth, lowers output impedance and reduces THD/IMD distortions. Does it make reproduction more faithful on average? I think it might. What sound you like has nothing to do with original question.
I'm working on this equation, you asked for, but it can take some time.
So your amp sounds best to you when you dial in a little feedback big deal. It does not tell you that all amps should sound better that way. It is very hard to design an amp without feedback. My hutch tells me that if your amp were designed to sound its best without feedback and then give you the choice you would prefer no feedback. This goes back to the Manley amp that allowed you to listen to their mono blocks in single ended mode or push pull mode. I liked it better in push pull mode and then thought that single ended amps suck. Oh not true, you can't design a circuit to sound good doing both you need different transformers and parts. So you see my friend your conclusions on a simple rotary switch is not viable. You need to keep an open mind which at that time I did not and now do.
That is a very informative article as are many of the replies, as usual.
Only thing of value I can think to add is yes, practically, it is how the whole system functions together that matters, not specific design paradigms applied to specific components. There are many ways to skin a cat. That phrase could never be more true than it is regarding the details of ways to deliver quality sound reproduction. Each will chose their preferred solution. VEndors will obviously also chose their approach and any vendor worth their salt will believe in their approach 110%. CEtain design approaches that are more unique or innovative, like no NF and one of my personal favorites, Walsh drivers, done well, help to make certain vendors stick out in the pack, which is important if you are in a business for the long haul.
When someone comes up with the undisputed mathematical equation for why I like what I like, please let me know! I am waiting patiently.....
Mt10425 and others have given very thoughtful answers and points of view. Dracule,you say you are`nt seeking validation but you responses suggest otherwise.There is no technical explanation to support your position,it`s simply your own preference as to what sound you happen to like.There`s no objective determination to seek in subjective issues as this is.Regardless of what forum or subject heading you choose to post on, the results/replies won`t change.You prefer the sound of NFB amps for the reasons you cite, it`s nothing more or less than that.Being condescending(as though you`ve seen the light now and other have`nt) to those who prefer a sound different from what you consider to be right or more"accurate" is pointless.Mt10425 is`nt off topic at all. Regards,
The degree of feedback needed is dependent on speaker that is driving. You make a valid point. The designer purposefully built feedback into the amp.
I don't necessarily understand the article fully as my technical knowledge is limited, but I can recognize a well thought out article when I see it. I wanted start a discussion on this subject, and see what others had to say. Like I said, this seems to be the wrong forum. Many here are subjectivists and do not care about objective analysis.
Dracule1: So you are looking for a technical explanation as to why you like and find more realistic your amp with NFB than with ZNFB?
I believe the technical explanation is in the article you provided a link to in your OP. Some may agree, others may disagree. I have read that article and others from the author. Some things I agree with and others I do not. He also states that there has been great disagreement on the issue. So why would you expect anything less?
If you agree with the article then fine. That's all there is to it.
Edwyun, I have an amp with adjustable feedback, so you can change feedback while keeping every other parameter constant. In this case, feedback can have a significant, if not drastic, effect on the sound. So stating feedback alone has little effect does not hold true in this case.
Mapman, I understand those who say just enjoy your gear, there is no best. I'm not looking for the best. And I am not looking for validation, nor am i unhappywith my gear. Some on this thread mistakenly make these assumptions about the intent of my thread. But that is not the point of this thread. I perceive NFB to be more realistic than ZNFB more often than not and I'm looking for a techical explanation. Perhaps I posted this in the wrong forum. Many have posted without reading the link which I think is pretty comprihensive on the subject. Tech Talk may be the more appropriate forum.
Mt, please start making sense. Start your own thread to vent whatever frustrations you have. You're just trolling here. You'll get no more response from me.
If you ge sucked into the music and it sounds bettef the longer you listen, you have a GOOD amp. Be happy. NF design is just one aspect. Alone, it means relatively little. No nfb designs in general might be better overall and there may be fewer nfb designs that sound "irritating", and ghere are certainly many mediocre products that use, nfb, but there are also many i have heard where fatigue whatever the cause is not an issue. No abolute good, bad, rights, wrongs here as is usually the case. If nf scares you do not buy products that use it. But if you have good ears you trust i would not let anybody scare yiu away from what sounds good based on limited theoretical arguments.
I don't assume anything. You're the one arguing with everyone who states personal preference is important to them. The topic is, and has been, is that your viewpoint is right (in an absolute manner, backed by some written babble which you ascribe to as being definitive). It's not a personal attack, I don't know you, nor do I care to. This is just more of the same drivel which shows up too often. Your 'having your fill of single driver fostex speakers' is irrelevant. Again, who cares?
Phaelon, you may be thinking of Rethem speakers which have powered subwoofer drivers with a wide bander taking care of the rest of the freq. they are coming out with low power ZNFB amp for their speakers. I think with powered subwoofers the low freq limitation of most ZNFB amps is negated while preserving the upper range qualities some people seek.
Mt, I'm sure your system sound awesome to you, but I've had my fill of single driver fostex speakers with low power tube amps which have significant drawbacks IMO. But you're getting off topic, and I do not see a reason to resort to personal attacks. Somehow you think you know more than I do and assume I am missing something important. Can you please stay on topic?
Im pretty sure that I recall some speaker manufactures recommending ZNFB amplifiers for their speakers. I always took it for granted that there was a scientifically based rationale for this. Could these be instances where NFB vs. ZNFB isnt a totally subjective decision, or are the speaker designers simply expressing their personal preference?
You're having some difficulty 'getting it'. It's not a 'type of sound'. I have a ten watt push/pull tube amp feeding 95db fostex single driver speakers. I've owned many different combinations over my life (60 this year). It took me until about two years ago to realize how much more enjoyable (FOR ME) my 3500 LPs sounded. I have been listening for more than four decades to this music previously. I don't expect you to understand what I hear. Here's the part you fail to comprehend. Who cares? It's about personal enjoyment. Real life...it's all perception...visual, auditory or tactile. Your search has no definitive end.
Kinjanki well said. That had been my experience with NFB as well.
I'm not looking for validation. I've been in this hobby for close to 30 years and I know what I like. I'm searching for technical explanation of why we hear what we hear.
I have listened to SET amps paired with high efficiency speakers. There's no denying the beautification of the midrange which can sound glorious and involving. But in the long run I know it's the way things sound in real life. And I do understand why some gravitate to this type of sound.
Ed: Pass does indeed up simpler circuits. His XA.5 and X.5 series apparently have but 2 gain stages. So, in that case would feedback be 'global' or 'local'? Also, Pass amps I see the specs for are about 26db gain...which is on the low side for people accustomed to 30db PLUS gain.
If I could afford it, I'd get Pass amps for my panels....I see a pair of XA30.5s at Reno which would be perfect for a biamp setup! (another can of worms)
Other posts address feedback without ever defining if they are speaking to Global or Local. It makes a big difference...
Many people like sound with some distortion that makes it more alive (distorted guitar vs clean guitar). My Benchmark DAC1 sounded too clean at first but it doesn't anymore (learned to listen). Many accused DAC1 of sounding sterile. When they compared different DACs in studio Benchmark was the closest to live sound but not favored by most of testers. One person expressed opinion that all instruments sound separately while he likes them more "together" (sound blob?). Reducing NFB increases distortion making sound more "alive". It also reduces bandwidth making bright systems sound more civilized. In addition it increases output impedance making impression of more bass on over-damped speakers with SS amp. All is a matter of taste. I prefer super clean highly resolving sound with very tight bass, while others might prefer some distortion or noise with rounder bass, calling sound more "organic".
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