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:
I 'm glad to see most of the replies are based on common sense and actual listening experiences.Which ever amplifier type sounds best to you, that's the one to buy. No need to cite this paper or that article to strengthen your preferences. I don't know why people need validation for their choices. For me, I'll take a well designed ZNFB SET amplifier anytime.It matters none at all if someone prefers something completely different, so what.Make a choice and stop worrying if others disagree. Different ears (and brains)will choose accordingly, imagine that. Regards,
it's adjustable on my amp (Mesa Baron,) so it is easy to hear the differences. I have ears, so I do not really care about the technical rationale. Some music sounds better with a little NFB (primarily studio produced rock), but most music sounds better without any NFB, at least with my system. In general, the sound is a bit less "alive" with NFB, but if a record is made with lots of compression, that hardly matters, and other factors come into play.
I am by no means a technical expert and have enjoyed both NFB amps and ZNFB designs, but I did want to note the perhaps it is a good idea to bring the human brain into this conversation (I am a neuroscientist, so I can't resist). Namely, it is fine that we are able to measure amps on the bench, but we also need to acknowledge that our ultimate enjoyment of an amp-speaker combo is based on how our brains process the signals that reach our ears. Psychoacoustics plays a major role that can't be overlooked, such as determining what frequencies we can hear most acutely and which order of distortion is more noticeable.
Also lurking in each individual brain are personal preferences. Nelson Pass' First Watt line of amps is a great example of a wide variety of design choices that are intended to both give a good match with certain types of speakers and to provide different flavors to account for different listener preferences.
"Curious, Have you ever listened to SET amps (hopefully with the proper speaker)?"
Onemug, I have listened to some SET amps and OTL amps, all operating in Class A (oh no, another amp design principle!). In my setup at the time, I didn't feel they had the overall extension at both extremes or bass slam I like. Perhaps I should have tried more expenses tube amps. =) But they were very good in the midrange and presence. I have since gone to a bi-amp system and eventually may use some SETs or OTLs amps for their benefits in the M/H frequencies (currently using Aleph 2s, which do have some SET characteristics)) and use SS amps for their bass slam in the L frequencies. It's all a balancing act.
Dracule1, I glanced over article and it seems to cover pretty much everything. I will read it in detail.
Stereophile also conducted experiment with adjustable feedback amplifier and they liked sound at minimum feedback. It doesn't mean that feedback at minimum was zero. It is virtually impossible to make SS amp without any feedback since even emitter resistors are form of feedback. The issue is not to use too much feedback. It comes to good very linear design to start with. Testing with sine waves doesn't show real picture (presence of TIM). You can have two amplifiers that measure identical but one is sounding very harsh/bright. Often amplifier that measures better sounds the worse. In early 70's before TIM was discovered SS amplifier's had huge negative feedback and THD in order of 0.001% (absolutely no need for that). In cases like that TIM can get so bad that output transistors choke on big overshoots (charge trapped at the junction) creating small gaps in music. Our brain fills them up but it makes us tired. I agree with Elizabeth that ZNFB is a catch phrase since amplifier with some amount of feedback will most likely sound better. NFB is a valuable tool that should not be used to cover poor design, cheap components etc.
Love your post and reside on your side of the fence. Curious, Have you ever listened to SET amps (hopefully with the proper speaker)? Pass is my favorite brand of ss amps.
fwiw, I don't think there is a correct side of the fence. We have our bias and whatever design gets our attention wins at least for the time being (audiophiles have been known to change their mind).
One of my amps has a variable feedback dial that goes from 0-12 db. I hear the bass firm up when I dial it in but it loses something in the overall sound that I bought the amp in the first place for.
Adjustable NFB is a great way to prove individual preference, not a technical absolute. You used the phrase 'more realistic sound'. To you. We're engaged (yet again) in a thread of trying to prove others to be less knowledgable, savvy and just downright wrong in their audio beliefs compared to the OP. Why? It's great you've seen the light. Is validation that important to you?
IMHO, NFB and ZNFB are different tools (among many, many others) that an amplifier designer has at his/her disposal to get the best performance (or let's say, for our discussion, the best faithful reproduction) given the amp's design parameters.
Some amps are designed to work better with NFB while other amps are designed to work better with ZNFB. So I don't think anyone can categorically say one method is better than the other for faithful reproduction, whatever that means.
Having said that, I personally prefer amps with the simplest circuit designs because I subjectively believe (rightly or wrongly) that the less capacitors (not power supply caps!), resistors, boards, etc., in the signal path means there is potentially less chance that there is going to be any change to the signal from input and output at the amp. That, to me, means ZNFB. But that also means that the amp design has to be inherently linear so that it doesn't need NFB to begin with.
I tend to prefer Nelson Pass designed amps - from Thresholds, Pass Labs, and First Watt. Almost all are ZFB designs. However, even Nelson Pass uses NFB in some of his amp designs because it works for that particular amp.
So, if you like a particular amp that was designed with NFB or with ZNFB, and it sounds good to you, then listen and enjoy it.
Elizabeth, I have and owned amps that have adjustable NFB, and you can clearly hear a difference between different levels of feedback. IME, the higher feedback setting produced more realistic sound. So your contention feedback has little to do with how a product sounds is in direct contradiction to my experience. Do you have any amps with adjustable feedback? If you do, I would be surprised if you can't hear a difference.
Hi Elizabeth, what is a current fad or "catchphrase" is not important to me, but the finding the underlying reason for "catchphrase" or current fad is. If I did't think NFB or ZNFB made a difference in sound, I wouldn't have made this inquiry.
""You compared the input signal to the output signal." Just how simplistic is this? So, I'll make this easy..."
Oh no you didn't. You didn't just try to dis me? ;) I don't think you are being condescending, so I'll give you benefit of the doubt and assume you are asking a genuine question. If that is the case, I thank you for your response.
"...if the two signals are exactly the same but one has 3% second harmonic distortion, and the other has .8% of the, more jarring, 7th harmonic distortion, which is the more "faithful"? Please answer my question."
Sure I'll answer. The result is the result and can't be disputed assuming you measured it correctly. What you're talking about is the INTERPRETATION of the result, which can be disputed. I do not know how a 3% 2nd harmonic distortion sounds relative to a 0.8% 7th harmonic distortion. But if the 7th harmonic distortion is say 80 dB down, I doubt I could ever hear it. Now 3% 2nd harmonic distortion may be audible, so I would consider that less faithful reproduction than a "jarring" 7th harmonic distortion I can't hear. Your interpretation may be entirely different than mine.
I can see how some might consider my input/output comparison simplistic. So let me explain. Why is it that a square wave is one of the industry standard of measurement of how an amplifier performs? Music does not contain square waves as far as I know. And I doubt if anyone considers a square wave a close facsimile of a music signal, which what an amplifier is suppose to reproduce. Wouldn't a more appropriate test signal be that of an instrument at certain frequency (eg, 1 kHz piano, guitar, or sax tone). Or even an instance of music containing many instruments. Is this technically too difficult to perform? I am relatively new at looking into the technical side of amplifier circuitry and performance, so my questions may seem naive to those who have years of technical experience. But this is one of the reasons why I started this thread. I would appreciate input from experienced individuals who would take the effort to be informative. As in any controversial thread, there will be individuals who will have nothing better to add other than dumb sarcastic or condescending comments.
Negative feedback reduces harmonic and intermodulation distortions, reduces output impedance and widens bandwidth. This can be easily measured. Negative feedback, especially a global one, can introduce Transient Intermodulation Distortion (discovered in 70's) that enhances odd order harmonics making sound bright and unpleasant. Loudness cues are in these harmonics and our ears are very sensitive to them. Transient Intermodulation can be completely avoided if bandwidth of the signal is limited at the input of the amplifier to value that amplifier had without feedback. For instance, if measured bandwidth is 30kHz without feedback then it has to be input limited to 30kHz after feedback is applied (that increased amps bandwidth). It is harder to design an amp that measures good with shallow feedback and often designer chooses to improve specifications at the cost of the sound. I would start width the best widest bandwidth most linear design and apply only enough feedback to reduce THD to 0.1%. After that I would temporarily disconnect feedback and measure bandwidth with very small signal. Finally I would limit bandwidth at the very input of the amp to this amount. It is a tradeoff between THD, IMD and the bandwidth. If you want to lower THD, IMD by use of feedback then you need more gain and amp has lower bandwidth at the higher gain. Bandwidth will increase greatly with feedback but not limiting it at the input to original one will enable TIM.
Transient Intermodulation is practically an overshoot (in time domain) caused by the fact that amplifier, having limited bandwidth, introduces signal delay. This delayed signal is subtracted at the input reducing gain of the amplifier. For instance 40dB negative feedback means that amplifier has 100 times bigger gain without the feedback. Feedback lowers this gain to normal except for the moment of fast transition of the signal because feedback signal is subtracted a moment too late (being delayed) and amplifier has for this short moment 100x higher gain. It causes overshoot of transitions (enhanced odd harmonics) but doesn't show at sinewaves, normally used to test THD and IMD.
Roger Modjeski has some interesting views on this topic and the application of negative feedback in designs. FWIW I have an amp he designed that uses negative feedback and another that doesn't. I enjoy them both quite a bit.
Charles1dad and Viridian, did you read the entire article in the link? I think he raises valid points about nature of distortion and feedback in amplifiers. We can't all be complete subjectivists here. Afterall, it's mostly hard science and technology have produced these wonderful high end products we enjoy, with a little bit of snake oil sprinkled in of course :)
This has been discussed many times with same division of opinions. The accuracy position has no absolute definition, nor does musicality and natural camp proponents.In the end we all choose components based on how they sound (our ears). Those who prefer the presentation of NFB designs should obviously buy them just as others who prefer ZNFB components should buy those. It's subjective and there's no avoiding that point.Trying to determine which side is right is fruitless. Buy what sounds best to you, you'll be happier.What does it matter if someone has different listening preferences. I'm glad there're multiple topology alternatives avaliable. There is no objective standard that is going to solve any thing. Regards,
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