Measurements and the Correlation of What We Hear

[mep]

By the way, to ensure clarity here, if you are stating I said something, please quote it exactly least we keep running about in circles here!

I said, many or most tube pre amps have 100% feedback circuits. You said, show me one.

And you still haven’t.

Someone posted a tube pre-amp link and that did not show the schematic but they praised their regulated power supply.

I said, well, thats one circuit that is 100% feedback.

And you were wrong. The power supply does not introduce any type of feedback of the circuit signal back into the circuit. If you want to keep saying it does, please show through engineering how this is possible.

Next, I found written evidence (I already have known this for 35 years) about cathode followers being 100% feedback.

Wrong again Tom. Having 100% feedback means you have unity gain. A cathode follower is not a unity gain device. So I don’t know what book you read that has “written evidence” that cathode followers cause 100% feedback. If the book really says that and it’s not your interpretation of what the book actually said, the book is wrong.

Now, not everybody agrees with the term "feedback" when it comes to cathode followers.

Now there’s an understatement-especially when you thrown in the 100% feedback number.

 

[Orb]

Not sure I have ever used percentages in feedback; what does "100% feedback" mean? 100% with respect to what? Or does it mean every stage has feedback, and/or in addition to global feedback? I am confused......

Yeah I agree Don,
because I am used to seeing the feedback calculated in terms of open loop gain with X dB of negative feedback (from the perspective of input stage-VAS-output stage).
So how would one decide the % when comparing very similar amp circuitry where one ulitises small amounts of feedback and the other around 70dB of feedback?
The effect of negative feedback will be different for these figures but in terms of semantics it seems it is being argued they are 100% feedback.
And on top of this we have the global feedback vs degeneration.
Just my take on an interesting topic.

Thanks
Orb

 

[fas42]

To throw a bit more into the mix, there's one very good reason why high levels of amplifying circuitry feedback have a bad rap. When designers look at audio circuits they tend to either look at the power supply in isolation, or at the audio circuit assuming a perfect power supply, very few seem to see these two sections of circuitry do not operate as separate entitities but are part of the whole. So in that sense Tom is right, feedback in the power supply IS part of the amplifier circuitry.

My point? If the power supply is dodgy then high levels of feedback in the amplifying side is working flat out to compensate for the deficiences there, let alone worry about what the speaker drivers and crossover networks are doing to the poor output stage. Quite often this battle is lost, hence crappy, compressed, sterile sound when you wind the volume up ...

Frank

 

[DonH50]

Your description here leads me to believe you're referring to Middlebrook's technique from 1975, normally implemented with a loop gain probe in SPICE. But if you try that technique with an emitter follower, it does not work to find the loop gain. To properly analyze the loop gain of an emitter follower, it's necessary to use Middlebrook's GFT, which is a much more recent development. It's not a well-known technique, and when I first tried to figure it out, it took me a while. I'd call the loop gain probe technique straightforward, but I'd also argue that the GFT is anything but. That's because there are different "GFT templates" required for different circuit configurations, and one must figure out the appropriate one to use for a given circuit. (Edit) With the GFT, there are also restrictions as to where the probe can be placed, while with the 1975 loop gain probe, anywhere inside the loop is fine.

100% feedback is a very common term. It's where the ratio of the signal fed back to the signal sampled is 1, such as a unity-gain op-amp circuit. Or an emitter follower (considering Vbe as the error voltage) .

Ah, you are right, I missed your direction. I'll have to download and read the GFT paper (easier to going through my old files). At one time thought I understood it , but it is not something I have used much. Using the (earlier method's) voltage and current transfer functions (Tv, Ti) leads to much better results (at least, closer to reality) than the simpler methods of breaking the loop and probing (e.g.) just voltage. I had forgotten that GFT requires special probe placement; brings back some painful memories. Designing high-speed circuits as I have, I usually used very little feedback, mostly just emitter degeneration, and now and then closed the loop around a single stage.

Unity-gain with 100% feedback as you describe I am familiar with; I was not sure how the term was being applied here. I have a vague memory of working problems that way, but the vast majority of my classwork (that I remember) and certainly career-work used feedback factor and loop gain terms not expressed in percentages. Maybe because I went to one of the other Southern CA schools for my grad degree...

Mark, in the sense that andy_c brings up, a follower is indeed a "100% feedback" circuit, but it is very rarely treated that way because there is no obvious external feedback loop. I probably have not analyzed it that way (as a feedback circuit) since grad school so maybe I can blame poor memory? I have always treated it as an "open-loop" circuit because of that (andy_c may well disagree). One of my instructors (S. Knorr, sharp guy, passed away a few years ago) analyzed followers (and everything else) using feedback theory.

Orb, a regulated supply has feedback (as I know you know), but few power amplifiers regulate their supply (tube or transister). While PSRR can relate power supply variations to signal variations, I am not sure how I'd relate the supply's feedback factor to the signal in any other manner (but it is not something I have thought about).

I admit by now I have forgotten what we are talking about; I have a dim memory of this thread beginning with Mark trying to clarify the relationship between hearing and specifications and/or measurements? At any rate, best I stay out of this one...

 

[Phelonious Ponk]

I know I should let this one lie, but I've just got to know...Mark, when, hypothetically speaking, Krell published specs for your new amp and rated THD @<0.003%...where do you think they got that number?

Tim

 

[fas42]

but this one is from the bible. The holy grail of tube audio.

Title:

Radiotron Designers Handbook
fourth edition, 1953
page 317, quote from the chapter entitled "negative feedback"

I quote: " As a result of 100 per cent, negative voltage feedback inherent in a cathode follower, both the distortion and output impedance may be very low." end quote.

If one has never heard of this book, then one has never heard of the "tube" bible.

Hmmmm, I wonder if this is an example of Tim's "facts", as mentioned in his tag line ...

Frank

 

[Gregadd]

"As a result of 100 per cent, negative voltage feedback inherent in a cathode follower, both the distortion and output impedance may be very low." end quote."

I certainly learned something here.


I think we may be mixing Apples and Oranges here. I don't like to deal in absolutes. The criticism I have encountered concerning negative feedback did not involve cathode followers. In fact it was concerning excessive amounts of global feedback to cure an otherwise incompetent solid state design. My favorite amp uses some local feedback. We all fall into that trap of "if less is better then none must be perfect." That's why we take two pills when one is all that is recommended.
The question is how much feedback? For what purpose. Is the cure worse than the problem

 

[andy_c]

The question is how much feedback? For what purpose. Is the cure worse than the problem

There's an article by Bruno Putzeys (PDF file here) that might shed some light on the subject.

 

[Phelonious Ponk]

Hmmmm, I wonder if this is an example of Tim's "facts", as mentioned in his tag line ...

Frank

Oh no...I had in mind things much more fundamental than that.

Tim

 

[mep]

So, I have to quote myself, because nothing new here for me to say, except, I found the book that is wrong according to Mark.
Also, that is enough research for me for this thread, as it took quite a while for me to find an authoritative, tube book, with an exact quote in it.

There are many quotes on the web, but this one is from the bible. The holy grail of tube audio.

Title:

Radiotron Designers Handbook
fourth edition, 1953
page 317, quote from the chapter entitled "negative feedback"

I quote: " As a result of 100 per cent, negative voltage feedback inherent in a cathode follower, both the distortion and output impedance may be very low." end quote.

If one has never heard of this book, then one has never heard of the "tube" bible.

Not only have I heard of it, I have it right in front of me, the exact same version you have. It used to belong to my uncle who is a retired EE. The book means a lot to me because of that. Anyway, you read what you wanted to read, but what you are saying is not correct in the context of the feedback controversy you stirred up when you said “almost all” tube preamps have 100% feedback because of using a cathode follower. What my extremely smart EE friend said the other day which I posted his comment is still true and is verified by what the “bible” says.

Again, the relevant part of what he said about cathode followers: “A follower of any type (cathode, emitter, source) is just a (near) unity gain buffer, and so there's really no feedback mechanism that achieves the unity gain; it's just a property of that topology. This is very different than say a unity gain configuration of an op-amp where the open-loop amplifier has very high gain but the feedback reduces it to 1. “ This statement basically mirrors what is said in the RDH: “As a result of 100 negative voltage feedback inherent in a cathode follower…” and again what my friend said;” So maybe it's just semantics, but if I don't have to check a Bode plot for the effect of the "feedback" (notably stability), I'd consider it to be an intrinsic property of the topology instead of a true feedback mechanism. The RDH goes on to say on page 317 that since the stage gain is always less than unity, the input voltage is always greater than the output voltage.

If you still want to fan the fires on the smokescreen of “most all” tube preamplifiers have 100% feedback, please find one preamp that meets your criteria for having 100% feedback and the designer of said preamp is still alive, I will write him and ask him how much feedback is in his design and publish his answer.

I will be glad to hear from anyone who can explain the operation of a cathode follower without invoking feedback, whether you call it intrinsic (as Marks EE did without, I might add, then explaining how it worked), or whatever, and if it is not 100%, then fine, show me or explain then what percent of feedback it is.

Tom, the “bible” you quoted said the same thing my friend did. The bible said it’s inherent in a cathode follower and my friend said it’s intrinsic. They mean the same thing. It is not a true feedback mechanism.

 

[Gregadd]

What does negative feedback accomplish?

The use of global negative feedback does several things: it flattens and extends the frequency response, it reduces distortion generated in the stages encompassed by the feedback loop, and it reduces the effective output impedance of the amplifier, which increases the damping factor. All of these things affect the tone in some manner.

The flattened, extended frequency response obviously changes the tonal character by removing "humps" in the output stage response and producing more high and low end frequencies.

The distortion reduction makes the amp sound cleaner and more "hi-fi", up to the point of clipping, with less output-stage generated noise.

Perhaps the main difference for the "feel" in a negative feedback amplifier, as opposed to a non-negative feedback amplifier, is the increased damping factor produced by the negative feedback loop. The decreased effective output impedance causes the amp to react less to the speakers. A speaker impedance curve is far from flat; it rises very high at the resonant frequency, then falls to the nominal impedance around 1kHz, and again rises as the frequency increases. This changing "reactive" load causes the amp output level to change with frequency and changes in speaker impedance (a dynamic thing that changes as the speakers are driven harder). Global negative feedback generally reduces this greatly. This can be good or bad, depending upon what you are looking for. Negative feedback makes the amp sound "tighter", particularly in the low end, where the speaker resonant hump has the most effect on amplifier output. This is better suited for pristine clean playing or a tight distorted tone, while a non-negative feedback amp has a "looser" feel, better suited to a bluesy, dynamic style of playing.

The other disadvantage of a negative feedback amplifier is that the transition from clean to distorted is much more abrupt, because the negative feedback tends to keep the amp distortion to a minimum until the output stage clips, at which point there is no "excess gain" available to keep the feedback loop operating properly. At this point, the feedback loop is broken, and the amp transitions to the full non-feedback forward gain, which means that the clipping occurs very abruptly. The non-negative feedback amp transitions much more smoothly into distortion, making it better for players who like to use their volume control to change from a clean to a distorted tone.

http://www.aikenamps.com/NegativeFeedback.htm

 

[Orb]

There's an article by Bruno Putzeys (PDF file here) that might shed some light on the subject.

I was going to link his masterclass document but decided against it, this is much better for this discussion.
Thanks for the link.

Really there should be a seperate thread on negative feedback as it is a very interesting subject, both technically and subjectively.
What adds to the interest IMO is the stance of various engineers, either for or against feedback - Nelson Pass being one of the famous for being critical of it, however I think he is in agreement with Bruno that if you do use it then use a lot.
Nelson's own paper is interesting on Negative Feedback.
Also there is an academic mathematical model paper showing different behaviours regarding feedback -although some of the circuit-models are not ideal.
Don, yeah good point in your responses.
Cheers
Orb

 

[fas42]

http://www.aikenamps.com/NegativeFeedback.htm

At first I thought this quote was hilarious, until I read enough and checked the link: this is about guitar amps, where the whole point is to use the amp as a musical instrument in its own right, what Tim talks of. If you want your system to be accurate, rather than a tone control or effects unit with built in amplifier, as Tom would say, then plenty of correctly applied feedback is a very straightforward method for achieving that.

Frank

 

[Gregadd]

At first I thought this quote was hilarious, until I read enough and checked the link: this is about guitar amps, where the whole point is to use the amp as a musical instrument in its own right, what Tim talks of. If you want your system to be accurate, rather than a tone control or effects unit with built in amplifier, as Tom would say, then plenty of correctly applied feedback is a very straightforward method for achieving that.

Frank

And the point would be that excessive feedback colors the sound. The antithesis of what some are arguing.

 

[Phelonious Ponk]

Wow...lower distortion, FR extension, better damping, minimum amp distortion before clipping. Gotta say this negative feedback stuff sounds pretty bad...

Tim

 

[MylesBAstor]

Wow...lower distortion, FR extension, better damping, minimum amp distortion before clipping. Gotta say this negative feedback stuff sounds pretty bad...

Tim

Until you play with equipment that has adjustable NFB or zero.

 

[Phelonious Ponk]

And the point would be that excessive feedback colors the sound. The antithesis of what some are arguing.

Greg, that's the opposite of what the article you linked said. It said feedback flattens and extends FR, lowers distortion, increases damping. That is not coloring the sound.

Tim

 

[fas42]

Until you play with equipment that has adjustable NFB or zero.

This highlights the dilemma that the industry has: if a system is only 98% on song then it can sound pretty horrible, so the trick, or easy solution, is throw lots of nice syrup over the sound to ameliorate the problem. The other technique, to get the system 100%, or even 99.8%, on song is much harder, far less favourable to having push button, over the counter, solutions so is largely ignored or disregarded ...

Frank

 

[MylesBAstor]

Greg, that's the opposite of what the article you linked said. It said feedback flattens and extends FR, lowers distortion, increases damping. That is not coloring the sound.

Tim

And you believe that NFB is a free lunch?

 

[fas42]

And you believe that NFB is a free lunch?

I think it's time to bring in one of these Discrimination or Villification Acts into the room, dunno which one, to help out poor old FB, he's being kicked around in the gutter quite a bit ...

Frank