PUDDING...
Hi,
LOL.
Yes and no the feedback artifact is audible, not necessarily the delay as such...although that too could play a role.
Is it? Subjective is more like it.
Yes, an objection, perhaps, would be that I hear it but can't measure it yet, objections to that?
I object to heaving dog ears, can't help that...
Hi,
LOL.
Yes and no the feedback artifact is audible, not necessarily the delay as such...although that too could play a role.
Is it? Subjective is more like it.
Yes, an objection, perhaps, would be that I hear it but can't measure it yet, objections to that?
I object to heaving dog ears, can't help that...
Steve Eddy said:
...NO...Mikek is not saying that waves 'propagate at infinite velocity'....he is suggesting wave propagation has nothing to do with electron flow, (or drift)...period...
...see university physics, (extended edition with modern physics), 9th edition, by profs' young and freedman, pg 800~803 .....this should show why my 'water in a hose' analogy is sound..
mikek said:
That's even worse.
The electromagnetic fields which make up the propagating wave are the direct result of electron drift! You can't have one without the other. The two are inseparable.
I'm afraid you're the one who needs to read it. And keep reading it until you actually understand it. Your water in a hose analogy is not sound at all.
The pressure wave does not propagate down the hose with infinite velocity either. The water molecules at the end of the hose will not begin to flow until the pressure wave has reached them, which will be some time after the pressure wave has been initiated at the beginning of the hose.
Pressure waves propagate through water just as they propagate through air (they propagate faster through water because it's more dense). So let's use an analogy you perhaps might more easily be able to grasp.
Let's say you're standing 50 yards away from me in an open field and you have a gun. You're sick and tired of hearing me talk about this stuff so you're going to shut me up once and for all.
You fire the gun.
The explosion initiates a pressure wave in the air, otherwise known as sound. Or less formally as "BANG!."
Do I hear the bang instantaneously? If your water hose analogy were correct, I would have to. But I don't. Why? Because the acoustic wave propagates with a finite velocity. Around 1,130 feet per second.
So, you're 50 yards away. That's 150 feet. 150 divided by 1,130 is 0.133. So there's a delay of 133 milliseconds from the time you fire the gun until the time I hear the bang.
The air molecules in the viscinity of my eardrum don't do anything they weren't doing before you fired the gun, they're just banging around randomly due to the thermal energy in the atmosphere. And they won't do anything different for another 133 milliseconds.
The acoustic wave is analogous to electromagnetic waves and the air molecules are analogous to the electrons. And just as there is a delay from the time you fire the gun until I hear it, there is a delay from the time a signal appears at the output of an amplifier until that signal reaches the input by way of the feedback path.
se
Sjeee………… ughhh,
A world without feedback does not exist. It is as simple as that.
Considering our well regarded audio hobby, even in a single triode amp stage there is feedback: The anode-grid feedback internal to the tube. And yes this involves delays. And what about the EMF of a voice coil in a loudspeaker interacting with the output impedance of an amp? Back to earth please
A world without feedback does not exist. It is as simple as that.
Considering our well regarded audio hobby, even in a single triode amp stage there is feedback: The anode-grid feedback internal to the tube. And yes this involves delays. And what about the EMF of a voice coil in a loudspeaker interacting with the output impedance of an amp? Back to earth please
Re: Re: Re: Re: Re: Re: Re: Re: Harold S. Black.
I wonder if you really understand anything about negative feedback. First off, I said there are several poles in the open loop gain. Secondly, a stable system does not have to be single pole system to be stable. I am not sure you understand the concept of phase margin.
I am puzzled by discussion of nonlinearity being academic since it lays at the core of this discussion. Rending the effects of nonlinearities feedback as negligible is a little broad since some on the DB listening test discussion revolved around the consideration of what level of distortion for two different amps made them objectively equal. The term negligible leaves this wide open for interpretation of what is actually negligible.
The "two hoots" statement I will leave alone other than say I am glad we are at least discussing a numerical quantify at some point in this post.
You made a statement about the limitations of Spice modeling without knowledge of the underlying concepts behind how a circuit actually works. I agree and assumed that you were a practicing engineer or maybe a graduate student, In either case, may I suggest revisiting a good book on control systems. Of course this suggestion is academic.
mikek said:
I wonder if you really understand anything about negative feedback. First off, I said there are several poles in the open loop gain. Secondly, a stable system does not have to be single pole system to be stable. I am not sure you understand the concept of phase margin.
I am puzzled by discussion of nonlinearity being academic since it lays at the core of this discussion. Rending the effects of nonlinearities feedback as negligible is a little broad since some on the DB listening test discussion revolved around the consideration of what level of distortion for two different amps made them objectively equal. The term negligible leaves this wide open for interpretation of what is actually negligible.
The "two hoots" statement I will leave alone other than say I am glad we are at least discussing a numerical quantify at some point in this post.
You made a statement about the limitations of Spice modeling without knowledge of the underlying concepts behind how a circuit actually works. I agree and assumed that you were a practicing engineer or maybe a graduate student, In either case, may I suggest revisiting a good book on control systems. Of course this suggestion is academic.
I think we all agree that a perfect amplifier will never be built. If I know would say we could try to agree on what errors and how much of it can be tolerated. But I am pretty sure this would end up in some kind of war
If I might add my opinion:
I think an amp should have reasonable overall feedback (i.e neither 0 dB nor 120dB). The open loop gain should have it's f3 between 10 kHz and 20 kHz in order to guarantee constant THD values over the audio frequency range (even if one would most probably not notice the difference between a 10kHz sinusoid and a rectangular of the same frequency).
The unity-gain point of the feedback loop should be significantly higher than the highest spectral content one is about to send into the amp.
It doesn't matter whether an amp has 1% or 0.0001% THD at full power since it is veeeery seldom used with it's full output power. More important is low THD and IMD between very low and low levels since an am is most of the time used within this range. Additionally the ear adds very little THD on it's own at low SPLs.
Regards
Charles
P.S. A test done by a German mag showed that the Pass X-series have very low THD at low levels, veeery gently rising with increasing output power. THD is almost frequency independant within the audio range and harmonics are rapidly falling in level with increasing order.
Congratulations Mr. Pass !!
If I might add my opinion:
I think an amp should have reasonable overall feedback (i.e neither 0 dB nor 120dB). The open loop gain should have it's f3 between 10 kHz and 20 kHz in order to guarantee constant THD values over the audio frequency range (even if one would most probably not notice the difference between a 10kHz sinusoid and a rectangular of the same frequency).
The unity-gain point of the feedback loop should be significantly higher than the highest spectral content one is about to send into the amp.
It doesn't matter whether an amp has 1% or 0.0001% THD at full power since it is veeeery seldom used with it's full output power. More important is low THD and IMD between very low and low levels since an am is most of the time used within this range. Additionally the ear adds very little THD on it's own at low SPLs.
Regards
Charles
P.S. A test done by a German mag showed that the Pass X-series have very low THD at low levels, veeery gently rising with increasing output power. THD is almost frequency independant within the audio range and harmonics are rapidly falling in level with increasing order.
Congratulations Mr. Pass !!
Re: Re: Re: Re: Re: Re: Re: Re: Re: Harold S. Black.
...Looking back at my control systems results, (a long time ago now), i reckon there was'nt i single occasion that i was'nt top of my class...
....beyond this, i think this particular discussion has outlived its usefullness.....
Fred Dieckmann said:
...Looking back at my control systems results, (a long time ago now), i reckon there was'nt i single occasion that i was'nt top of my class...
....beyond this, i think this particular discussion has outlived its usefullness.....
Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Harold S. Black.
Long time ago when I did my control theory, there were many students that were far on top of me. They got always high numbers for their theoretical proofs: By filling out the right formulas at the right places. They did rehearsal over and over in advance for it. But actually they did not understand a jot what it was actually about. Nearly all failed to get a practical temperature controller working (because there was a few seconds delay in the control loop )
Hmm, weak point Mikek,mikek said:
...Looking back at my control systems results, (a long time ago now), i reckon there was'nt i single occasion that i was'nt top of my class...
Long time ago when I did my control theory, there were many students that were far on top of me. They got always high numbers for their theoretical proofs: By filling out the right formulas at the right places. They did rehearsal over and over in advance for it. But actually they did not understand a jot what it was actually about. Nearly all failed to get a practical temperature controller working (because there was a few seconds delay in the control loop
)Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Harold S. Black.
Run away! Run away! Run away! Run away!
P.S. You misspelled "wasn't" and "usefulness", and "I" is capitalized....
mikek said:
...Looking back at my control systems results, (a long time ago now), i reckon there was'nt i single occasion that i was'nt top of my class...
....beyond this, i think this particular discussion has outlived its usefullness.....
Run away! Run away! Run away! Run away!
P.S. You misspelled "wasn't" and "usefulness", and "I" is capitalized....
Attachments
My gosh, this was one of the more useless threads I have seen on here in a long time...... [There, that should add to the inflamatory nature of this thread!]
Am I missing something here? Why are people discussing the amount of time it takes for a "feedback" signal to propogate through a small piece of wire? Isn't that completely missing the point? Is not the greater issue the propogation time (or in reality the open loop bandwidth at a given frequency) through the amplifier itself that is at issue? The time it takes for the signal to propogate back to the input difference amp from the output is insignificant [1.5nanosec?] for all intents and purposes. Think of it this way. You have an amplifier with no signal. Hence the input is near ground and the output is near ground. You then apply a signal to the output. The output, almost instantly, starts changing. However, how fast it changes, is determined by the open-loop gain. The change at the output is fed back to the input and which causes the output to change and so on and so forth. Of course this is a continous process.
So some points to make:
If you had infinite open-loop bandwidth and infinite open-loop gain, then an NFB amplifier would be perfect. However, we don't so we need to make a good trade-off between open loop gain, open loop bandwidth, and how good it sounds (which is a combination of THD with sine waves that have only some bearing on music), overall amplifier stability (or is this thing going to be able to accurately reproduce the output into a speaker), etc. We will through into this whole mix, that there are actually pure delay elements in amplifiers. Most capacitors, whether they are real capacitors, or a semiconductor junction have some level of dielectric absorption and hence can add in essentially pure delay.
If water was completely uncompressible, my water hose was infinitely stiff, and I had control over air pressure in real time, then the water would stop running just as quickly as it started running when I either turned the tap on or off. The reason why it doesn't in real life is that water is slightly compressible, you are starting the water running with 50psi, and only stopping it with 14 (air), and the hose is essentially a balloon that stores energy that is must release (as water coming out of the hose), when you turn the tap off. I am not sure what that had to to with the sound of amplifiers, but it was interesting.
Phase_accurate, I agree in principal with most of your recommendations w.r.t. NFB design, but I think you are already applying assumptions when you create them. I don't think you need constant THD over the audio bandwidth, though everything I have designed does. That is because I use low overall feedback which basically forces me to have a high F3 in open-loop to have good performance. I think 1% THD at full power is too high. While I may not ever have my amplifier turned that high, transients in the music do occur. However, I completely agree that low power THD is critical and this is not something that high open-loop gain, high feedback factor NFB amplifiers general have as a feature. That is likely because what you are amplifying is really the error signal, and if you have a high open-loop gain, that means the error signal is small. Couple that with a small input signal and that error signal becomes very small. Just a thought.....
I find that with real loads (and some unreal ones I try), it is easier to make low feedback designs very stable, i.e. little to no ringing. Interesting enough, no ringing in an amplifier does not necessarily mean lowest THD, or total error.
..... and for the record, I did just fine in my Control's class, I have built commercial temperature controllers using sophisticated control loops, and my wife thinks my much higher measured distortion amplifier (at max power at least) sounds better than the one that is measures about 25 times lower.
To that I will add that THD at maximum power is an imperfect measurement for how something will sound. I can take a signal, add in a perfect but delayed version of that signal, and measure the THD, and it will be 0 with many measurement systems. It won't sound to good though.
Alvaius
Am I missing something here? Why are people discussing the amount of time it takes for a "feedback" signal to propogate through a small piece of wire? Isn't that completely missing the point? Is not the greater issue the propogation time (or in reality the open loop bandwidth at a given frequency) through the amplifier itself that is at issue? The time it takes for the signal to propogate back to the input difference amp from the output is insignificant [1.5nanosec?] for all intents and purposes. Think of it this way. You have an amplifier with no signal. Hence the input is near ground and the output is near ground. You then apply a signal to the output. The output, almost instantly, starts changing. However, how fast it changes, is determined by the open-loop gain. The change at the output is fed back to the input and which causes the output to change and so on and so forth. Of course this is a continous process.
So some points to make:
If you had infinite open-loop bandwidth and infinite open-loop gain, then an NFB amplifier would be perfect. However, we don't so we need to make a good trade-off between open loop gain, open loop bandwidth, and how good it sounds (which is a combination of THD with sine waves that have only some bearing on music), overall amplifier stability (or is this thing going to be able to accurately reproduce the output into a speaker), etc. We will through into this whole mix, that there are actually pure delay elements in amplifiers. Most capacitors, whether they are real capacitors, or a semiconductor junction have some level of dielectric absorption and hence can add in essentially pure delay.
If water was completely uncompressible, my water hose was infinitely stiff, and I had control over air pressure in real time, then the water would stop running just as quickly as it started running when I either turned the tap on or off. The reason why it doesn't in real life is that water is slightly compressible, you are starting the water running with 50psi, and only stopping it with 14 (air), and the hose is essentially a balloon that stores energy that is must release (as water coming out of the hose), when you turn the tap off. I am not sure what that had to to with the sound of amplifiers, but it was interesting.
Phase_accurate, I agree in principal with most of your recommendations w.r.t. NFB design, but I think you are already applying assumptions when you create them. I don't think you need constant THD over the audio bandwidth, though everything I have designed does. That is because I use low overall feedback which basically forces me to have a high F3 in open-loop to have good performance. I think 1% THD at full power is too high. While I may not ever have my amplifier turned that high, transients in the music do occur. However, I completely agree that low power THD is critical and this is not something that high open-loop gain, high feedback factor NFB amplifiers general have as a feature. That is likely because what you are amplifying is really the error signal, and if you have a high open-loop gain, that means the error signal is small. Couple that with a small input signal and that error signal becomes very small. Just a thought.....
I find that with real loads (and some unreal ones I try), it is easier to make low feedback designs very stable, i.e. little to no ringing. Interesting enough, no ringing in an amplifier does not necessarily mean lowest THD, or total error.
..... and for the record, I did just fine in my Control's class, I have built commercial temperature controllers using sophisticated control loops, and my wife thinks my much higher measured distortion amplifier (at max power at least) sounds better than the one that is measures about 25 times lower.
To that I will add that THD at maximum power is an imperfect measurement for how something will sound. I can take a signal, add in a perfect but delayed version of that signal, and measure the THD, and it will be 0 with many measurement systems. It won't sound to good though.
Alvaius
Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Harold S. Black.
....including practicals...
Pjotr said:
Hmm, weak point Mikek,
Long time ago when I did my control theory, there were many students that were far on top of me. They got always high numbers for their theoretical proofs: By filling out the right formulas at the right places. They did rehearsal over and over in advance for it. But actually they did not understand a jot what it was actually about. Nearly all failed to get a practical temperature controller working (because there was a few seconds delay in the control loop
....including practicals...
cheevers..
quote:
Originally posted by Brett
The psychoacoustic research shows that we cannot distinguish quite high levels of low order harmonic distortion. It would be interesting to hear a DBT with you to add a small amount (below the perception threshold say <3%) to your beloved Onkyo, to see whether you can actually hear it. I bet you couldn't, because if you could it would fly in the face of 70 years of research by such 'tweakers' as Shorter.
The main premise of the document was to reconcile what is heard with what is measured. Because a meter reads a certain value of something does not mean it has any relevance at all to the purpose of the device.
And as it was passed by MIT, hardly, I would have thought a tweak school, I think it is fair to assume that the technical analysis is in fact correct. How about you approach it on point instead of taking the easy way out and simply bash Stereophile or a connection to them?
I have been reading Crowhurst's writings since I was at Uni two decades ago. The Engineer I trained under had a huge catalogue of articles in technical magazines from that time (40's - 80's). I have yet to see you or anyone else discredit successfully what he (and others such as Shorter) wrote then. Shorter, working for the BBC labs was part of the great tradition of researchers which developed low disortion, low colouration speakers that still stand up well today, so simply to dismiss them based upon your percetion of what you think is correct, only serves to illuminate your closed mind. In those days the BBC was one of the pre-eminent acoustic research facilities that has ever existed, and all their work was based upon rigorous investigation. Simply because it disagrees with your worldview it has no merit. Hmmm.
The reference to Stereophile at the head was merely a segue into the technical discussion. If the technicalities didn't hold up to scrutiny, do you think it would have passed?
My reading of your response is that as soon as you saw reference to Stereophile, you turned off your thinking and went into Borg mode, and failed to read the later technical analysis.
I am no great fan of Stereophile, or any of the magazines. I subscribe to the UK hifi+ because it's entertainment (also partly a gift), and has great music reviews and articles.
As for majorities, most people drive cheap cars, watch too much trash TV, listen to boomboxes and eat too much junk food. So should we take a majority view on what a performance automobile is, a great movie, a realistic sounding hifi or fine cuisine? Most people aren't interested in what is primarily a hobby to a small section of the population. My brother is into surfing in a big way, and whilst I can (just) ride a board, I can tell little dfference between them. That does not mean that he can't, with about a million waves behind him, and numerous boards ridden.
If your point was to try to discredit me, you failed. I have nothing tied up in the hi end industry at all.
In a word, bull. For ANY person to undertake an analysis of a subject, first they must make a premise. You notice something that doesn't tally, and then you look for reasons why. It's irrelevant if you hear a difference, or you measure it on a meter as the starting point for an investigation. In this case it was that the the measured results don't reconcile with a simple one bit THD reading. The analysis makes a good case as to why, and it ties in auditory research from other fields unrelated to 'hi fi', as well as a fair amount of previous research. It is primarily a technical document, and has a ton of references to previous academic research. Take it apart on point, or you'll simply show that you are merely full of hot air.
...i have had the oportunity to examine cheevers thesis, (thanks Brett), and i fear the initial assumptions made, and references cited to justify the work are fataly flawed.
He gives prominance to a subjective review of the cary amp. in 'stereophile', whose lack of intellectual rigor and consistency is legend....for instance, the cary amps. lack of global feedback is given fulsome credit for the amps. performance, but we need only recall the Halcro dubbed the best amp. in the world by the same periodical, uses a hefty feedback factor.
Mr. Cheever cites Matti Otala's work on TIM, but sadly appears to
not to be cognisant of the fact that TIM, SID have been demonstrated to be a red herring of the Monster variety.....see P. Baxandal series in wireless world 1978-1979, whose name is conspicuosly missing from this thesis....
...considering that the author subsequently cites Dr. Cabot of Audio Precision at a later stage, i find it inexplicable that he failed to mention an extremely important AES paper by the Dr. 'Comparison of non-linear distortion measurement method' ,which is freely available for download, subject to registration at:
http://audioprecision.com/publicati...pers/index2.htm
I have difficulty in accepting that this ommision was accidental, as many figures in this thesis are infact directly copied from other publications on the audio precision website.....
quote:
Originally posted by Brett
The psychoacoustic research shows that we cannot distinguish quite high levels of low order harmonic distortion. It would be interesting to hear a DBT with you to add a small amount (below the perception threshold say <3%) to your beloved Onkyo, to see whether you can actually hear it. I bet you couldn't, because if you could it would fly in the face of 70 years of research by such 'tweakers' as Shorter.
The main premise of the document was to reconcile what is heard with what is measured. Because a meter reads a certain value of something does not mean it has any relevance at all to the purpose of the device.
And as it was passed by MIT, hardly, I would have thought a tweak school, I think it is fair to assume that the technical analysis is in fact correct. How about you approach it on point instead of taking the easy way out and simply bash Stereophile or a connection to them?
I have been reading Crowhurst's writings since I was at Uni two decades ago. The Engineer I trained under had a huge catalogue of articles in technical magazines from that time (40's - 80's). I have yet to see you or anyone else discredit successfully what he (and others such as Shorter) wrote then. Shorter, working for the BBC labs was part of the great tradition of researchers which developed low disortion, low colouration speakers that still stand up well today, so simply to dismiss them based upon your percetion of what you think is correct, only serves to illuminate your closed mind. In those days the BBC was one of the pre-eminent acoustic research facilities that has ever existed, and all their work was based upon rigorous investigation. Simply because it disagrees with your worldview it has no merit. Hmmm.
The reference to Stereophile at the head was merely a segue into the technical discussion. If the technicalities didn't hold up to scrutiny, do you think it would have passed?
My reading of your response is that as soon as you saw reference to Stereophile, you turned off your thinking and went into Borg mode, and failed to read the later technical analysis.
I am no great fan of Stereophile, or any of the magazines. I subscribe to the UK hifi+ because it's entertainment (also partly a gift), and has great music reviews and articles.
As for majorities, most people drive cheap cars, watch too much trash TV, listen to boomboxes and eat too much junk food. So should we take a majority view on what a performance automobile is, a great movie, a realistic sounding hifi or fine cuisine? Most people aren't interested in what is primarily a hobby to a small section of the population. My brother is into surfing in a big way, and whilst I can (just) ride a board, I can tell little dfference between them. That does not mean that he can't, with about a million waves behind him, and numerous boards ridden.
If your point was to try to discredit me, you failed. I have nothing tied up in the hi end industry at all.
In a word, bull. For ANY person to undertake an analysis of a subject, first they must make a premise. You notice something that doesn't tally, and then you look for reasons why. It's irrelevant if you hear a difference, or you measure it on a meter as the starting point for an investigation. In this case it was that the the measured results don't reconcile with a simple one bit THD reading. The analysis makes a good case as to why, and it ties in auditory research from other fields unrelated to 'hi fi', as well as a fair amount of previous research. It is primarily a technical document, and has a ton of references to previous academic research. Take it apart on point, or you'll simply show that you are merely full of hot air.
...i have had the oportunity to examine cheevers thesis, (thanks Brett), and i fear the initial assumptions made, and references cited to justify the work are fataly flawed.
He gives prominance to a subjective review of the cary amp. in 'stereophile', whose lack of intellectual rigor and consistency is legend....for instance, the cary amps. lack of global feedback is given fulsome credit for the amps. performance, but we need only recall the Halcro dubbed the best amp. in the world by the same periodical, uses a hefty feedback factor.
Mr. Cheever cites Matti Otala's work on TIM, but sadly appears to
not to be cognisant of the fact that TIM, SID have been demonstrated to be a red herring of the Monster variety.....see P. Baxandal series in wireless world 1978-1979, whose name is conspicuosly missing from this thesis....
...considering that the author subsequently cites Dr. Cabot of Audio Precision at a later stage, i find it inexplicable that he failed to mention an extremely important AES paper by the Dr. 'Comparison of non-linear distortion measurement method' ,which is freely available for download, subject to registration at:
http://audioprecision.com/publicati...pers/index2.htm
I have difficulty in accepting that this ommision was accidental, as many figures in this thesis are infact directly copied from other publications on the audio precision website.....
.......everybody knows i need help.......Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Harold S. Black.
I seem to remember getting a mediocre grade in Control Systems,
but I did escape knowing what phase margin is. (post 37)
I see what you mean about discussion, since you now have launched into multiple post monologs.
You know what happened to the last one to utter "Dave, This conversation can serve no useful purpose."
mikek said:
...Looking back at my control systems results, (a long time ago now), i reckon there was'nt i single occasion that i was'nt top of my class...
....beyond this, i think this particular discussion has outlived its usefullness.....
I seem to remember getting a mediocre grade in Control Systems,
but I did escape knowing what phase margin is. (post 37)
I see what you mean about discussion, since you now have launched into multiple post monologs.
You know what happened to the last one to utter "Dave, This conversation can serve no useful purpose."
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