Well said, dvv. Otala got the ball rolling, so to speak, but it must be remembered that he was blazing a trail that was relatively unknown, when it came to audio, at least.
He was fortunate that he had access to a government laboratory for his measurements and people to do virtually hundreds of spectrum measurements in his quest for understand TIM and how it related to slew rate, and other qualities in superior audio reproduction.
Matti Otala was not the first to clue me in about use of feedback. Richard Heyser, told me more than 40 years ago, that global feedback was problematic, and should be avoided, if possible. He discovered this, accidently more than 10 years previously before telling me, or back in the 1950's. Richard was very adamant about it. You see, he used his ears, as well as his mind, to determine what is audible.
Matti was NOT against negative feedback, he was FOR high open loop bandwidth. This proved to NOT be the primary source of TIM, itself, but it seemed to still make for better amplifiers. I have had a version of his original design since 1976, and still have one working in my lab. It is amazingly musical, especially for how it was fabricated, and what parts it was made with. Something is just 'right' about it.
Of course a 50KHz open loop bandwidth would be wonderful. Good luck TRYING to achieve it! '-)
He was fortunate that he had access to a government laboratory for his measurements and people to do virtually hundreds of spectrum measurements in his quest for understand TIM and how it related to slew rate, and other qualities in superior audio reproduction.
Matti Otala was not the first to clue me in about use of feedback. Richard Heyser, told me more than 40 years ago, that global feedback was problematic, and should be avoided, if possible. He discovered this, accidently more than 10 years previously before telling me, or back in the 1950's. Richard was very adamant about it. You see, he used his ears, as well as his mind, to determine what is audible.
Matti was NOT against negative feedback, he was FOR high open loop bandwidth. This proved to NOT be the primary source of TIM, itself, but it seemed to still make for better amplifiers. I have had a version of his original design since 1976, and still have one working in my lab. It is amazingly musical, especially for how it was fabricated, and what parts it was made with. Something is just 'right' about it.
Of course a 50KHz open loop bandwidth would be wonderful. Good luck TRYING to achieve it! '-)
What I find a bit puzzling: we have wonderful tools now, which when used in an informed fashion with good models give remarkably accurate results. Let's use them! Of course some do.
If the secret is in momentary overloads, or we suspect that it is, then simulate overloads. If part of that overloading is running out of current to rapidly charge internal capacitances, probe this, and see what happens when you change things.
When all we had were IBM PCs and pocket calculators to use personally, we had to do other things. But those days are gone forever --- although many have gone overboard in the other direction, and run to their Pentium machines to simulate a simple voltage divider.
I had a close friend recently say How do you know if it works or not if you don't build it? This was someone who studied electrical engineering in the beginning, a long time ago, but thereafter got into other areas of audio, and in his student days did not have the tools.
I said I might build a portion of a circuit if I suspected the results of simulation, or found something that was just too critical in terms of certain ill-defined parasitics to easily model. And I lack a full 3-D electromagnetic capability, so a good deal of rough estimation and prudent layout may be brought to bear and then tested for high frequency circuits/systems. But for a great many things the simulators are quite reliable.
If you have hands-on experience and well trained imagination no simulation is needed. But in order to obtain this skills you need to work with breadboards and measurement tools. Like pocket calculators deprived people from ability to calculate numbers, simulators deprived people from ability to see, feel, and hear circuit diagrams.
I think it is an iterative process. I agree that calculators have resulted in an inability to calculate for many, but are nonetheless useful. Similarly, if you start simulating before any experience or imagination you will stunt your growth. One of the things I like about the late Peter Dunn's book Gateways into Electronics is the absence of simulations.
It's interesting to be in a group of scientists and try to get anyone to calculate the simplest things without a calculator. Another funny was when one of the Mars Curiosity people was asked to give the touchdown time as UT. He was unwilling to attempt to figure it out.
I avoided simulators for years and designed lots of equipment without them. There was a good deal of bench work and tweaking, but most of the time was with pencil and paper and calculators. But when I added the simulators to the suite of tools it was quite liberating and powerful.
It's interesting to be in a group of scientists and try to get anyone to calculate the simplest things without a calculator. Another funny was when one of the Mars Curiosity people was asked to give the touchdown time as UT. He was unwilling to attempt to figure it out.
I avoided simulators for years and designed lots of equipment without them. There was a good deal of bench work and tweaking, but most of the time was with pencil and paper and calculators. But when I added the simulators to the suite of tools it was quite liberating and powerful.
Of course, everything can be of help. I had a slide rule on my belt since 1959, and used it mainly until 1973, when I got an HP35. It was wonderful to have a scientific pocket calculator, and I derived all my circuits using it back in the early and middle 70's. I did NOT have to go back to my slide rule, or mechanical calculator. What a labor saving device the scientific calculator was!
Wavebourn is of course right about losing some skills to having calculators and computers doing math for us. I mean, when was the last time I had to do a square root on a single sheet of paper?
I do find that excessive dependence on SPICE is a drawback, and one reason I normally don't use it.
Wavebourn is of course right about losing some skills to having calculators and computers doing math for us. I mean, when was the last time I had to do a square root on a single sheet of paper?
I do find that excessive dependence on SPICE is a drawback, and one reason I normally don't use it.
Exactly! When you added the tool to your already developed talent, instead of replacing your talent by simulators, it "was quite liberating and powerful".
Being a total rookie, I find Spice helps me understand a lot as I can't really visualize things yet. Getting there. Maybe 20 more years. I would have to believe innovation comes from the mind; sims just help verify quicker. However, my first experience with Spice it identified a problem all the smart, and they were smart, engineers did not notice.
I am lucky to own an example of the XX, along with the matching preamplifier. Some small repairs mean that I have not had a chance to listen to it yet, but I look forward to hearing it. I also have the smaller X-1 amplifier and matching X-11 preamplifier, both of which sound very good indeed.
I often think it my duty to try and get young engineers to think and analyze sans simulator but on the other hand these next generations are smarter and will face different challenges than we did.
All this new group atmosphere and idea sharing may lend itself to a more visual learning where simulators aren't very different from graph paper.
I think these issues will work themselves out just fine (p.s. I don't allow myself to use a simulator at home).
Thanks
-Antonio
All this new group atmosphere and idea sharing may lend itself to a more visual learning where simulators aren't very different from graph paper.
I think these issues will work themselves out just fine (p.s. I don't allow myself to use a simulator at home).
Thanks
-Antonio
The key sentence for me here is: "Otala got the ball rolling". Exactly.
I never thought Otala had actually invented anything, I have always considered him a person who observed, thought about it and took some pains to systematize it, to turn it into a theory one could deal with. TIM did not occure when Otala said it existed, it was there all the time, he just found a way to see it as a mechanism which yielded trouble. While TIM was not only the result of poor bandwidth, I don't think anyone can deny that this was one of the possible, and at the time frequent reasons for poor sounding amp. Those were the days of beiving that global NFB was a panacea - what the heck, just hit it with lots of feedback and it'll come out just fine.
Even assuming Otala got some things either not quite right, or even downright wrong, no-one can deny that he got the ball rolling indeed, and then some. Even those who denied his views were people who did some heavy thinking along the way, which they had NOT done before Otala.
I could be caustic here and say that it's strange how many people appeared set on denying his theories, not realizing that they became someone because he made them into someone by giving them though food for denial.
It is always infinitely simpler and easier to sit back and deny than to sit back and create. In all truth, some formulations in Otala's papers were so phrased that they almost begged discussions.
And I agree with John here, Otala may not be altogether right, but some of his principles do help make amps simply sound better. Such is my experience.
But I am baffled by John's closing, two sentence paragraph. Why do you think a 50 kHz open loop full power bandwidth is so hard, John? Mind you, I am not saying it's easy as a pie, but I also do not think it all that hard. After all, Otala's model TIMless amp, as pblished in IEEE and as subsequently manufactured by Per Abrahamson and his Electrocompaniet company had an OL FPB of 100 kHz - now, THAT is hard to achieve without seriously compromising stability into complex loads, but even that can be done. If it was possible in 1977, it should be also possible in 2012.
Although, Otala said that this bandwidth should be "at least 20 kHz" ...
In my general view, as it somehow always turns out, it's more of a question of overall balance. Global NFB is GOOD if it's relatively moderate, say no more than 26 dB (a loose pointer, no more), and if your OLFPB is at least say 30 kHz, not to be satisfied with the barest of minimums. Finding the right sonic balance between local and global NFB, which will obviously vary with circuit topology and other aspects, so there is no simple one-fit-all formula.
Smarter? No. Different challenges? Yes.magnoman said:
My experience is that the cream of new talent are every bit as smart as the best of the older generation, but they are now less well educated. This is because the average has declined, so courses have had to be simplified so they don't fail too many students.
I remember chatting to someone a few years ago. He was bright, and had a good EE degree from a good university. When he started work, doing real engineering, he and his colleagues were surprised to discover that he knew almost nothing about noise because his course had taught him almost nothing about noise. I was surprised too, because Johnson noise (and its derivation using the transmission line model) is first-year stuff for physicists (or it was 30 years ago - I don't know about nowadays).
Before using simulators for real, all newbies should be forced to simulate an LTP with a perfect CCS tail. Then build one and measure it, and explain the huge differences in CMRR and even-order distortion performance.
But it's people, Wave, not tools.
Look at a simple hammer. You can waste time with it by cracking walnuts, you can build a house with it, or you can kill a man with it. All with the same hammer. So it's not the hammer, but the man using it.
All the tools we have now are all wrong if taken as absolute. They are not an end unto themselves, they are tools, and as any tool, they are only as good as the man using them.
In other words, if the PC is understood as a tool which is here to help us by saving us from menial work, that's just fine beacuse the thought process is still our own, and has not been relegated to a machine "because it's faster".
It's a simple fact that my simulator will allow me to make a fault free electrical circuit, but that says nothing about how that circuit will sound. I have had it sound absolutely awful, so it was back to the drawing board. On a few occasions, using the same tools, I have hed it produce great sounding circuits, but I never lost sight of the simple (at least simple to me) fact that it only does what I tell it to do far better than I could protoboard the sam circuit.
But I must clean here. The best aspect of it is that allows me to create SPICE models of my own choosing. I test drove that point once, and indeed got different results that from the simulator model database. After that, I downloaded an unholy collection of transistors I tend to use plus some new ones and created my own database, as per the manufacturers' data sheets.
Ever since then, over the last 7 years, my suimulator hos not been wrong once, not even once. It it says the circuit will work, it has worked each and every time. If anything, it's a bit conservative, and I just love that - if it says the circuit will hit say 300 kHz, you can bet your bottom dollar that as a model on the testbench it will hit at least 330 kHz, and more often 350 kHz.
I much prefer that to overoptimistic results, always have. If nothing else, it keeps building of models and test bench measuring them still a lot of fun, there you are investigating by how much it shortchanged you this time.
Hoqwever, in the end, it's all up to how I hear it under real world conditions, and I have never in my life had a circuit made which got it just right the first time, all to the one had to be reworked, some more, some less. The ultimate judge are my ears, such as they are.
The typical LTP of old was 1 mA tail current with no extra emitter degeneration . I have played with various currents and found 1 mA not such a bad choice . The choice then to run more current or make the VAS a Darlington or whatever needs to be considered . I suspect Douglas Self struggled with this and came to no absolute conclusion . Anyone brave enough to suggest an optimum LTP current ? I will start the ball rolling and say 2 mA . Is 10 mA excessive ?
I know what I have just said is a bit daft . However in motorcar engineering it was "suggested " 330 cc to be an optimum capacity per cylinder if petrol . I believe this was said in the 1920's and might have been Citroen . There is evidence to say it is a good rule of thumb today . I forget how square the engine is at 330 cc ( bore and stroke ) . The parameters being for example , piston velocity , economy of fuel and parts , power and torque , vibration . For a simple hybrid car 990cc would be practical and fast .
John suggested 50 V / uS @ 100 W as desirable . We arrived at 22 000 uF x 2 as the PSU for the same . Optimum tail current and perhaps bandwidth would be useful . I suspect 100 W @ 8R is also a good universal power level . I will suggest 5Hz - 3 dB and 100 kHz - 3db as a good minimum bandwidth . To ice the cake I see distortion below 0.1% as still the hi fi standard with as littler high order distortion as possible . 0.1% THD @ 100 W . Preferably the amplifier should behave as a class A design ( op amps more or less do these days ) .
I know what I have just said is a bit daft . However in motorcar engineering it was "suggested " 330 cc to be an optimum capacity per cylinder if petrol . I believe this was said in the 1920's and might have been Citroen . There is evidence to say it is a good rule of thumb today . I forget how square the engine is at 330 cc ( bore and stroke ) . The parameters being for example , piston velocity , economy of fuel and parts , power and torque , vibration . For a simple hybrid car 990cc would be practical and fast .
John suggested 50 V / uS @ 100 W as desirable . We arrived at 22 000 uF x 2 as the PSU for the same . Optimum tail current and perhaps bandwidth would be useful . I suspect 100 W @ 8R is also a good universal power level . I will suggest 5Hz - 3 dB and 100 kHz - 3db as a good minimum bandwidth . To ice the cake I see distortion below 0.1% as still the hi fi standard with as littler high order distortion as possible . 0.1% THD @ 100 W . Preferably the amplifier should behave as a class A design ( op amps more or less do these days ) .
I know . I think 2 mA is as far as this will go . I don't think many will go wrong with that . I did see somewhere a very lengthy analysis of the Douglas Self's designs . It argued that noise could be improved . It did come to an optimum current . If I can find it I will post it . It was based on assumptions of typical pre amp output impedance and typical uses . I was very impressed by Mr Self saying he was a little nervous when bootstrapping the input to have lower noise ( 2 x 1K input from memory ) . I assumed all was simulation in his books ! I do think noise is important as does he . I was told that lower noise transistor amplifiers sound warmer . I have no way of knowing if it is true . It seems no bad thing to believe . Noise gating is not an alternate .
actually the 10-20 dB is the range where feedback is "worst" - where "harmonic multiplication" gives the highest peak in the feedback generated new, higher harmonics when looping a low order open loop distortion
much higher feedback is needed to have the new harmonics all fall with increasing order as we are told is necessary
the "harmonic multiplication" is fundamental feedback math - applies to degeneration as well as "local loop" or "global" feedback for those who think there is a distinction - it is not in this property
much higher feedback is needed to have the new harmonics all fall with increasing order as we are told is necessary
the "harmonic multiplication" is fundamental feedback math - applies to degeneration as well as "local loop" or "global" feedback for those who think there is a distinction - it is not in this property
Taking an op amp as a mini amplifier I find some curious things . A cheap and nice chip is MC33078/9 . I use it with a gain of 62 and also a gain of 1 . Both sound exceedingly nice like this . A gain of 3 not so much my cup of tea ( two different uses ) . This is not evidence , it just reminds me of your statement . Lets be clear the gain of 1 sounds truly excellent . Gain of 62 to 100 superb considering how it looks on paper ( <250 uV in and 60 dB noise if lucky ) . Gain of 30 was not special either . I am very happy to be shot down in flames for this . If not you then I should do it .
Although troublesome should we expect power amplifiers to be usable as big op amps with a gain of 1 ? The output stage is that . I said before if the output stage is preceded by a big op amp it could have a high degree of feedback and the output stage enough global to the op amp like stage to maintain low output impedance .
just to be clear it is 10-20 dB of feedback factor, not the closed loop gain that we are talking about with the "harmonic multiplication" math
I have seen "unity gain" op amp datasheet open loop curves with a bump in the phase that gives less phase margin at low closed loop gain of 3-5 than at unity gain or much higher closed loop gain
I have seen "unity gain" op amp datasheet open loop curves with a bump in the phase that gives less phase margin at low closed loop gain of 3-5 than at unity gain or much higher closed loop gain
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