Sound Quality Vs. Measurements

[john curl]

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! '-)

[bcarso]

Originally Posted by jcx

actually Cordell did a lot to beat to death Otala's specific prescription for avoiding TIM/PIM with "flat loop gain" – with better explanation, Cordell built hardware both dedicated test hardware and low PIM but high feedback amp

CordellAudio.com - A MOSFET Power Amplifier with Error Correction

both Walt Jung and Marshall Leach were early Otala enthusiasts - later came to the conclusion that you don't have to use Otala's "flat loop gain" prescription - high feedback, sloping loop gains can still have vanishingly low TIM/PIM

perhaps Otala deserves credit for sparking the renewed slew analysis effort in audio amps but his actual recommendation, initial model have been shown lacking

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.

[Wavebourn]

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.

[bcarso]

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.

[john curl]

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]

Originally Posted by bcarso

But when I added the simulators to the suite of tools it was quite liberating and powerful.

Exactly! When you added the tool to your already developed talent, instead of replacing your talent by simulators, it "was quite liberating and powerful".

[tvrgeek]

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.

[Audionutz]

Originally Posted by dvv

There, you see, Nige? Exactly what I've been telling you all along.

And you ask me why I think Beritish made amps sound slow and sometimes quite dead - band limiting is useful only way up there at or above 200 kHz. Most British amps use band limiting both down below at or below 20 Hz and up above from 50 kHz or so. The logic being "There's nothing there that we want." Not quite so.

I apologise for coming in so late to a subtopic I sort of initiated, but this forum has a wild way of sometimes NOT alerting me to new posts, and it has been dead silent for the last 6 days or so. I came in today to ask what happened, somebody die, or something, and I find you eager little beavers are at it full force.

John, thans for the hint, GK7 thanks for the link. I downloaded it so I can study it later, I don't want to just run through it.

From what I gathered from the discussion here, it seems that the criteria suggested in the Nationl Seminvonductor Audio cookbook, 0.5 V/uS per PEAK volt output is, as I took it even then, too conservative. Right from the start, I assumed double that value, i.e. 1 V/uS per peak output volt, or 40 V/uS for a nominally 100 W/8 Ohms. More can't hurt, unless it compromises something else.

I sort of knew H/K got it right when they made HK 680 integrated amps, nominally delivering 85/130 W 8/4 Ohms, but giving it a slew rate of 80 V/uS. Richard Miller, who is credited as the chief of its design team, obviously agrees with Otala, Jung & Curl. Hardly surprising, given that H/K employed Otala for some 6 or so years.

On a purely personal level, I am very pleased with this turn of events, I have always thought and spoken out loud that H/K is the one company to watch among all mass producing manufacturers, they were always at least one cut, and often more, in front of the rest of the pack. Citation XX is still thought by many to be one of the best power amps ever made by anyone, but unfortunately, few were made and it's really rare. I never even saw one live.

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.

[magnoman]

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

[dvv]

Originally Posted by john curl

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! '-)

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.