Well , this should get their juices flowing ....
"There are only advantages and no disadvantages to applying stratospheric amounts of negative feedback in an amplifier. The only hard part is figuring out how to do it. The more feedback, the better it sounds provided that it’s never less than 30dB at any audio frequency."
:
Wayne; nothing is wrong in feedback itself. I myself for stratospheric amounts. But when we apply feedback around real things that are inertial we have to deal with problems created by it. But as soon as this problems are under control, amount of feedback itself does not matter, What matters, results, like amplification factor we are getting, input/output resistances, their linearities, and so on. When you say control say satellites on orbit you still solve the same problems: how precise you want to control it, VS how stable is your control system VS how fast it is. You want as more stratospheric as you can get feedback, to control the satellite more precise. But "want" and "can" are quite different things. Do you hear me now?
Have you ever played with analog computers solving differential equations? We did. Great experience, unforgettable. However, today SIMs can be used for such purposes...
Err, Ok john , i 'm sure i will wrap my head around it in another 10 or 12 yrs ..
DVV,
Thanks for the response , IMO 3 prs is not enuff for an amp operating at 2 ohms , My current audio phase, see me with only large low z speakers( ribbons or esl) in my non main systems i have other regular speakers ( 8 ohm/4 ohm coil) and i can tell you any amp with ONLY 3 output prs running at 2 ohm is toast , well unless less than 300 watts.
I do agree with your other assumptions, and do favor big constant voltage amplifiers over small , always have, same for speakers. I'm not a fan of pam pam sound or noise and detest dainty hi-fi stuff, (no 12 watt nor single speaker for me) i have had a few of those over the years ( highly rated dainty amplifiers), the box arrive on monday, shipped back out on tuesday. The worst was an amp I was after for Years , made famous by it's designer's disappearance, a big lug of an amp , ran like a little girl and hid on my low -Z speaker, destroyed by a pr of adcom 565's , that i had sitting @ the time...
I have a rule , if i need help to lift the amp , it could be a keeper
next is the test...
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He forgot to mention which exact "performance" he measures: "has no bearing at all on actual audio performance".
Remember my example?
Actually, it was the same idea as Peter Walker patented in his famous Current Dumping patent. I did not know about him and his patent, so instead of working around slow opamp and power transistors implementing "The Bridge" like he did, I went differently, but the main idea is still the same: approximation of overall transfer function by summing 2 functions, adjusting summing result finally by additional feedback.
It would be interesting to see how The Author solved this problem without multiple feedback loops.
Go get him wave ........
Go get him wave ........
Thanks, you do it better.
I explained my point of view: I am not superstitious about amount of feedback, and I am not compulsive regarding numbers. It does not matter how deep is feedback, end results matter. Like, I eat regarding how much I want and need to eat to feel comfortable. Of course, some people who have obesity need to calculate numbers of calories, fats, etc..., but I don't need to do that.
Hee hee. (general comment on how Putzeys' piece sparked a great outpouring).
The part of Bruno's excellent article I most enjoyed was his mention of "suggesting a method of wrapping almost 60dB of loop gain over the full audio range around a valve amplifier using a third order loop. Whenever it was stable it sounded immaculate." [my italics].
Also his history of how feedback got a bad rap is fascinating.
Does this mean I love feedback above all else? Well... I don't dispute the reports of listeners. If they are reporting what they heard honestly, such reports fall into the realm of being philosophically "incorrigible" --- I can't crawl into their heads and experience precisely what they experience.
And I know the many ways that feedback can go wrong, very wrong. When tvr gets his 120 up and running, he will look at waveforms and will probably be concerned with things that happen over temperature and at overload, where many times things become momentarily oscillatory or otherwise "stuck" in some fashion.
JC mentioned somewhere (I think in one of the Blowtorch threads) about the robustness of outputs that are just resistive with collecting-electrode drive (i.e. not emitter followers for example) for driving lines. These are ways of sidestepping oscillatory behavior almost independently of what vagaries of loading are imposed, whereas emitter/source followers are notorious for turning into Colpitts oscillators without plenty of care.
Anyway, thanks to everyone for these contributions. I am enjoying this a lot.
Brad
The part of Bruno's excellent article I most enjoyed was his mention of "suggesting a method of wrapping almost 60dB of loop gain over the full audio range around a valve amplifier using a third order loop. Whenever it was stable it sounded immaculate." [my italics].
Also his history of how feedback got a bad rap is fascinating.
Does this mean I love feedback above all else? Well... I don't dispute the reports of listeners. If they are reporting what they heard honestly, such reports fall into the realm of being philosophically "incorrigible" --- I can't crawl into their heads and experience precisely what they experience.
And I know the many ways that feedback can go wrong, very wrong. When tvr gets his 120 up and running, he will look at waveforms and will probably be concerned with things that happen over temperature and at overload, where many times things become momentarily oscillatory or otherwise "stuck" in some fashion.
JC mentioned somewhere (I think in one of the Blowtorch threads) about the robustness of outputs that are just resistive with collecting-electrode drive (i.e. not emitter followers for example) for driving lines. These are ways of sidestepping oscillatory behavior almost independently of what vagaries of loading are imposed, whereas emitter/source followers are notorious for turning into Colpitts oscillators without plenty of care.
Anyway, thanks to everyone for these contributions. I am enjoying this a lot.
Brad
Err, Ok john , i 'm sure i will wrap my head around it in another 10 or 12 yrs ..
DVV,
Thanks for the response , IMO 3 prs is not enuff for an amp operating at 2 ohms , My current audio phase, see me with only large low z speakers( ribbons or esl) in my non main systems i have other regular speakers ( 8 ohm/4 ohm coil) and i can tell you any amp with ONLY 3 output prs running at 2 ohm is toast , well unless less than 300 watts.
I do agree with your other assumptions, and do favor big constant voltage amplifiers over small , always have, same for speakers. I'm not a fan of pam pam sound or noise and detest dainty hi-fi stuff, (no 12 watt nor single speaker for me) i have had a few of those over the years ( highly rated dainty amplifiers), the box arrive on monday, shipped back out on tuesday. The worst was an amp I was after for Years , made famous by it's designer's disappearance, a big lug of an amp , ran like a little girl and hid on my low -Z speaker, destroyed by a pr of adcom 565's , that i had sitting @ the time...
I have a rule , if i need help to lift the amp , it could be a keeper
No problemo, to each his own is what I say.
Bear in mind that I am talking about an amplifier made to play music, not reproduce steaty state sine waves into a lab resistor. This means that its COTINUOUS power will never be over 25W/8 Ohms, to accomodate those transients which will come along.
Given that I have rather efficient loudspeakers, 92 dB/2.83V/1m, and that my room is small (app. 170 ft2), by the time I get to 5 Watts steady state, my window panes are swinging along with the music. By 10 Watts, I have SWAT people raiding me, and the neighbours are evacuating fast (I live in a condo). The truth of the matter is, a decent 50W per side amp will do just fine, and I have a collection of power amps. My trusty and beloved Marantz 170 DC from 1978, practically rebuilt and beefed up, is said to deliver 85 W/8 Ohms, and it just never goes over 2 Watts according to its power meters; allowing for their not too good a dynamic response (a fad in those days), even if that is actuallay 5 Watts, I am still way below its actual capability.
Attaching my Karan Acoustics KA-i180 integrated amp, rated at 180/250W into 8/4 Ohms is really overdoing it, but, as we both know, big sound is usually delivered by big amps, just as usually only big seakers will deliver a big speaker sound. Well, I got both.
So, as I see it, you really need BIG power in transients only, and what I have in mind is a REASONABLE REAL WORLD solution, not an example of a perfect amp.
To you, I would suggest you hop over here and I'll introduce you a friend of mine, Milan Karan, and his top power amp model. It weighs in at around 260 lbs and delivers 1,200W into 8 Ohms, and a hell of a lot more into lower impedances. A gift at around $50k. He has to ship it in a wooden crate.
Plus great food, great girls, an added bonus.
"Have you ever played with analog computers solving differential equations? We did. Great experience, unforgettable. However, today SIMs can be used for such purposes..."
God, I remember being shown that some 40 years ago!
Patch panel op-amps (Philbrick modules) modeling a
f'x + f''y + f'''z = 0 function
The "= 0" bit was modelled by grounding it, AFAIR!
Tricky scaling to keep every stage within +/- 10V
God, I remember being shown that some 40 years ago!
Patch panel op-amps (Philbrick modules) modeling a
f'x + f''y + f'''z = 0 function
The "= 0" bit was modelled by grounding it, AFAIR!
Tricky scaling to keep every stage within +/- 10V
Wayne, I just thought of something ...
If we say that about the average power device used today is say Toshiba's 2SA1943/2SC5200, each rated at 150W, in between those small 90, 100 and 120 W plastic devices used in the industry, and the more serious Sanken and ON Semi platic devices rated at 200 and 230 W, my choice would be a rough equvalent of 9 pairs of thiose Toshiba devices.
I just looked up their Data Sheets, and their power derating number is also 1.43. So, at 85 degrees, their actual real world power would be just (1.43 x 60 = 85.8 W; 150 - 85.8 =) 64.2 W, or about 1/3 of my initial Motoroloa/ON Semi devices. It will take three of them to equal my one.
The only advantage they would offer is the fact that the heat source would be split into 3 sources with them, which would make my heat sinks more efficient at cooling them down.
For that, I pay with additional problems with the Miller effect, which I really neither want, nor need.
Subjectively, I do not believe they are anywhere nearly as robust as the Motorola trannies. I've been using Motorlola output devices for a long time, and I know from experience they will survive what many other transistors will not, and that will be a lot more than Motorola says they will. Which is why you will find them in many (even most) professional grade amplifiers, such as Bryston, Crown, Electro Voice, at al. No sexy Sankens there.
If we say that about the average power device used today is say Toshiba's 2SA1943/2SC5200, each rated at 150W, in between those small 90, 100 and 120 W plastic devices used in the industry, and the more serious Sanken and ON Semi platic devices rated at 200 and 230 W, my choice would be a rough equvalent of 9 pairs of thiose Toshiba devices.
I just looked up their Data Sheets, and their power derating number is also 1.43. So, at 85 degrees, their actual real world power would be just (1.43 x 60 = 85.8 W; 150 - 85.8 =) 64.2 W, or about 1/3 of my initial Motoroloa/ON Semi devices. It will take three of them to equal my one.
The only advantage they would offer is the fact that the heat source would be split into 3 sources with them, which would make my heat sinks more efficient at cooling them down.
For that, I pay with additional problems with the Miller effect, which I really neither want, nor need.
Subjectively, I do not believe they are anywhere nearly as robust as the Motorola trannies. I've been using Motorlola output devices for a long time, and I know from experience they will survive what many other transistors will not, and that will be a lot more than Motorola says they will. Which is why you will find them in many (even most) professional grade amplifiers, such as Bryston, Crown, Electro Voice, at al. No sexy Sankens there.
In still earlier days, using tube opamps, the range was typically +/- 100V
Those folks also paid a lot of attention to dielectric absorption.
...when you carefully dose input signal and load so it does not screw up any of active/passive elements of the amp that are in common feedback loop.
That's why steady sinewave is good to measure and report how close the amp is to the wire with gain.
In still earlier days, using tube opamps, the range was typically +/- 100V
Here is opamp:
...when you carefully dose input signal and load so it does not screw up any of active/passive elements of the amp that are in common feedback loop.
In other words, good commonsense engineering that we all learned, ehh Wave?
jan
Perhaps you are right, but that is quite the opposite of my experience, As I know it, amplifiers which use a lot of feedback are prone to be harsh and rather dead.
Amplifiers which use zero (overall) feedback tend to sound sort of unfinished to me, as if there's something missing.
As a general rule, it is my view that the best can be expected of amplifiers which use between 20 and 30 dB of overall feedback.
But of course, these are not rules carved in stone, I have heard exceptions to the above general outlines.
In other words, good commonsense engineering that we all learned, ehh Wave?
Common sense, particular sense, all have to be considered. However, if you draw straight lines like in the article you can come to one conclusions, if you draw lines for every subsystem in the system you can get to another conclusion. The axiom is still valid, "The system made from optimal subsystems can not be optimal". From another hand, each subsystem have to be optimized to perform functions that are used in order to get the resulting function. But this is no way proof that the axiom is not valid.
Like in my example, class A and class C output amps had properties defined by feedbacks such a way their transfer functions are smoothly glued together working in parallel on the load. Then, errors of "glue" were minimized by overall feedback.
Yes. Feedback alone is never a panacea. One thing we tend to forget is whatever we do to the feedback loop, the amp itself always works open loop! In a sense, the only thing (global) feedback can do is manipulate the effective input signal such that the output is as perfect as possible a replica of the input signal.
So you must be aware of what it is that will be thrown at the amp itself, and make sure it doesn't overload or clip internally and doesn't slew limit.
Taking any old amp and wrapping it in a feedback loop doesn't gurantee good sound.
But that has been done for almost 3/4 of a century so isn't very exciting anymore.
jan
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