Originally Posted by firev1
With tubes, using NFB plays only a minor role in reducing THD as compared to the solid state design, where NFB is needed and is vital in producing a low distortion amp. Btw reading the last few pages, I don't think NFB increases bandwidth? In fact, the reason why tubes play badly with NFB is because many designers had NFB factors too high without consideration for the limited bandwidth of the transformer coupled tube circuit. Lastly, even if tubes are indeed more efficient, the fact they run in Class A would mean they waste a lot more power than today's modern amp designs.
As for on Dr Otala's work, here is what Douglas Self has to say of it,"Another objective was the elimination of so-called transient intermodulation distortion,
which after many years of argument and futile debate has at last been accepted to mean nothing more than
old-fashioned slew-rate limiting." - Audio Engineering Explained.
For his low feedback design, Dr Otala made use of heavy local feedback to reduce the overall feedback, so it isn't very accurate to call it a zero feedback amp. On another note, Otala use of 20 db global feedback across the frequency band for his prototype so the Otala amplifier that was eventually produced added 10 db more feedback for 30db across the frequency band.
Tubes and Negative Feedback, the short answer is, it depends on a bunch of factors:
Yes I agree, when you take into account the effect of the output transformer, then the bandwidth is reduced and usually must be reduced to keep the amp stable, i.e stop it from oscillating.
In addition, because most tube are more linear than most transistors, hence, normally tube amps don't need as much feedback. In addition, tubes amps are generally simpler, i.e, designed with less open loop gain. Complicated question! So I'll stick with "it depends"
You certainly didn't read me saying that tubes are more efficient than transistors. They all need a heater element which blows any hopes of low efficiency out of the water and out the window. Sounds like you read sumthin' out of context? Or maybe not? Which is cool, this thread is getting a bit looooong. I'm sure you are aware that output stage of most tube power amps do not run in Class A.
But I agree, whether they run in Class A or not, they are less efficient than "equivalent" transistor amps.
Personally, I don't hate Class A so I have no axe to grind.
Yes, I am aware that more than a few designers question Otala's work.
If you read Douglas Self's book on amp design he actually uses a few techniques that Otala suggested! i.e. local feedback in the power amp input stage.
They may disagree with his results, but his work continues to be extremely influential in modern power amp design. For example, high output current amps.
He seems to be the prophet that a few designers love to hate.
You certainly didn't read me saying the Otata based designs were zero feeback.
You are probably thinking about the Eletrocompant amps, they were designed with his input, as I understand it, but were not actually designed by him. Maybe someone knows something I don't know?
Otala wrote many papers on TIM and reducing TIM.
He never said: "use this much feedback". He only made suggestions. You could argue he suggested using feedback in moderation. I can send you some of his papers if you like??
Originally Posted by qusp
not sure whos posts you are referring to with the statement of NFB increasing bandwidth because I certainly didnt say it, it lowers it normally, I was under the impression thats one of the reasons it increases stability in some amps
maybe someone abbreviated No Feedback as NFB and you took the wrong meaning?
Feedback and bandwidth: the answer is "it depends".
For example, if you take an existing power amp with global feedback and remove the Global feedback (assuming you can!), you will see the bandwdth decrease and the gain increase. This is called Open Loop gain and Open Loop Bandwidth.
My statement is a bit misleading, when you design an amplifier you design it with certain goals and constraints in mind.
It's a bit disingenious to say OK, now I will remove the feedback from an amp design to run with 37.5 dB of Negative feedback (for example). May as well remove the turbo charger from an engine designed to work with a turbo charger and expect it to be a fantastic non-turbo charged engine. It ain't gonna happen. You are changing the design intent and will now have to re-design the engine or the amp.
Try running an Op Amp without feedback, the bandwidth will drop like a stone. Take a look at any Op Amp spec sheet.
Negative Feedback also stabilizes gain. etc. etc.
Feedback: Fred Waldhauer.
I would argue that if you like the sound of an amp and it drives your speakers properly then I wouldn't spend a lot of time looking under the hood or pondering how much feedback it has. If it's an excellent amp then the designer probably used the right amount of Global and Local Negative Feedback, etc, etc, etc.