Is there anybody built a non feedback amplifier?? - Page 24 - diyAudio
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Old 25th February 2004, 11:18 PM   #231
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by Tube_Dude

The output mosfets are used as folowers or as comum source?

If they are not used as folowers how the 2-3 Ohms output impedance is achieved and if they are used as folowers why the ouput transformer?

As common source, obviously. These MOSFETs do have a triode-like output charachteristics = low output impedance in a CS mode. He uses these with quite high voltage power supply and an output transformer from a PP valve amplifier.

Essentially it is a transistor equivalent of a class A PP triode valve circuit without any negative feedback.

When (and if) he will get his circuit on the net I'll post a link

x-pro
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Old 25th February 2004, 11:20 PM   #232
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Quote:
Originally posted by Steve Eddy


If they're not used as followers, the 2-3 ohm output impedance can be achieved with the output transformers. A step-down transformer reduces the source impedance by the square of the turns ratio.

In other words, if you have a step-down transformer with a 4:1 turns ratio, the source impedance of the circuit driving the transformer will be reduced by a factor of 16.

se
Yes...but a drain of a Mosfet behave like a current source...in the hundreds of KOhms even if reduced by a factor of 16....
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Old 25th February 2004, 11:47 PM   #233
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Quote:
Originally posted by Tube_Dude
Yes...but a drain of a Mosfet behave like a current source...in the hundreds of KOhms even if reduced by a factor of 16....
But if he's using them common-source, the output impedance will be roughly that of the drain resistor.

Seems like he's kinda sorta got a Son of Zen with an output transformer.

se
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Old 26th February 2004, 12:20 AM   #234
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Charles,

Well, after all that I still can't help thinking that if the factors that can affect the sound adversly when NFB is used are really fully understood there is no reason why they cannot be overcome to the extent that the sound can equal that of a well designed amp without NFB.

I know that this is quite a bold statement but I just have a feeling that if every element is attended to it may be possible.

As I write this I am listening to our BBC Radio 3 culture / music channel through my current amp and although there are some things that could be done better I honestly can't hear exagerated detail and I think I know what it sounds like.

I don't know how your amps would sound incomparison. When I finnish my next effort I will have to take it along to one of the dealers here that are just begining to stock your stuff and see If I am deluding myself !!!

Your sentiment earlier to this kind of thinking was why not just build an amp without feedback. Well the honest answer to this question is that I would not really know where to begin, like the post a few pages ago demonstrated, building an amp such as this can come out sounding quite dreadful if you don't know what you are doing. Anyway I have been developing my ideas for class A with NFB for years now and I am curious to see how good they can get so as they say on one of our UK quiz shows " Iv'e started so I'll finnish "

You were not keen to mention the factors that you thought might be responsible for NFB messing up the sound so I start the ball rolling as see what reaction I get. They are expressed here as possible solutions.

John Linsley hood is of the opinion that stability is very important to achieve a smooth sound. So for me a good NFB amp has to be fast, perhaps very fast. This means that compensation, if needed, can operate well away from the audio band. this should overcome the time delay problem that might somehow change the charactor of the sound.

every part of the amp including the o/p stage must be very quiet. I think choke regulation and very low open loop Z class A o/p will help here. Class AB o/p's are always noisy ( switching transistors ) and should therefore be avoided if NFB is used.

The feedback cct must be free as much as is possible of memory distortion which could play all kinds of havoc depending on what distortion it is attempting to deal with.

I think simplicity is also vital.

not to mention a short FB path

That's about all I can think of at present

Oh of course I must not forget a little o/p stage emitter degeneration - I have tried this this already and the effect was really quite wonderful.....

regards

mike
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Old 26th February 2004, 12:33 AM   #235
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Quote:
Originally posted by jam
X-Pro,

A schematic would be nice.

Steve,

Enough with transformers...................................... .

Try this
http://duteela.et.tudelft.nl/~wout/o...edback_LNA.pdf

Regards,
Jam

I hate it when I see this. Ulrich Rhode did this 25 years ago and he is not even referenced. And yes, that's THE Rhode.
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Old 26th February 2004, 12:44 AM   #236
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Originally posted by x-pro


In a strict sense, yes . Even in not so strict sense, ignoring internal and parasitic feedbacks, still yes, as any follower IS a negative feedback circuit. To create a really (let's call it "no any additional feedbacks" = NAAF ) circuit you have to use only common emitter/source/cathode circuits without ANY degeneration. It is reasonably easy to create a voltage amplifier like this if you don't need much power. However it is very difficult to make a NAAF power amplifier using transistors. There are quite a lot of valve one's thought.

There is an amplifer build (literally in last few weeks) by a Russian DIY-er in Moscow using NAAF approach - it is class A push-pull using two Russian HF MOSFETs, quite linear on their own, and a transformer output. He's got about 10W output into 8 Ohm, about 2 - 3 Ohm output impedance and really no any additional feedbacks, even local degeneration. He likes the sound a lot

x-pro


x-pro,

You say that: To create a really (let's call it "no any additional feedbacks" = NAAF ) circuit you have to use only common emitter/source/cathode circuits without ANY degeneration

What is an CC without degeneration?

Does a CE alone also have some kind of "feedback"?

Why do you say that a CC have feedback, isn't that an inherent property(you know holes & electrones...) as well as the increasing voltage over the resistor will cause the current to increase, the only diffrent thing is that the resistor acts quite linearly?


Cheers!
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Old 26th February 2004, 01:33 AM   #237
SY is offline SY  United States
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Well, after all that I still can't help thinking that if the factors that can affect the sound adversly when NFB is used are really fully understood there is no reason why they cannot be overcome to the extent that the sound can equal that of a well designed amp without NFB.
Vice versa is much more difficult!

Quote:
To create a really (let's call it "no any additional feedbacks" = NAAF ) circuit you have to use only common emitter/source/cathode circuits without ANY degeneration.
How do you get the Rbe down to zero?
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Old 26th February 2004, 04:35 AM   #238
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Originally posted by scott wurcer
I hate it when I see this. Ulrich Rhode did this 25 years ago and he is not even referenced. And yes, that's THE Rhode.
Hmmmm. Look who joined our lovely little tea party. None other than Scott Wurcer. I'm sure that most of you have never heard of him, but he is *quite* an experienced designer.

His resume includes the Analog Devices AD797 and AD711. Probably knows a thing or two about feedback. Plus he's a heck of nice guy. One time I had a question about the internal circuit of an older Analog Devices IC. This part had been designed before the current era of PCs and they couldn't find the hard copy documentation very easily. So this guy pops the top off of a metal cased one, studies it under the microscope, and not only answers my question about the internal circuit but also gives me the resistor values!

I asked him how in the heck he could figure out the resistor values by looking at it. I figure maybe he probed the device with some fantastic piece of nanotechnology, but the actual answer was much simpler. He has designed so many ICs that he can calculate a resistor value simply by observing the physical dimensions and knowing the bulk resistivity of the undoped silicon. I guess that shows it's not his first BBQ....

By the way, you may want to look at his introduction here:

http://www.diyaudio.com/forums/showt...319#post311319

Seems he's not only designed some of the best performing op-amps on the planet, but also something about a phono stage featuring.....no, he can't be serious. No, he must be lying. No, he must be putting us on, knowing we're such a bunch of gullible audiophools. Anyone who knows *anything* about *real* electronics knows that it just ain't possible.

So forget that post. It's just a hoax. Instead here's a circuit for the original poster. After all, that was what he asked for...

This is a prototype of the Ayre V-3. It's pretty close to what went into production over 10 years ago. We discontinued it about 4 years ago. Our current designs are much better, but this is as good a starting point as any.

Enjoy,
Charles Hansen

PS -- To the moderators: Why not allow PDFs to be attached? The zip archive doesn't achieve any meaningful compression, and just makes it a pain in the neck to save and unzip before viewing.
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Old 26th February 2004, 04:47 AM   #239
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Quote:
Originally posted by mikelm
Well, after all that I still can't help thinking that if the factors that can affect the sound adversly when NFB is used are really fully understood there is no reason why they cannot be overcome to the extent that the sound can equal that of a well designed amp without NFB.
Hello Mike,

Well, I just don't quite get where you're coming from. Why go to all that trouble? Why not just build an amp without feedback?

Best regards,
Charles Hansen
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Old 26th February 2004, 05:16 AM   #240
SY is offline SY  United States
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To the moderators: Why not allow PDFs to be attached?
Outside of our powers. That is reserved for the Architect of All Coding (i.e., Audiofreak).

Scott Wurcer has killed more of my liver cells than I care to remember.
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