Current feedback - Voltage feedback, how do I see the difference? - Page 2 - diyAudio
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Old 21st June 2004, 01:05 PM   #11
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by AKSA
....... as I see it, at no stage would either diff pair device switch off completely until clip, or on those rare occasions when the feedback loop loses control.

Cheers


Hugh
There is no diff. stage in 'current feedback' arrangement....

In voltage feedback circuit, diff. stage normally used...and this goes into current clip when the tail source current is switched entirely from one member of the diff. stage to the other...

This will will not necessarily occur when the amp. goes into voltage clip at the TIS output....

...as 1st stage current clip is entirely a function of frequency, and the size of your minor loop comp. cap....

......assuming of course, that you've adopted single-pole minor-loop comp. in the first place........
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Old 21st June 2004, 01:10 PM   #12
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Ouroboros
You can run a current-feedback input stage in class-A, but it does mean that the dissipation in the current-mirror stages is rather high.

Normally the input stage will run in class AB, to ensure there's plenty of current available to charge and discharge the compensation cap between the output of the current mirrors and ground.
Correct in all respects....
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Old 21st June 2004, 01:47 PM   #13
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John,

Sorry if I was unclear. I know both you and Gilbert are native
speakers of english. By different languages I meant the kind
of differences in terminology that often arises even between
people in neighbouring fields of technology. What I meant
was that perhaps there were people like Gilbert who knew
about the phenomenon, as he claims, but that he didn't
realize TIM as described by audio people had anything to
do with this. Or perhaps that is not the case? Anyway, as
Upupa Epops said, it is not really that important who was
first. It was unnecessary of me to mention it in the first
place since I didn't mean to start a debate on whether
Gilbert was right or not on this.
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Old 21st June 2004, 04:30 PM   #14
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Quote:
Originally posted by mikeks
Would avoid 'current' feedback in audio applications, as the input stage operates in class-AB at the feedback node.
Why would you care, since you can't hear the difference anyway?
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Old 21st June 2004, 05:08 PM   #15
dimitri is offline dimitri  United States
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Quote:
Why would you care, since you can't hear the difference anyway?
"Oh, sure" john curl (r)
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Old 21st June 2004, 06:37 PM   #16
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This is the situation, folks. There are many feedback related events that generate distortion. TIM (SID), PIM, load instability, and higher order conversion from lower order nonlinearities.
A sufficent slew rate, coupled with a linear input stage, eliminates just TIM.
Please remember, that 30 years ago, when we were wondering what was wrong with our audio designs, 0.5 V/us was considered by MANY COMPANIES as sufficent. This included tape recorders, studio boards, phono input stages, etc, etc. We had to FIGHT to get people to understand that simple slew rate was a worse case situation, AND that the distortion started building 5-10 times BELOW slew rate.
After 100's or even 1000's of measurements and giving papers at the AES, etc, people begrudgingly started to make faster IC's at low cost, as well as faster power amps, and the problem began to recede.
Then you get the 'academics' who want to take over, by criticizing previous work, and renaming the distortion mechanism.
And so it goes.
As far as I understand, two stage compensation is rarely used in most audio designs.
Once, 25 years ago, I developed a discrete 600 ohm driver for Sound Technology that could do VERY LOW DISTORTION at 100KHz and below. I used 2 stage compensation in order to get a 100KHz open loop bandwidth in order to have MAXIMUM feedback available at 100KHz. So far as I know, this circuit resides in the next generation ST distortion analyzer, after my own equipment.
Folks, let's give credit where credit is due, and ignore the small stuff that you don't completely know all about.
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Old 21st June 2004, 06:45 PM   #17
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Charles Hansen


Why would you care, since you can't hear the difference anyway?

Neither can you....

Nobody...but nobody can hear the difference between a current feedback amp. , a voltage feedback amp, etc....

If all these units are calibrated to deliver exactly the same voltage swing across the same transducer.....below clip....and each possesses low THD....say below 100ppm across the audio band.......throughout the voltage swing.....NOBODY!


This is common sense.....scientifically proven common sense....

Why is it sooo hard to make simple factual statements in this field without having them disputed??

Subjectivism reminds me of the Vatican...Galileo....and the later's simple factual statement......the earth goes round the sun.....

This nearly got him killed by the 'infallible' pope....Same with this subjectivist nonsense.....Cannot make a statement of fact ...proven fact without somebody disputing it....because he can....


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Old 21st June 2004, 06:50 PM   #18
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Quote:
Originally posted by mikeks
Why is it sooo hard to make simple factual statements in this field without having them disputed??


because if it were easy, a lot of people wouldn't be able to make a living off of selling otherwise average amps at extrodinary prices.

Why is it sooo hard to understand so simple of a fact? Why would you keep disputing those "infallible" claims of super human hearing?

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Old 21st June 2004, 06:53 PM   #19
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by millwood


mikeks, you aren't suggesting that current feedback applies to class-ab at the input stage, are you?
No....'current feedback' obtain their high slew by running the 'small signal' stage in class AB....

This has nothing to do with the class of operation of the output stage.....



Quote:
Originally posted by millwood


....the jlh1969 for example has a current feedback input stage which operates in class a (the input stage).
No.......it has a voltage feedback input stage.....

He merely chose a single common-emitter stage over the usual diff. stage....
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Old 21st June 2004, 07:23 PM   #20
jam is offline jam  United States
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Mike,

You just blew it..................................
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