Is the CFB topology superior, and why?

[dadod]

Originally Posted by homemodder

Just look at the CFB opamps by National

What kind is LM4562? Tell me one CFG to compare with it.

[catalin]

I don't understand why you are taking in consideration the harmonic distortion beeing the most important .Personaly I don't make a difference under 0.06 % .
As I said already there are others to consider ...
Nobody want's to talk about the phase or about the negative feedback speed ?

[dadod]

Originally Posted by catalin

I don't understand why you are taking in consideration the harmonic distortion beeing the most important .Personaly I don't make a difference under 0.06 % .
As I said already there are others to consider ...
Nobody want's to talk about the phase or about the negative feedback speed ?

What is the negative feedback speed, you on slippery path here?

[catalin]

Is the speed of the feedback circuit before the substracting point .To have a good feedback correction this needs to be more faster than the speed of the corrected signal circuit or the speed of the signal circuit .
Can somebody post a open loop gain&phase response for VFB and CFB ?

[Elvee]

Originally Posted by catalin

Elvee ,I think that all 3 "preconceptions" are theoretical correct .
Please say where I am wrong in these theories .

Let us try, or at least begin to ...

Originally Posted by catalin

Fact 1:
Nobody talks about transfer functions; about that we have a first pole at low freq in VFB and the first pole in CFB is more more higher .This will shift the phase for VFB below 20khz with -90 degrees ,but in CFB ,because the pole is at a freq very high ,we have no phase shifting in all audio band !
So even the gain is high at VFB at low freq ,the phase is shifting from 0 to -90 in audio band ;It starts with 180 in DC to very low freq ,then at about 10-200Hz we have 45 degrees(the pole) ,and after we have 90degres at 1-3 khz.
All these in open loop .

So if the designers consider the open loop gain for thd why they don't consider also the open loop phase ?

1/ Absolute phase is irrelevant, it simply amounts to transmission delay.
2/ Variation of phase with frequency is not only innocuous, it is required to minimize group delay (the relevant parameter): if it is not proportional to frequency, it will cause phase distortion.
3/ Anyway, what counts are the residual errors (group delay, THD, etc) once feedback has been applied. Otherwise, comparisons are meaningless: if you have an open-loop group delay of 1ns and a loop gain of 20dB, this will be ten times worst than an OL group delay of 10ns but 60dB loop gain

Why accuphase amps with CFB sounds better ?

I would call that a personal opinion, not exactly a fact

Also phase for negative feedback input in VFB is turned by 2 poles but in CFB is turned only once and this is at high freq.If in VFB we have 2 common emitter stages for the negative feedback ,in CFB we have a common emitter and a common base .

So logical is to choose the circuit with less transformation of the negative feedback signals .Because the non-linear distortion is important .

I fail to see any logic

So the CFB at this hour has high speed ,high bandwith ,excelent phase response .
The VFB has speed but not as much as CFB ,low bandwith ,poor phase response .

That is a purely anecdotal and qualitative appreciation (and not a very clear one)

Fact 2 :
What about the THD ?
Well , we can say that the THD is almost the same with VFB and CFB ,but at low freq .

Here again, what counts is the end result: if an amplifier shows a 20x increase in distortion from 1KHz to 20KHz, it will still be better than a flat one having an higher overall THD

Is more important the THD at high freq than at low freq ?
I am thinking only at the fact that the low frequencies in music is at least 3-5 times higher in amplitude than the high freq and that the SNR is also higher at low F and low at high F.
So to have a good separation between the instruments we need more low thd at high freq compared to the thd at low freq .The small amplitude signals in music are the high freq signals majority !

When we listen music we are searching to have good separation of instruments ,we are searching also to hear the complex pieces clear ,clear means that the high amplitude signals don't distort the small signals .The brain recognize the high amplitude easy but the low amplitudes (which is the high frequencies ) is a little more hard to understand .Also the small amplitude signals(HF) have a smaller SNR .
So then we need more accuracy at high frequency .

Not very clear either, but it goes against all accepted psychoacoustic data and evidences: the ear gradually looses its ability to detect distortions outside the 400 to 4KHz range, is less effective at low levels and is sensitive to masking

Fact 3.

Negative feedback circuit should have a high speed ?
What speed will have the negative feedback signal to compensate the output signal in CFB compared to VFB ?

In order to make meaningful and fair comparisons, post the .asc of your CFB:



I will convert it into its exact VFB-dual image

[catalin]

VFB


CFB


standard.zip is the model library for the BJT transistors

[Lazy Cat]

Originally Posted by catalin

So what topology should we choose ?

CFB naturally

I listened to lots of VFB amps in the past, some made by DIY, some from well known producers - high-end market leaders, than some day I made SSA in its simplest form and was fascinated by its sound since ever ... the rest is history.

[Lazy Cat]

Yes, tnx Catalin, It Is All About This Graph.

OLG phase and freq response, it is clear as sunny day after the storm.

[danielwritesbac]

Wow, Catlin, the VFB graph is just wickedly bad compensation overdo, and the CFB is very slightly undercompensated but your RF filter or feedback cap could trim off that slight excess. At least with the CFB there is a tiny excess instead of a severe shortage.

In the case of the CFB, the amp is running faster than the outputs, and the simple fix to align matters is set the amp to match those two curves perfectly, or use a slightly faster output device. With MJL21194, 4mhz, I got Circlophone pushed to 3.5mhz Phase Linear, but any faster amp settings (compensation reductions = faster) made that "exceeding" graph like shown above with more treble in than out. If you don't those two lines on your graph perfectly, treble quality will suffer a bit. The exceeding is a lovely problem to have because the fix is so very simple. Please adjust to fell the amp before exceeding the outputs.

This is an unfair race what with one example having no brakes and the other with the parking brake on. In the case of the VFB, you've got the parking brake on. Some kind of heavy handed compensation overdo mistake on that one. The pre sections are actually trying, but the more it tries to amplify, the worse it gets? I'm not sure what that's for.

[catalin]

Originally Posted by danielwritesbac

Wow, Catlin, the VFB graph is just wickedly bad compensation overdo

Can you say what you mean here ? I see that there are 55 degrees reserve .Minimum 45 are ok but not always the phase reserve it says things about the stability .Also it needs study with square wave response .
For example the "helpful " transitional Miller compensation does not add nothing at the phase reserve but will dramaticaly distort the square wave response with capacitive loading (1uF for example)


But here we are just talking about the CFB vs VFB and the pictures are just for the quality comparision .
And I don't understand why the curve with gain must be on the same place like the one with phase .Maybe you want to say another thing .
Thank you .