Is the CFB topology superior, and why? - Page 30 - diyAudio
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Old 4th November 2012, 02:07 PM   #291
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Quote:
Originally Posted by Lazy Cat View Post
Can someone explain why we need extra transistor in FB path if the output impedance from where FB is taken is low Z?
Any nonlinear element inserted in FB path makes the whole regulation process to deal with extra unnecessary corrections that wouldn't be needed if this element wouldn't be inserted.
That's my point from the beginning of this thread, both in the linear point of view and in the time domain.
And we both provide exactly the same argues.
But it seems we are talking in a desert, for some contributers.
No one pretend that it is not possible to reach the same level of performance (or better) with other amps, using better parts, other more complex FB implementations (error correction, Nested feedback loops, inverting amps where feedback is applied before - input at low Z etc...).
We need to be serious and compare what can be compared.

What i said from the beginning, providing an example to test-it, is: everything equal around, a current feedback implementation will always give better results than VFB in the same amp topology.
It measure better, and lot of listeners pretend that it sound better.
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Last edited by Esperado; 4th November 2012 at 02:27 PM.
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Old 4th November 2012, 05:21 PM   #292
jcx is online now jcx  United States
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there are few costs from the "added" diff pair Q - small signal Q at today's hotter bias are running at 100s of MHz ft - meaningless added delay compared to commonly used power output Q with ft from low MHz to low 10s of MHz ft

the better matching of like polarity Q gives better distortion cancelation in a diff pair vs the CFB complementary input

there is no good reason I know of to believe CFB has inherently lower audio distortion for similar number of gain devices, bias levels, compensation - certainly not the 10x factor you claim

I strongly suspect your comparison is "unfair" due to easily "debugged" reasons like bias, compensation, test level differences between the circuits

you refuse to clean up your sim which you claim "proves" your assertion - until then, and we can compare clean sims I see no reason to believe you


I am not dismissing CFA - in fact I use the TPA6120/THS6012 every place it makes sense

Last edited by jcx; 4th November 2012 at 05:40 PM.
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Old 4th November 2012, 05:49 PM   #293
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Originally Posted by jcx View Post
you refuse to clean up your sim
I don't know what you are talking about. You are welcome to "clean" anything you want and demonstrate the contrary, or provide any sims of yours to do it.
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Old 4th November 2012, 05:56 PM   #294
MiiB is offline MiiB  Denmark
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Ok.. then look at it this way.. what is the signal above the differential pair..?? modulated current.. then why not inject the feedback as current modulation, rather than going through a V/I conversion.. that has to pass through not one but two transistors to blend with the signal. You can go through a 1000 sims.. It'll tell you nothing more than common sense can give you just by looking at the nature of things
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Old 4th November 2012, 06:31 PM   #295
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Quote:
Originally Posted by MiiB View Post
Ok.. then look at it this way.. what is the signal above the differential pair..?? modulated current.. then why not inject the feedback as current modulation, rather than going through a V/I conversion.. that has to pass through not one but two transistors to blend with the signal. You can go through a 1000 sims.. It'll tell you nothing more than common sense can give you just by looking at the nature of things
Devil being in the details, can you show the schematic you are referring to?

But as already stated many times, adding a transistor provides a first order compensation of non-linearities, meaning in practice you get a ~20dB improvement practically for free from the point of view of closed-loop margins, and other parameters
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Old 4th November 2012, 07:02 PM   #296
wahab is offline wahab  Algeria
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Originally Posted by Esperado View Post
I don't know what you are talking about. You are welcome to "clean" anything you want and demonstrate the contrary, or provide any sims of yours to do it.
The sims says that the VAS current is negligible and consequently
exponentialy rising with temperature to still a negligible value
of about 180uA at 70°C.

At 0°C the VAS current is so low that the closed loop bandwith
is about 200Hz , rising to 10khz at 20°C , 200Khz at 40°C and
1.5Mhz at 60°C with a slight frequency bump at 1.1mhz
due to the VAS current/transconductance starting to magnify
high frequency poles.

In a few words , this amp will inherently and very audibly
sound much better once it has warmed and the hotter the better ,
though normal VAS current will produce a big peak at a few mhz.

The distorsion figures are nowhere near the 10X claimed
improved ratio , in fact there s no improvement at all
and as a whole it perform not as well as the original
crescendo for most significant parameters.
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Last edited by wahab; 4th November 2012 at 07:08 PM.
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Old 4th November 2012, 08:16 PM   #297
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Quote:
Originally Posted by Elvee View Post
But as already stated many times, adding a transistor provides a first order compensation of non-linearities, meaning in practice you get a ~20dB
What a misunderstanding of closed loop systems !! (stated many times ?)
Let look at a differential VFB amp.
The distortion produced by the +V input will be present in the output.
Remove-it from the feedback signal by a similar distortion in opposite phase in the -V stage, and you simply induce this distortion will be removed from the feedback signal, so never corrected by the feedback loop.
Adding this distortion in the signal path, you will correct two times this distortion, so get the same amount, but in phase opposition.
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Old 4th November 2012, 08:42 PM   #298
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Quote:
Originally Posted by wahab View Post
The sims says that the VAS current is negligible and consequently exponentialy rising with temperature to still a negligible value
of about 180uA at 70°C.
Not at all what i measured on my real amp and with LTSpice. I wonder where is the problem. Models ? Different Spice program ?
Currents are quite the same in the VFB and the CFB on my LTSPICE.
~300µa in the first stage, ~10ma in the driver, ~150ma in each output device.
I don't know where to simul temperatures.

Sincerely, i don't understand, i don't use sims enough and not experienced to them at all.
Can-you try to improve-it with your parts and see what's happens, or explain-me the differences you measure ?
(If you can do-it by pm or by mail, i would be very thankful to you)

[edit] the driver stage on your schematic is not the same than mine (zeners and pot to tune the quiescent)
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Last edited by Esperado; 4th November 2012 at 09:05 PM.
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Old 4th November 2012, 09:04 PM   #299
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Esperado View Post
.
~300µa in the first stage, ~10ma in the driver,
Theses two numbers dont correlate at all when looking at the schematic.

If the input stage current is 300uA then you will have
1.4K x 0.0003A = 0.42V biasing voltage trough the 1.4K resistors
connected in the input stage collectors.

How this 0.42V , that is the biaising voltage of the VAS common
emitter , can yield 100mV through the 10R emitter resistors
since a transistor has 0.65V Vbe..??....

In my sims the input stage current is 356uA , i increased the 1.4K
resistors to 1.5K wich will slightly increase the VAS current but
not enough to have it properly working in class A.

Indeed , just doing the maths mentaly rapidly suggest theses
defects.
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Old 4th November 2012, 09:27 PM   #300
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Quote:
Originally Posted by wahab View Post
...
The value i gave where from the VFB( original amp) simulation. where the value of R5 was 6.8K
On my CFB sim, i have 600µa across the R5 1.4k, means 68,55V on the Q7 base, and 69,27V on the emitter. VBE is correct.
With 69.39-69.27 = 120 mv and 12mv at the sides of the R12 10ohms, everything looks perfectly coherent.

ps: i tried on my sims to get the current as close as possible on the VAS, for fair comparison. In real life, as i said, i have optimized my CFB amp for best results...
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Last edited by Esperado; 4th November 2012 at 09:44 PM.
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