Is the CFB topology superior, and why? - Page 35 - diyAudio
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Old 6th November 2012, 06:52 AM   #341
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Lazy Cat View Post
Look wahab all these guys who likes CFB amps probably started with VFB amp as usual, than probably explore different VFB designes, made different amps from simple to complex VFB topologies, listened, compared, searching continued and one day they heard CFB amp. End of the story.
I also heard CFB amps of all kind well before it became a fashionism in this forum, yet , it didnt end the story at all.

As for the VFB amps , there are a lot , including in this forum ,
wich are genuine lemons thanks to bad engineering habits ,
so yes , better an average CFB than a badly designed VFB,
although there are also CFB lemons by the tons.

Last edited by wahab; 6th November 2012 at 06:55 AM.
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Old 6th November 2012, 07:35 AM   #342
wahab is offline wahab  Algeria
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Quote:
Originally Posted by MiiB View Post
Nico Built the simple SSA and really liked it.
A story from the Arab world :

When all animals reunited , they decided to ask the donkey female
about who has the most beatifull children.
The donkey answered that in all honnesty she found her own children
to be the most awsome.....


Quote:
Originally Posted by MiiB View Post
All The best amplifiers i have heard has turned out to be CFB.

It seems like those that listens to things come to the same direction. There just seems to be something good/nice about the way the CFB amplifiers work.
"There seems" is just a subjective position.
What is there exactly , as what is audible is forcibly measurable.

Quote:
Originally Posted by MiiB View Post
Now instead of wonder why and try to find good valid explanation, some people run to the mine is bigger than yours arguments by pulling out the low distortion card, as if that is the only truth. Ok sound perception and musical performance which must be the final goal has diversities and can be challenged for not being objective. But in the end it's the only objective that really matters...!!
The distorsion card as you call it is more than a single card as it
has other meanings that simply THD.

Low THD amps will almost invariably yield low IMD as well , wich is way
more intrusive non linearity and is a better scale than the classical THD
to evaluate an amp s ability to reproduce a signal without re shaping it
musicaly speaking , what we call High Fidelity.

Quote:
Originally Posted by MiiB View Post
By pulling out the tools we already know we get now where. looking at things with an open mindset may just take you somewhere.
As already said , what is audible is measurable so if an amp
that measure worse sound better then it means that distorsions
can make an amp sound good the same way an aphex aural exciter
make a poor recording sounding good.

Surely that someone having one of thoses amps that sound well
thanks to harmonics enhancement will refuse to aknowledge that
his amp has poor absolute perfs, instead he will say that it sound
good for reasons that are still unknown but not because of its
lack of linearity.
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Old 6th November 2012, 10:15 AM   #343
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by wahab View Post
As already said , what is audible is measurable
Yes, what is audible is measurable. But there are many measures, not only THD. Even probably more measures that has not been known to have an audible effect.

Quote:
Originally Posted by wahab View Post
so if an amp that measure worse sound better then it means that distorsions
can make an amp sound good the same way an aphex aural exciter
make a poor recording sounding good.
Yes, distortion can make an amp sound good, subjectively. But there are many other measures instead of THD, and there is no one measure/formula that summarize them all.

Which one is better: (A) DF=1, THD=0.001% (B) DF=100, THD=0.01%.

If that is easy (or impossible), can you make it harder by changing the composition? And it is only 2 variables!

Quote:
Originally Posted by wahab View Post
Surely that someone having one of thoses amps that sound well
- thanks to harmonics enhancement - will refuse to aknowledge that
his amp has poor absolute perfs,
What is ABSOLUTE performance
And what is harmonics enhancement
I think you have an issue with different tastes regarding second or low or even order harmonics. Why not fill your creative mind with something higher, something that is still "unknown" (instead of trying to look smart around idiots), like Rod Elliot new post regarding inter-modulation of symmetrical versus asymmetrical input signal.

Quote:
Originally Posted by wahab View Post
instead he will say that it sound good for reasons that are still unknown but not because of its lack of linearity.
There are many things in audio that is hard to explain scientifically, but the phenomena exist. Same like strange things in life, like meta-physical things (such as how rusted nails can grow inside your body). You can try to explain those using your best ability and your best knowledge about Physics, but those who think they are the real experts in Physics will laugh at you.

There is indeed something about CFB amps that I haven't heard in VFB amps. If you think that this "something" with CFB is related to harmonics (soundwise), I think no, not at all. I think regardless of validity I'm satisfied with my knowledge about this, but I don't want to discuss it because the real experts will laugh.

I believe that those real experts also cannot explain it if they can observe the phenomena. But they can't even observe the phenomena. So it's like they are saying "Prove it if God/Ghost exist, because it doesn't".
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Old 6th November 2012, 10:47 AM   #344
AndrewT is offline AndrewT  Scotland
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After reading many of these arguments, I have to ask what the Thread title asks:

Why?

If the CFB sounds superior, then why?
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Old 6th November 2012, 10:55 AM   #345
Elvee is offline Elvee  Belgium
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Here is the comparison between the two versions of amplifier from LTspice's examples.
The test conditions have been properly normalized, and the silly impedance levels corrected (at least for the VFB; for the CFB I have kept the values chosen by Esperado, even though they are unusually low).

Some remarks about the way the initial comparison was made: with the very low feedback resistor values, the CFB was in fact on the verge of being VFB: the emitter resistance of the transistor at ~5mA is ~5ohm, with the equivalent resistance of the FB network under 10ohm.
This means it classifies as CFB, but just.
By contrast, the ridiculously high values in the "VFB" version made it in fact CFB too, and even more so than the one that is supposed to be so: the input resistance of the transistor is 800ohm, much less than the FB network.
This means that in fact, the comparison was not between VFB and CFB, but between two half-baked CFB.
Another problem was the level, which was so high that it brought the VAS into presaturation.

The problems have been fixed, at least partially, the levels exactly equalized, and the phase margins have been set at 50°, meaning the comp capacitor is 8.2p for CFB and 22p for VFB.
THD is 0.13% for CFB, and 0.107% for VFB. Not striking, because in this amplifier, the main source of distortion is elsewhere: it is shown by the high level of 2nd harmonic, which remains high with the VFB, when it should almost vanish. The poor quasi output stage is to blame.
Next is the bandwidth: the CFB has a -3dB point at 32MHz, against 25MHz for the VFB.
Thus, a moderate improvement, traded for a moderate degradation of linearity.
But the poor performance of the amplifier hides part of the picture; which is why the sim I made earlier concentrated only on the parts of interest: the VFB and CFB cores.

Now, if we want, we can add a degeneration to the VFB to mimic CFB: third pic.
The difference is becoming much smaller, and it is indicative of the true speed penalty brought by the second transistor in the loop.
Attached Images
File Type: png CVcompt.png (147.4 KB, 101 views)
File Type: png CVcompf.png (141.1 KB, 101 views)
File Type: png CVeq.png (115.2 KB, 99 views)
Attached Files
File Type: asc audioampMod.asc (8.7 KB, 2 views)
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Old 6th November 2012, 10:56 AM   #346
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Quote:
Originally Posted by Lazy Cat View Post
Look wahab all these guys who likes CFB amps probably started with VFB amp as usual, than probably explore different VFB designes, made different amps from simple to complex VFB topologies, listened, compared, their search continued and one day they heard CFB amp. End of the story.
I listened to VF amps all my life and I was always perplexed why products with so good specs sound so dull! Most of the time it is sound that passes by the listener, never connecting to the pleasure centers of perception. You can't say exactly what is wrong with VF but listener remains disinterested in musical emotion. I can not say that VF amps are downright bad, they are simply not sounding good enough. They are mediocre.
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Old 6th November 2012, 12:15 PM   #347
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It has been discussed from the beginning that the differences appear to be the starting points and not the end point. The Vf brings higher non-linearity but a lot of open loop gain to use for correction (via either global or local feedback) compared to the Cf which has lower non-linearity and less gain (which can be increased by the feedback impedance but this is not free).
At the completion of a competent design(s) from both sides it would be surprising if most of the parameters were not nearly identical. Depending on the constraints one approach may be more suited than the other. May be more helpful to dig into the details and try to characterize where each is more appropriate.

The slew rate argument bothers me, I just cant see having more than needed as a benefit especially if its purely a class-ab operation. If the Cf amp really turned off one of the input transistors to get that added current just how far behind the input did it get?

Thanks
-Antonio
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Old 6th November 2012, 12:23 PM   #348
AndrewT is offline AndrewT  Scotland
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There seems to be a consensus that the most signal slew needed for a 100W into 8ohms amplifier is about 5V/us.

If one then adjusts the amplifier to just be able to meet this slewrate then one finds that it distorts.
From experiments it seems that by increasing the amplifiers slew rate up until it is about 10times the maximum signal slew that this added distortion reaches a minimum.

End of consensus.

That seems to imply that 50V/us is required for fast music signals, if one wants to reach that distortion minima for music reproduction..
And increasing beyond the 10times achieves little if anything of an improvement.
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Old 6th November 2012, 12:44 PM   #349
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Andrew

Indeed, and this would appear to be a result of large signal bandwidth requirements which should be met. That is, is there ever a need to even measure the slew rate of an audio amplifier? I can see where it may serve as an indirect metric and or for diagnostics.
If one could specify requirements for their desired audio amplifier is it appropriate to include slew rate?

Thanks
-Antonio
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Old 6th November 2012, 01:01 PM   #350
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Jay View Post
snip
Your point of view is interesting but since you have some maths formal
training and are used to simulators why not try to check a few claims
and bring a stone to the build that would have way more weight than
subjective assumptions.

Electronics is a science backed field and in this respect it accept
to be questionned with practical experiences so any claim can be
checked and in the subject that interest us CFB supporters still fall
short of proving their claim of so called superiority of this topology
using scientifical yet simple arguments.
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