Is the CFB topology superior, and why? - Page 20 - diyAudio
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Old 19th October 2012, 01:20 PM   #191
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Why does the inverting transistor add (and not partially cancel) distortion?

Daniel, Why does the cfb have more gain capacity, is it this inverting transistors extra phase near the it's ft or is its output impedance, or something else completely?

Thanks
-Antonio
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Old 19th October 2012, 01:42 PM   #192
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Quote:
Originally Posted by magnoman View Post
Why does the inverting transistor add (and not partially cancel) distortion?
We use the feedback signal to cancel (partially) errors. If you add error (distortion) to this signal, this error will be subtracted from the original signal, means added in opposite phase.
Too, the delay added by the speed limitation of this extra transistor will reduce error cancellation at high frequency , because you subtract a signal to an other witch in not exactly in phase opposition That explain several phenomenas, like reduced damping factor and increased distortion at high frequencies..
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Old 19th October 2012, 01:58 PM   #193
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You are attenuating local negative feedback to that 1st emitter.

Increased gain, increased impedance, and increased distortion
(unless a global loop is closed elsewhere, then it doesn't matter),
and less sharpness to the knee of the local pole.

I've used this before to attenuate feedback to the cathode of a
Coincertina phase splitter. Giving +Mu/2 and -Mu/2 gains, and
true equal impedance phase splitting.

Last edited by kenpeter; 19th October 2012 at 02:02 PM.
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Old 19th October 2012, 03:55 PM   #194
jcx is offline jcx  United States
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Quote:
Originally Posted by Esperado View Post
???
It is exactly the contrary.
See here the difference, on the same amp between the two configurations: www.esperado.fr - Le crescendo revisité
It is easy to understand why, as current feedback remove one pole in the loop (the input inverting stage) , it increase the bandwidth.
It is easy to understand why, because Current .Feedback is directly removed from the original signal, resultant signal to be amplified has lower level -> lower distortion. And why the inverting stage of a voltage feedback, adding his own distortion to the feedback signal; add total distortion.

And, even on a good amp schematic, you can really and definitively hear the difference. Both in preamps and power amps.

Curent feedback has advantages in near all the domains: Open loop bandwidth (Slew rate), HF phase shift, H.D. and IM, so... transparency. And only one negative side effect: reduced PSSR that you can cure with accurate schematic design.
I downloaded the Ltspice - missing zener definitions

http://www.esperado.fr/images/storie...-crescendo.zip

used diode models from: www.ece.utah.edu/~ece2280/diode.lib

CFA sim doesn't bias correctly, VAS is cutoff

I'm traveling for the next week so may not debug the sim any further - care to correct it for the curious here?


quickly found, fixed input collector R value to web schematic- now the VAS bias is nearly 4x as hot as the VFB amp - is that fair?
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Old 19th October 2012, 05:20 PM   #195
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Quote:
Originally Posted by jcx View Post
I downloaded the Ltspice
I don't understand your problem ( I'm not an expert in simulation software, i work in real world, old style).
I run LTspice at home, and this sims runs ok on my computer. Zener had been defined with the included LTspices zeners models. And currents are ok and as expected.

The story is i had made this mod in *real* first, and it worked fine after tuning. Since this time it runs in my system and i am too lazy to open-it and look at the real values.

I had designed this spice months later to respond at a request, and i'm not sure it reflect the exact real values i used. But i'm sure it is not so far, and it measure quite similar. Distortion apart, i can't tell because they are under the limits of my measuring instruments.
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Old 19th October 2012, 05:36 PM   #196
jcx is offline jcx  United States
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I keep Ltspice updated, maybe they've changed their lib components since you ran it - or you added models to your LTspice directories and forgot

It can't be verified that the sim is "fair" if you aren't going to fix the sim values as you claim they need to be to relect your circuits

I and others find you claim of orders of magnitude better 1kHz distortion fro the CFA topology unreasonable - stepping through the circuit values, sim results will help us learn what is going on

Last edited by jcx; 19th October 2012 at 05:51 PM.
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Old 19th October 2012, 06:57 PM   #197
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Quote:
Originally Posted by jcx View Post
... sim results will help us learn what is going on
What i can do to help ? The added models are in the file. All others where parts of original library, as far i remember... My LTspice version is 4.16p
... I do not like this program ... capricious...with a so stupid interface...
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Old 19th October 2012, 08:41 PM   #198
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I have showed my simulation of the crescendo in other thread(SSA) and result were good but far from what Esperado claim.
THD20k at 70W/8R 0.013422%, at 8W/8R 0.019876%
THD1k at 70W/8R, 0.000984%, at 8W/8R 0.000565%
Strange that THD20k increased at lower power.
Esperado spice models for the laterals were wrong.
dado
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Old 19th October 2012, 09:44 PM   #199
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Originally Posted by dadod View Post
Esperado spice models for the laterals were wrong.
They are models for the original Hex from International Rectifier (My parts, not Lateral).
They where picked in the Bob Cordell site, and i recently tried an another source for the same references with very similar results.

Again, the absolute values of distortions and slew rates, has no interest, as they will not reflect the real disparities. Only the difference between the two schematics, because we are in the same landscape, with the same models. Agree ?

But i don't understand those differences. It just convince-me a little more to never lie on Spice simuls for other things than evaluate raw values. or filters fast calculations.
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Old 19th October 2012, 10:53 PM   #200
wahab is offline wahab  Algeria
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Quote:
Originally Posted by jcx View Post
CFA sim doesn't bias correctly, VAS is cutoff
I had the same result using another simulator , the VAS work in class C ,
otherwise the amp would oscillate.

I asked for real life measurement of the said VAS current ,
the answer is still on the waiting months later.....
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