Is the CFB topology superior, and why? - Page 19 - diyAudio
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Old 18th October 2012, 09:41 AM   #181
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Quote:
Originally Posted by tommy1000 View Post
Same design as the Revox, I was given two channels stripped from a amp with damaged case. At the moment they are lying in the scrapheap in my backyard as they are awful sounding amps. Some really bad revievs on them around the net too which I concur with. They only sounded decent when biased to class A.
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Old 18th October 2012, 09:56 AM   #182
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Originally Posted by Joachim Gerhard View Post
The VFB design shown doesnt need to be in class AB to outperform CFB designs although they are more complex. Marantz is using that VFB design in some of their top of the range.
Good example is lm6171 circuit.
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Old 18th October 2012, 09:59 AM   #183
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Originally Posted by homemodder View Post
Same design as the Revox.
No. And the price was not the same neither. And i confirm, the Revox amp was not so good. Where symmetrical input, good power supply and selected paired parts makes all the difference.
Is was the same for the tape recorders, they where not playing in the same playground.
Btw: What are-you trying to do systematically arguing against all i can write ?
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Last edited by Esperado; 18th October 2012 at 10:07 AM.
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Old 18th October 2012, 11:13 AM   #184
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Here is some more information on the Studer A68:
Das STUDER und ReVox Infoportal | STUDER A68 - Analoge Studioendstufe

Here is one studio that uses it:
Sear Sound NYC, NY
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Old 18th October 2012, 11:24 AM   #185
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Some can claim indefinitely that VFB is better than CFB. And even provide bad designed examples to demonstrate-it. Or examples looking like current feedback, but not real CFB.

I said that CFB are more sensible to power supply quality. This is a major point where a bad supply can ruin all the sonic quality you can expect (specially in respect to dynamic behavior).

As far i'm concerned, and during 4 decades of work, CFB topology, when it was possible, had always given to me better results, both on measurements and in subjective listening arguments. That is the only reason why i appreciate this topology.

I have provided a link with two examples you can simulate. The VFB example is yet damn fast, and not degraded to twist the demonstration. The Current feedback mod was tuned for optimal result as well, with the exact same components. The result is clear:
VFB slewrate: 222V/µs CFB slew rate: 1200V/µs
VFB bandwidth: 300Khz, CFB bandwidth: 5Mhz
VFB THD : 0.0023% CFB THD: 0.000204%
IM is divided by 10 too when CFB.
And in real world, the improvement was subtle, yes, but positive, no doubt about that.

Is that talkative enough, homemodder ? Why don't you play with those simul, and try to understand, or, better try build an SSA, for example, to enjoy in real how it can sound GOOD, as reported by all who had build one ? Instead of posting useless opinions in forums ?
Where is the interest to publish every where and each time someone is speaking about CFB that VFB "can be better", with no technical argument ?

You can explore Error correction too, an other interesting topology, with nice results...

If you want to enjoy electronic for audio and improve your skill, do the same that i've done during all my (long) life: Learn, experiment, as objectively as you can, improve, imagine new solutions, break your certitudes all mornings and experiment again. And never stay stuck or believe anything you have read here or here.
Make your mind by yourself.
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Old 18th October 2012, 11:34 AM   #186
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Quote:
Originally Posted by tommy1000 View Post
Here is one studio that uses it:
Sear Sound NYC, NY
Oh, my God, i hope they had changed the electrochemical Caps several times during all those decades !
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Old 18th October 2012, 11:36 PM   #187
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CFB does appear to have two excellent conveniences. It has higher gain capacity, convenient for medium size power amplifiers. With orthodox measuring, you're measuring either strengths of VFB or the weaknesses of CFB (conveniently reduces reliance on listening tests). The severity of the divergence depends on the amount of gain. And that gives me an idea:

I guess that further comparisons would be more interesting with unity gain buffers? The wipes out the gain question apples versus oranges debates. What do you think of that? I'm in need of a buffer. I have a computer. Any normal computer (not equipped with an audiophile card) needs a buffer for assuring level frequency response (even if it is only just to find out if you needed it--you'll certainly find out), so does VFB or CFB do better for a buffer?

If buffer was a dumb question, then howabout comparing low gain preamplifiers run from superreg power? That would be the next stage after the buffer anyway.
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Old 19th October 2012, 03:52 AM   #188
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Quote:
Originally Posted by Esperado View Post
CFB THD: 0.000204%
2ppm THD without non switching circuitry and error correction
is just plainly impossible moreover if you re talking about your modded
Crescendo that use a classical two amplifying stages topology.
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Old 19th October 2012, 12:09 PM   #189
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Originally Posted by wahab View Post
2ppm THD without non switching circuitry and error correction
is just plainly impossible moreover if you re talking about your modded
Crescendo that use a classical two amplifying stages topology.
Wahab, you don't understand? It is simulation. To evaluate. Abstract ! Provided for you can verify.
In real life, the distortion is, of course, higher, but i am not able to provide CFB real numbers, HD and IM are under my (old) measurements instruments capability. Just i assure-you they are reduced in a proportion looking more or less like in the model.
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Last edited by Esperado; 19th October 2012 at 12:25 PM.
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Old 19th October 2012, 12:31 PM   #190
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Originally Posted by danielwritesbac View Post
so does VFB or CFB do better for a buffer?
One thing is for sure: if you use a separate transistor (inverting input) to carry the feedback, you will add its own distortion.
At gain 1 i don't see any advantages concerning bandwidth. . The difference will be in the slew rate ?
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Last edited by Esperado; 19th October 2012 at 12:35 PM.
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