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Old 17th June 2006, 05:39 PM   #1
ash_dac is offline ash_dac  United Kingdom
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Question Current feedback - not suitable for audio ?

Hi,

I have been looking at the Accuphase amplifier design, and reading chapter 9 of http://focus.ti.com/lit/an/slod006b/slod006b.pdf

CFA's disadvantages:-

-poor precision
-poor common mode rejection (due to mismatched input impedances)
-poor powersupply rejection
-difficult to apply (stray capacitance..)

CFA's advantages:-

-high slew rate
-wide bandwidth

Has anyone listened to an amplifier that uses current feedback in the design?

Do the advantages of increased slew rate, and bandwidth outweigh the disadvantages ?
 
Old 17th June 2006, 05:50 PM   #2
jcx is offline jcx  United States
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a few people seem to like the JLH amp

also TI's own TPA6120 datasheet appears to show superior audio performance #
 
Old 17th June 2006, 05:53 PM   #3
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Quote:
Do the advantages of increased slew rate, and bandwidth outweigh the disadvantages ?
You can answer your own question. What slew rate and bandwidth do you require? Then make two designs that meet this, look at the other critical aspects and choose the better one.
The topology doesn't really define anything...it just creates a different set of problems to solve. You might ask which feedback method is "easiest" to achieve necessary performance. But again the answer depends on the skill of the designer. Same with FETs, valves, bipolars and so on.
 
Old 17th June 2006, 05:55 PM   #4
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DartZeel's core voltage magnification is CFB, and it's the component of the year Confusing, me too.....
 
Old 17th June 2006, 07:12 PM   #5
forr is offline forr  France
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There already were some discussions at DiyAudio about the so called "current feedback" . You may have a look to them. You'll learn that it's an inadequatly named technique and that nothing can make the mathematical difference between between a voltage and a current.

Dartzeel : component of the year ? Ever seen the schematics ?
Probably the worst I ever saw. Overwhelming.
 
Old 17th June 2006, 07:13 PM   #6
PMA is offline PMA  Europe
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Excellent preamp or power amp can be designed based on current feedback topology. The only necessary condition is a competent designer.
 
Old 17th June 2006, 07:17 PM   #7
PMA is offline PMA  Europe
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Quote:
Originally posted by forr
There already were some discussions at DiyAudio about the so called "current feedback" . You may have a look to them. You'll learn that it's an inadequatly named technique and that nothing can make the mathematical difference between between a voltage and a current.
I would respectfully disagree. Don't look at DIY discussions

(like here: Current feedback - Voltage feedback, how do I see the difference? ),

but better at Kester's, Jung's and Mancini's papers, articles and books.
 
Old 17th June 2006, 07:45 PM   #8
poobah is offline poobah  United States
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Current feedback is often the only available means when very high speeds are concerned.

 
Old 17th June 2006, 09:58 PM   #9
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Default Re: Current feedback - not suitable for audio ?

Quote:
Originally posted by ash_dac

CFA's disadvantages:-

-poor precision
What do you mean? Offset? Distortion? I don't think (mis-called) cfb has any inborn limitation to distortion figures. It can be higher or lower than typical vfb. Offset can be higher though.
Quote:
-poor common mode rejection (due to mismatched input impedances)
It does not very much matter in typical audio applications.

Quote:
-poor powersupply rejection
I don't think so. It rather depends.


Quote:
-difficult to apply (stray capacitance..)
I don't understand. Can you say something more?


Quote:
-high slew rate
-wide bandwidth
Chose of differential stage being 'voltage' or 'current' type does not limit slew and bandwidth of audio circuits. You can get more than enough using both. For video, measurements, yes, cfb op-amps can have better slew rate (several kV/us) and ultra low settling times.

I use a version of current feedback of low feedback factor and am satisfied with results, both from engineer and listener point of view.

best regards
Adam
 
Old 17th June 2006, 10:43 PM   #10
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Nelson Pass's Aleph (and XA) designs use current feedback.
People rant and complain about current feedback. It can be done and done well. It can also be done poorly...like most everything.

Grey
 

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