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Old 30th September 2012, 12:14 AM   #111
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The question is do you really need such a thing for audio, when the difficulties of current feedback are mitigated by buffering with an emitter follower, ( the inverting input transistor of the ltp acts as an emitter follower, and not as a common emitter).

The answer is no.

As has been demonstrated time after time you only need a gbp of 8-10MHz. for audio, and there are perfectly adequate vfb chips that can do this, and they have been around for many years.

There are simply no practical benefits from using cfb devices in audio, and some practical difficulties, because they are more difficult to use.
rcw

Last edited by rcw666; 30th September 2012 at 12:16 AM. Reason: ambiguity
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Old 30th September 2012, 12:28 AM   #112
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Catalin

I think I'm beginning to understand what your saying.
If one is being dominated by a secondary pole of a simple LTP then using a cacscode version can reduce this effect and allow a higher bandwidth, and that inherently a cfb type stage lacks at least this same secondary pole.
But generalizing this as vfb versus cfb may not be the gist of the arguement.

rcw66: what are the pratical difficulties in using a cfb, and of which emitter follower are you referring to. edit (re-read your post got the emitter follower reference), thanks


Thanks
-antonio

Last edited by magnoman; 30th September 2012 at 12:54 AM.
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Old 30th September 2012, 12:44 AM   #113
catalin is offline catalin  Romania
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@rcw
If you have a current mirror(majority has) in the Ltp than you have 2 paralel stages for the neg feedback.One is follower but the same stage is common emitter .From here the mirror will add a pole and a phase shift .So we gain good thd with the mirror but we loose bandwidth and phase .

Why should we use CFB in audio ?
I will respond you with a request .
Please share a VFB schematic with a common 100pF miller cap for compensation and current mirror in ltp.And then please measure the phase at 20 khz .In closed loop .
You will see then why .

@magnoman ,yes correct .But to use the cascode or common base only for the negative feedback .Because if it is used also/toghether by the input signal the advantage is 0 .
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Last edited by catalin; 30th September 2012 at 01:13 AM.
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Old 30th September 2012, 01:58 AM   #114
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Quote:
Originally Posted by catalin View Post
@magnoman ,yes correct .But to use the cascode or common base only for the negative feedback .Because if it is used also/toghether by the input signal the advantage is 0 .
Catalin

Any chance you have a circuit or reference to this type of single side cascode?
I was imagining something common like even used within the 741 input stage.

Thanks
-antonio
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Old 30th September 2012, 02:10 AM   #115
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by danielwritesbac View Post
Observation: I have noted that perceived soundstage size in monophonic changes size depending on feedback current.
do you really mean 'sound stage' and 'mono' ? - please expand a little (I am a recent builder of a mono system)
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Old 30th September 2012, 02:31 AM   #116
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I would like to see a double blind test of this device that improves a mono sound stage by increasing feedback current, I suspect a urine extraction exercise.

If you post your data about your assertions Catalin I would be interested, I however am not going to jump through hoops at your behest.
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Old 30th September 2012, 09:07 AM   #117
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Quote:
Originally Posted by magnoman View Post
Lazy Cat
I'm still not getting this, can you or Catalin expand on what you mean by an additional nonlinear stage?
Fundamentally isn't the error current developed the same and subject to the same transistor characteristics? The difference being only that vfb has a limited current?
Quote:
Originally Posted by magnoman View Post
Catalin
I think I'm beginning to understand what your saying.
If one is being dominated by a secondary pole of a simple LTP then using a cacscode version can reduce this effect and allow a higher bandwidth, and that inherently a cfb type stage lacks at least this same secondary pole.
But generalizing this as vfb versus cfb may not be the gist of the arguement.

Thanks
-antonio
Hi Antonio

I'm glad that the topic attracted you and you're beginning to understand, nevertheles we could still have eternal theoretical discussion about VFB vs. CFB.

My suggestion would be more of a practical nature: try to build simple-basic 6 transistors VFB and simple-basic 6 transistors CFB amp and compare them sonically.

After this experience you'll also understand why top technical specification & complex VFB amplifiers sounds dull.
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Old 30th September 2012, 12:19 PM   #118
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by catalin View Post
@rcw
If you have a current mirror(majority has) in the Ltp than you have 2 paralel stages for the neg feedback.One is follower but the same stage is common emitter .From here the mirror will add a pole and a phase shift .So we gain good thd with the mirror but we loose bandwidth and phase .

Why should we use CFB in audio ?
I will respond you with a request .
Please share a VFB schematic with a common 100pF miller cap for compensation and current mirror in ltp.And then please measure the phase at 20 khz .In closed loop .
You will see then why .

@magnoman ,yes correct .But to use the cascode or common base only for the negative feedback .Because if it is used also/toghether by the input signal the advantage is 0 .
Why do you think VFB (or CFB for that matter) needs to stick to a frozen, preestablished scheme?
VFB simply means the voltage feedback mechanism is dominant. A topology usually associated with CFB can be converted lossessly to VFB, as I have shown.
Maybe you are not convinced the process is lossless, despite I went much further than most, by showing that the group delay remains unchanged in the transformation.

In fact, contrary to what Lazycat states, the process is not only lossless, but has gain: by including a non-linear element in the feedback resembling the one in the main path, the overall distortion is reduced. Since this is an open loop process, it affects in no way other good or bad characteristics of the looped system.

You can compare the merits of one specific type of topology against another one, but this is not related to the fundamental question of this thread: what are the merits of root CFB against the merits of root VFB.
If think there are none: it all boils done to idiosyncrasies usually associated with one or the other topology (or should I say philosophy?).
Typical are the completely worthless "vanity" graphs of phase vs. frequency shown: they simply look nice and make the poster feel good, but they bring no useful information, and they hide the most important aspect (in case it is relevant, which I am not even convinced of), the variation of group delay with frequency.

From a purely anecdotal point of view, I have heard systems that sounded good but were purely trash, from an objective point of view, and also some that were flawless, but didn't sound extraordinary (good, clean, OK, but no more)

Subjective impressions can be very misleading and as engineers, we first have to do our homework properly, and only then look into contentious details if we have time to.
Reversing the order of things leads to pure gold mains plugs and similar quirks.
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Old 30th September 2012, 12:44 PM   #119
catalin is offline catalin  Romania
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Elvee, you want to bring a deviation to the true. You forget that the vfb is uncompensated.Therefore the group delay is the same.But an uncompensated vfb will not work ever.
So please compensate the vfb and I will bring also a cfb with good phase reserve.Let see than the real group delay.

Let 's do the things correctly and not loose time.

Thank you
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Old 30th September 2012, 01:14 PM   #120
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I read Elvee's comment, and as he indicates it would be hard to support any particular theory here since the only difference in CFB opposed to VFB could be the inherent low or higher impedance of circuit.

Whether this would make a sonic difference at all is very debatable and subjectivity never have a definitive outcome.

Better and worse are measurable and relative. Subjectivity on the other hand merely indicates an emotion or opinion.
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