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Old 3rd September 2012, 04:02 PM   #31
Salas is offline Salas  Greece
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Quote:
Originally Posted by rcw666 View Post
If you can actually hear the difference between a 741 op amp and your pet device with a statistically significant figure of more than fifty percent then you might convince me and the bulk of other scientists on the planet, until then your assertions are entertaining but irrelevant.
rce
Hmmm some notable guys in Natsemi seem to had been listening to their audio chips for evaluating far more subtle variations between their advanced designs. Older relative post.
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Old 3rd September 2012, 05:56 PM   #32
wahab is offline wahab  Algeria
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Quote:
Originally Posted by homemodder View Post
The first statement is incorrect, VFB has as many advantages as it does have disadvantages over CFB. Common Wahab you know that.
VFB has more advantages than disadvantages over CFB.
Quote:
Originally Posted by homemodder View Post
How does slower slewrate argue for sound quality ??? Until Edmond showed TMC compensation it was impossible for a VFB amp to come close to THD20 performance that can be obtained by CFB.

Better THD was obtained by CFB amps and this has been the case ever since CFB made its appearence, only recently performance is much closer
I beg to differ in this matter.

CFBs logicaly gave gave better perfs than the old first kind of VFB
amps because it had two gain stage instead of a single inverting
gain stage.

Latter topologies using input differential did equal and even best
the classsical CFBs.

The slew rate limitation is often due to poor design but in principle,
slew rates of hundreds V/uS are quite feasible with VFB topology,
although it is somewhat pointeless in audio gears.
.[/QUOTE]
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Old 3rd September 2012, 10:33 PM   #33
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The point is that if we take the slew rate we need as being where, f = 2pifVp, then if we want to design an amplifier to put out a 10V peak signal at 100kHz. we need a slew rate of 6V/us.

Exactly why one would need more than this is not clear to me, and the vast majority of others, and the 5534 has a slew rate that is more than twice this.

To adequately amplify the 10MHz. sine wave I am with the posted circuit you need at least 45V/us, now current feedback devices start to make sense, other wise it is technical nonsense to say that you need this sort of figure to amplify audio signals, however vehement your assertions to the contrary might be.
rcw
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Old 3rd September 2012, 10:48 PM   #34
Bonsai is offline Bonsai  Taiwan
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I think both topologies have their merits. However one advantage (and plenty will disagree with this) is that in CFB th loop gain bandwidth is generally greater than the audio bandwidth, so you can apply the same amount of feedback across the whole audio band. If you believe PIMD is an issue, you can either design you VFB for very high loop gain (this reduces PIMD - see Cordell for example) or you can go the CFB route.

I will publish a little amplifier in a week or so on my site - CFB. Slew rate 250V/us, 700kHz -3dB and loop gain -3dB of circa 60kHz. Figures like his are not possible with VFB without a lot of effort.

I think slew rate is important, but beyond a certain point, does it add any value.

Which one is best? Both of course!

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Old 4th September 2012, 03:43 AM   #35
seanvn is offline seanvn  Viet Nam
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The CFP is an interesting circuit. It can give much more current gain than a Darlington pair if you are doing an impedance transformation. It is prone to UHF oscillation and it is a good idea to use a stopper resistor at the base of the PNP transistor. It is also a bit noisy. A very good basic 2 transistor circuit is the bootstrapped amplifier. There are quite a few tricks you can pull with that circuit.
In its basic form it is rather unusual because it is an example of positive feedback actually increasing linearity. You can connect an emitter resistor to the first transistor without actually reducing the gain very much, improving linearity even more. You can also use negative feedback around the outside of the amplifier giving even greater linearity.
The use of both positive and negative feedback in an amplifier can be both viable and very useful. If I had time I would provide a clear circuit example but maybe you can understand from some of the examples you can download from here: EvoSpice 4.1 & 4.2
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Old 4th September 2012, 04:32 PM   #36
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Quote:
Originally Posted by wahab View Post
VFB has more advantages than disadvantages over CFB.


I beg to differ in this matter.

CFBs logicaly gave gave better perfs than the old first kind of VFB
amps because it had two gain stage instead of a single inverting
gain stage.

Latter topologies using input differential did equal and even best
the classsical CFBs.

The slew rate limitation is often due to poor design but in principle,
slew rates of hundreds V/uS are quite feasible with VFB topology,
although it is somewhat pointeless in audio gears.
.
[/QUOTE]

Name the advantages and disadvantages of VFB, Ill add the disadvantages youre not thinking of.

CFB outperformed all generations of VFB, even now the latest outperform the VFB if we look at opamps. Please name me one VFB opamp that could equal even the old 1980s comlinear opamps. Two gain stages ???, could you demonstrate.

Traditional VFB topology cant and never will reach the speed that can be attained by CFB, theory dictates. VFB can have larger slewrate at the cost of very high currents in the LTP, CFB on the other hand can probably reach 10 times that high if the same current is used.
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Old 4th September 2012, 04:52 PM   #37
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Quote:
Originally Posted by rcw666 View Post
The point is that if we take the slew rate we need as being where, f = 2pifVp, then if we want to design an amplifier to put out a 10V peak signal at 100kHz. we need a slew rate of 6V/us.

Exactly why one would need more than this is not clear to me, and the vast majority of others, and the 5534 has a slew rate that is more than twice this.

To adequately amplify the 10MHz. sine wave I am with the posted circuit you need at least 45V/us, now current feedback devices start to make sense, other wise it is technical nonsense to say that you need this sort of figure to amplify audio signals, however vehement your assertions to the contrary might be.
rcw
Again the 'classical' mistake of assuming that the to-be-reproduced-audio-signal is all that the amplifier is working with. No! Any active feedback system needs to and runs at much higher frequencies than required for the signal-to-be-created-by-feed-back.

This means: The better the slew-rate, the faster the loop control frequency is. The faster loop control frequency, the better harmonics and other interference suppression.
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Old 4th September 2012, 05:24 PM   #38
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by homemodder View Post
Traditional VFB topology cant and never will reach the speed that can be attained by CFB, theory dictates.
In theory, any CFB amplifier can be converted into a VFB one having at least identical performances.
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Old 4th September 2012, 08:22 PM   #39
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The only thing I can say Magic Box is that making bold assertions of that sort withought any technical justification for them is really not good enough.

Exactly where is my "classical mistake" and what references do you have to back up your assertions, there are non that I can find. This is the sort of thing you sometimes hear being asserted, withought any shred of evidence at all, in the more lunatic fringes of audio.
rcw
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Old 4th September 2012, 09:14 PM   #40
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by rcw666 View Post
The point is that if we take the slew rate we need as being where, f = 2pifVp, then if we want to design an amplifier to put out a 10V peak signal at 100kHz. we need a slew rate of 6V/us.
Of course there's nothing wrong with the basics. What happened usually is that real world situation is always more complex (than implementing one or two basic formula).

Quote:
Originally Posted by rcw666 View Post
Exactly why one would need more than this is not clear to me, and the vast majority of others, and the 5534 has a slew rate that is more than twice this.
Quote:
Originally Posted by rcw666 View Post
The only thing I can say Magic Box is that making bold assertions of that sort withought any technical justification for them is really not good enough.
Your bold assertion, explicitly or not, is that the scientists, or the vast majority of the more clever scientists, are having exactly the same thinking as you...

Please show us your high level of technical knowledge to justify that...
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