Simple Symetrical Amplifier

[Edmond Stuart]

I think that Wahab was talking about a different distortion mechanism, for example commode distortion of the IPS. Also remember that one half of the IPS stays outside the NFB loop. So, even with 1000dB loop gain, distortion stemming from this 'half' will not decreased.

Cheers,
E.

[jan.didden]

Yes, agreed, but if you read from the start, the issue was different.
In fact there were several, one being that nfb cannot lower distortion of a stage before the gain stage, which is not true.

But I think I made my point, so I have no desire for yet another discussion.
BTW Isn't it time for your daily beer

jan

[Lazy Cat]

Input stage in SSA is in a deep class A, VAS input current is hundreds times lower than IPS quiescent current in that case both periods of a NFB signal completely affect/interact with the input signal in a linear manner. CFB has major advantage compared to other types of diff inputs and that is direct subtraction at the same junction as input. Input node is high impedance and CFB from the output low impedance, so naturally correct impedance matching as it should be. I don't see the need the output signal should be send to diff input via another high impedance device as with LTP-s.

All in all that natural impedance match and subtraction in a simplest possible way can be clearly heard as superior sound to more standard LTP IPS.

[Jay]

Originally Posted by FdW

And? is there anything else that matters? Maybe my point was that I totally agree to Jan's point.

Of course everyone will agree with Jan, even probably with everyone else. That's if you tried to read between the lines and never tried to find flaws in semantics. Once you tried to find flaws in somebody's statement, he might in turn try to find flaws in your statement.

Originally Posted by FdW

And? is there anything else that matters?

I don't know the point of your question, but of course there is something matters here. THD is measured after the final stage, right? But it doesn't tell you the open loop condition. 0.01% open loop can sometimes be better than 0.001% closed loop. I mean, the condition before the NFB is applied is very important, and it isn't showed in the standard THD figure. I already posted my opinion about JFET-input double LTP topology, why I don't like it.

[FdW]

Originally Posted by Jay

Of course everyone will agree with Jan, even probably with everyone else. That's if you tried to read between the lines and never tried to find flaws in semantics. Once you tried to find flaws in somebody's statement, he might in turn try to find flaws in your statement.

This is poppycock, why would every one agree with Jan? Why would every one agree with everyone else? And why would that be probable? Why would you search for faults (alone) and not for truth... I will not discuss this any further. (these are questions for myth-busters).

Originally Posted by Jay

I don't know the point of your question, but of course there is something matters here. THD is measured after the final stage, right? But it doesn't tell you the open loop condition. 0.01% open loop can sometimes be better than 0.001% closed loop. I mean, the condition before the NFB is applied is very important, and it isn't showed in the standard THD figure. I already posted my opinion about JFET-input double LTP topology, why I don't like it.

In the end the only thing of interest is what comes out (in relation to what was put in) the observer (listener/judge) is not interested in the number and type of stages in-between. There are people that claim that zero (stages in-between) is the best (and they may have a point) but I would say that any number of undetectable stages is fine also. It has nothing to do with type, it has to do with detectability and desirability (or total THD). P.s. the questions where rhetorical.

[Edmond Stuart]

Originally Posted by janneman

[...]
BTW Isn't it time for your daily beer

jan

After dropping my post I went right to Hoppe.

[wahab]

Originally Posted by Edmond Stuart

I think that Wahab was talking about a different distortion mechanism, for example commode distortion of the IPS. Also remember that one half of the IPS stays outside the NFB loop. So, even with 1000dB loop gain, distortion stemming from this 'half' will not decreased.

Cheers,
E.

Hi , Edmond

I was talking effectively of the impossibility for a GNFB loop
comprising several stages to reduce distorsion of a given stage
using available loop gain provided by the following stages.

To simplify , the IPS gain used for GNFB can reduce distorsion of the VAS
as well as the one produced by the OS , while eventual loop gain quantity
that originate from the VAS can reduce distorsion for the VAS and the OS.

If the OS has also some gain that is used for GNFB , this loop gain
quantity could only reduce the distorsion of the OS.

[wahab]

Originally Posted by janneman

In fact there were several, one being that nfb cannot lower distortion of a stage before the gain stage, which is not true.

That was the point , indeed , and we do not agree about it.

Originally Posted by janneman

But I think I made my point, so I have no desire for yet another discussion.
BTW Isn't it time for your daily beer

jan

Well , i did settle for coffee , do not see any will of contradiction in it...

[Jay]

Originally Posted by FdW

P.s. the questions where rhetorical.

I know it was rhetorical. And now I know your point, exactly as I guessed.

This is nothing new. There has been 2 camps since long time ago. I belong to one camp, and you the other one . Trying to find out which is wrong is useless. Trying to find out the truth is useful (such intention can be seen from open mindset, questioning own self).

Originally Posted by FdW

In the end the only thing of interest is what comes out (in relation to what was put in)

The problem is that THD has nothing to do (not 100%) with what is perceived or heard. Yes, we care only with what comes out in relation with what was put in. But remember, in music we never put in pure sine waves.

Originally Posted by FdW

the observer (listener/judge) is not interested in the number and type of stages in-between. There are people that claim that zero (stages in-between) is the best (and they may have a point) but I would say that any number of undetectable stages is fine also. It has nothing to do with type, it has to do with detectability and desirability (or total THD). P.s. the questions where rhetorical.

Yes, the observer is only interested in the quality of the sound. But observers have different experience in observing the sound. Some people find certain phenomena and have hard times to find the answer/reason for the existence of the phenomena, while some others do not find the phenomena exist.

So the blame is on those who see the phenomena because they cannot find the proper explanation. Sometimes, complex minds just make mistakes, and they can look like idiot minds, just because they stretch their mind.

[Bigun]

Originally Posted by wahab

I was talking effectively of the impossibility for a GNFB loop
comprising several stages to reduce distorsion of a given stage
using available loop gain provided by the following stages.

To simplify , the IPS gain used for GNFB can reduce distorsion of the VAS
as well as the one produced by the OS , while eventual loop gain quantity
that originate from the VAS can reduce distorsion for the VAS and the OS.

If the OS has also some gain that is used for GNFB , this loop gain
quantity could only reduce the distorsion of the OS.

I'm afraid that according to my understanding (I don't claim to have a perfect understanding of anything in this world) most, if not all of this, is completely wrong and will be confusing.

The thing about nfb is that it acts to reduce distortion from the composite of everything inside it's loop. The feedback loop is 'blind' to what is inside the loop, it 'cares' only about the net result of everything inside the loop that affects the signal at the output (i.e. wherever the nfb take off point is). There is no merit in discussion about the levels of distortion of some pieces of what is inside the loop when the loop is closed. It only makes sense to talk about the distortion of what each piece inside the loop contributes to the result when the feedback loop is open.

When the nfb loop is closed, the distortion produced by each piece inside the loop doesn't change since nothing physically has changed to these pieces (no doubt there are exemptions, but perhaps only under circumstances such as clipping). If the middle piece of the circuit adds 1% distortion to the signal passing through it with nfb loop open, it will still add 1% distortion to the signal passing through it with the nfb closed.

As has already been said, the nfb changes the input signal to the circuitry inside the nfb loop. Once that signal progresses through the circuitry inside the loop the nfb, closed or not, has no influence.

A rather unscientific way of thinking about it, although not entirely accurate, is to say that the gnu loop 'looks' at the distortion at the output, then applies a reverse copy of this distortion back to the input so as to cancel out the distortion generated by the circuitry inside the nfb loop. It doesn't 'see' or know what is inside the loop, it only 'sees' what appears at the output. Because it has to 'look' at the distortion at the output there must be some distortion at the output for it to work - so paradoxically it can never remove all the distortion (feedforward can pull that trick off, but it's harder to do).