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Bob Cordell Interview: Error Correction
Bob Cordell Interview: Error Correction
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Old 28th November 2006, 06:24 PM   #941
ingrast is offline ingrast  Uruguay
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Default Re: Re: Final contribution

Quote:
Originally posted by Bob Cordell



Thank you for your participation. I think I agree with everything you have said.

Bob

Thanks Bob!!

I am silently following the other thread, being neither experienced nor expert in audio hardware, it is most educational.

Rodolfo
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Old 28th November 2006, 06:26 PM   #942
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally posted by andy_c


Hi Jan,

Just edit out the punctuation mark after the ".pdf" in the URL.

On another subject, I think the ongoing controversy isn't much of a controversy at all when all is said and done. Rather, I see it as different people having different ways of looking at the same problem. One person's concept is another person's implementation detail and vice versa. Regardless of whether error correction is a concept in itself or just a particular implementation of a general feedback system, the "con-plementation" is an incredibly useful one.

Andy, you are exactly right. Looking at the error correction in multiple ways gives multiple insights, all of which are useful.

It is fascinating the different ways that this circuit can be looked at, and especially fascinating that it posesses the behavior, as the pot is moved, of reducing error, going through essentially zero, and then coming out on the other side with reversed error sign.

There is a lot in the semantics here, I suspect a lot in the definition of loop gain here, and finally, a lot in the details of the implementation that happen to work out very synergistically in the Hawksford architecture. The Devil is in the details....

If I had successfully convinced myself that the Hawksford scheme was nothing more than NFB in disguise, I probably would never have ventured to build it into my amplifier and achieve such good results. I always like the story about the engineer who did something good because someone forgot to tell him it would not work.

Cheers,
Bob
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Old 28th November 2006, 06:28 PM   #943
mikeks is offline mikeks  United Kingdom
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Hi Rodolfo,

Your contribution is undoubtedly appreciated; it is not my intention to give the appearance of flippant ''fire from the waist'' type retorts.

If this was the impression conveyed, i apologise unreservedly; I hope I have not unduly offended you in this respect.

Now, if you consider the rationale for the 1/K block given here, i think you'll see that there really is nothing mysterious about it.
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Old 28th November 2006, 06:37 PM   #944
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Bob Cordell

....... fascinating that it possesses the behaviour, as the pot is moved, of reducing error, going through essentially zero, and then coming out on the other side with reversed error sign.

If I had successfully convinced myself that the Hawksford scheme was nothing more than NFB in disguise, I probably would never have ventured to build it into my amplifier and achieve such good results. I always like the story about the engineer who did something good because someone forgot to tell him it would not work.

Cheers,
Bob

I agree. However, i have difficulty classifying error-cancellation-by-feedback as classical NFB in the traditional sense, since the derived quantity to be fed back is differentially extracted.

It is the later attribute, i suggest at this point, which explains why only a finite loop-gain (as opposed to infinite loop-gain in the classical form of NFB) is required for a nominal 100% error cancellation.

Moreover, in contrast to classical NFB, the error correction arrangement appears to possess the attribute that, at balance, loop-gain increases with increasing error extracted, as shown here.

This attribute would go a long way to explaining this system's remarkable output-stage distortion reducing properties.

What do folks think? Bob?

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Old 28th November 2006, 06:51 PM   #945
andy_c is offline andy_c  United States
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Quote:
Originally posted by Bob Cordell
It is fascinating the different ways that this circuit can be looked at, and especially fascinating that it posesses the behavior, as the pot is moved, of reducing error, going through essentially zero, and then coming out on the other side with reversed error sign.

There is a lot in the semantics here, I suspect a lot in the definition of loop gain here, and finally, a lot in the details of the implementation that happen to work out very synergistically in the Hawksford architecture. The Devil is in the details....

If I had successfully convinced myself that the Hawksford scheme was nothing more than NFB in disguise, I probably would never have ventured to build it into my amplifier and achieve such good results. I always like the story about the engineer who did something good because someone forgot to tell him it would not work.
I agree completely. One thing that bugs me is that some (maybe all?) of the equivalency arguments rely purely on linear system theory. I thought the whole purpose of error correction, at least for unity gain power amp output stages, was to keep the nominal gain and bandwidth of the stage intact insofar as is possible, while at the same time linearizing it. After all, if you try to make linear gain corrections, the dynamic range of the error correction circuitry gets partly used up by an essentially useless gain correction. Typical negative feedback is pretty radical by comparison. I like to look at it as an implementation technique whereby significant distortion reduction can be accomplished in return for having to deal with a pretty tame loop stability concern.
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Old 28th November 2006, 07:04 PM   #946
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by andy_c
After all, if you try to make linear gain corrections, the dynamic range of the error correction circuitry gets partly used up by an essentially useless gain correction.

I think i agree with you here, since, in contrast to classical NFB, the error correction arrangement appears to possess the attribute that, at balance, loop-gain increases with increasing error extracted, as shown here.

This implies, it seems to me, that the system is actually one of active negative feedback.

What think ye?
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Old 28th November 2006, 07:12 PM   #947
andy_c is offline andy_c  United States
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Quote:
Originally posted by mikeks
What think ye?
I think ye liketh the scratch2 icon .

But seriously, classifying exactly what it is seems less useful than figuring out what cool things can be done with it... unless such a study could reveal yet more applications or ways to improve the existing ones.
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Old 28th November 2006, 07:17 PM   #948
mikeks is offline mikeks  United Kingdom
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I suppose you're right.

What bugs me is the Lipshitz/Vanderkooy assertion in that paper that no true cancellation can occur with this arangement unless you have infinite loop gain.

I just cannot see it, and it is constantly gnawing away at moi.
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Old 28th November 2006, 07:27 PM   #949
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by mikeks
..... rationale for the 1/K block given here, i think you'll see that there really is nothing mysterious about it.
Oh well, I cannot fail to tease you, again firing from the waist !!!

Seriously now, this issue harks back from post 888 where I suggested Fig. 1 and 2 were not equivalent and let for you to find out. Of course they are *algebraically* equivalent (algebra is a more compact and spiffy way of putting it anyway), but how on earth do you reroute a signal in bolck diagram from output to input (and insert 1/K1) in the real world.

Again, this may be a personal limitation, by I shudder at the idea of depicting a bock diagram including components that cannot possibly exist, no matter the fact it is basically an abstraction.

Rodolfo
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Old 28th November 2006, 07:29 PM   #950
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by ingrast

...... but how on earth do you reroute a signal in bolck diagram from output to input (and insert 1/K1) in the real world.

Rodolfo

The 1/K1 term merely shows that you need attenuation of K1, instead of a gain of K1.

Remember your (1/A') attenuator, here?
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