'Local Feedback and Nonlinearity of Multistage Feedback Amplifiers'

[mikeks]

Check your mail...
Cheers.

[Geoff]

Originally posted by mikeks
Good reason to avoid Linsley-Hood's 1983(WW) design methinks...

Mike

I don't appear to have a JLH power amp design from WW 1983 in my index (http://www.tcaas.btinternet.co.uk/jlharticles.htm). If your date is correct, could you please let me have the relevant details of the article so that I can add it to the index.

Thanks

Geoff

[mikeks]

..Hi Geoff,

..that should have been 1982.....Apologies for inconvenience caused....

Cheers.

[Geoff]

Originally posted by mikeks
..Hi Geoff,

..that should have been 1982.....Apologies for inconvenience caused....

Cheers.

No problem, there was no inconvenience. I was just interested in determining whether I had omitted an article from the index or, more importantly, which JLH design you considered to be below par (and why).

Geoff

[mikeks]

..that positive feedback within the global negative feedback loop is sometimes recommended by some authors in theoretical treatise....e.g:

http://grove.ece.iastate.edu/docs/Papers/20020507.pdf

...i remain unconvinced, as in the example given, the authors blithely refuse to consider the effect clipping or otherwise disabling the global feedback loop may have on their carefully worked out analysis......

[ppl]

Like any technique in science, there will be critics of that theory or method. Myself I like Multi-Loop Nested feedback in Audio Amplifiers. I use this technique Exclusively with IC op amp and Buffer composite Applications. The technique I use was popularized by Walt Jung from his Tools and Tips articles in EDN the most relevant is http://www.elecdesign.com/Articles/A...7206/7206.html http://www.elecdesign.com/Files/29/7206/Figure_02.gif Schematic.

The next article describes the real problem as I see it and this is the limited open loop bandwidth of the Gain stage in this instance an IC OP-AMP. Most of these have the open loop gain start to decline at 10-100 Hz. Others will have lower open Loop gain and feature a wide bandwidth. These types of devices can have a flat gain vs. frequency to 10 KHz or more and I have seen a few flat up to 100KHZ. I agree that with these devices and a suitably fast output stage that Multi-loop loses a lot of its appeal. However if one was to consider that most of these fast devices are not practical in audio and as noise is concerned you still want the benefit of a High feedback factor then Multi-Loop becomes attractive. Myself I would much rather have a constant 01% THD from 20-20,000 Hz than a THD vs frequency that is say 0.0001% from 20-100 HZ rising to 0.01% at 20 KHz. I believe that the Ear perceives this Rising distortion vs. frequency as added Brightness because the harmonics are artificially raised in relation to the Fundamental starting at some predefined corner frequency
http://www.elecdesign.com/Articles/A...7207/7207.html

Walt Jung is not the only person nor is he the first to advocate Multiple gain stages each operation at a reduced gain via local feedback and then enclose this in an overall feedback loop Malcolm Hawksford also advocates this approach in his article on Fuzzy distortion. http://www.elecdesign.com/Articles/A...7207/7207.html
This is a short 10 page article and I refer you to page 8 figure 7 for his Basic Multi-Loop Feedback architecture.

[mikeks]

Originally posted by ppl
Like any technique in science, there will be critics of that theory or method. Myself I like Multi-Loop Nested feedback in Audio Amplifiers........

Nothing intrinsically wrong with nested feedback loops old chap.......in fact i love them.....

http://diyaudio.com/forums/showthrea...482&highlight=

...But these have to be carefully designed, otherwise you may have a wonderfull oscillator.......

[Pan]

ppl,

Having listened to class A BJT amps with (I believe) relatively flat THD all the way up to 100k or so and comparing them to FET amps with a raising curve at 1k or so...

(pause, breath... long sentence..)

has led me to a similar conclusion as you. But... to my ears the FET amps with higher dist. does not sound bright but slightly soft and "muschy/foggy" with reduced energy in high frequency attacks/transients.

/Peter

[ppl]

Pam this is also my observations also and i belive that since FET is a Sq law device that this raises the evean order harminics lending to a warm soggy top end however the BJT with its pedomintly odd order distortion component and following an Exponential curve ted to sound Bright.

Oh yes instability can result if all the poles and zeros are not dotted so to speak

[andy_c]

Originally posted by AKSA
What was that amusing quote about academia? 'And the fighting was bitter, and malicious, and never ending because the stakes were so low'.

Ah, but academia was so much fun back then! I guess I'm just a frustrated academic, 'cause I wish I had stayed with it. The only time I've ever been literally pursued by women was my last semester in grad school, when I taught an EE course that was required for many non-EE engineering majors. Man! That was back when jogging shorts and heels were in . Whoever the guy was that started making fun of women who wear jogging shorts and heels, I'd like to go back in time and give him a piece of my mind. Something like "Shut up or they'll stop wearing them you idiot!!!". Too late though.

How did I get on this subject? Oh well. We now return control of your TV set to you...