Output stages with gain Enhance Slew rate

[mikek]

Originally posted by Fred Dieckmann

http://www.a-and-t-labs.com/K6_Sw_Am...atic/k6amp.pdf

(This amp has some design features that are a big enough liabilities to make the topology selection pretty irrelevant. This is based on actual experience and not schematic browsing by the way.)

......yeah....like dubious quiescent stability.......already mentioned at the start of the thread....but i think you've missed the point....

[Fred Dieckmann]

I think I miss most of your points. I wonder why that is..........

[mikek]

Originally posted by Fred Dieckmann
First off, Slew rate:

From http://www.passdiy.com/pdf/a40.pdf by Nelson Pass

The combination produced phenomenal high frequency power which hit -25dB (reference peaks below 20kHz) at 50kHz. If the amplifier were clipped, I measured the 30V/us recovery, but with unclipped performance, the highest measured slew was
1.5V/us.

In other recordings and with other cartridges, .5V/us peak slew was more common and was generally associated with cymbals and synthesizers but not piano or vocal material. I found this interesting as I had always imagined higher slews than I found. It certainly makes one wonder whether the value of high
slew rate specifications isn't like damping factor, where very high values
show diminishing returns in sonic performance.

Second........
Walt Jung built an amplifier with gain in the output stage in this article: Walt Jung´s AD823/812 amplifier. This is not even a power amplifier but is a composite op amp preamp circuit for preamp line level use. Both the op amps use unity gain output stages if I am not mistaken.

Last and least.......

If people on going to provide links, please link what you are referring to so one doesn't have to wade through the web site to see what you found of interest For example:

http://www.hafler.com/techsupport/index.asp?ID=3 and tell us which of the couple dozen manuals you are referencing.

http://www.a-and-t-labs.com/K6_Sw_Am...atic/k6amp.pdf

(This amp has some design features that are a big enough liabilities to make the topology selection pretty irrelevant. This is based on actual experience and not schematic browsing by the way.)

The "I wrote a Spice model or drew a schematic" amplifier design school is now be reduced to include block diagrams! Everyone raise their hand that has actually built a real amplifier. I am reminded of the news "analysis" programs where a bunch a journalist sit around and speculate over issues that they have 1% of the information about.

No mention of the Nelson Pass designs which are some of the best amps with gain around the output stage?

You don't appear to have a relevant point to make in relation to the small technical fact i pointed out as the purpose of the thread.....?

No...i didn't think so...

[Fred Dieckmann]

"The combination produced phenomenal high frequency power which hit -25dB (reference peaks below 20kHz) at 50kHz. If the amplifier were clipped, I measured the 30V/us recovery, but with unclipped performance, the highest measured slew was
1.5V/us."


Maybe the need for high slew rate is much over stated....... I am having a great day. Why do you ask?

Fred Dieckmann*

* still having trouble with spelling I see........

[mikek]

Originally posted by Jocko Homo

Maybe you should more closely read the old stack of AES Journals that I suspect you might have. Marshall Leach ahs written articles explaining input degeneration, and if his advice is followed, it will not have the problems that you allude to.

Actually i have read all of prof leach's , and prof. cherry's work in JAES, and other periodicals....

...there are of course matters raised in each case that i do not accept...for sound technical reasons......But that is an entirely different subject...for another thread perhaps?

...but i cannot resist one obvious example...Prof leach uses feedfoward compensation in his amps...this is neither desirable, nor required.....

.....for one thing it shunts the output stage out of the global feedback loop, causing an unnesessary increase in THD.....

....for another, feedfoward compensation almost invariably introduces pole-zero doublets that will needlessly increase the settling time of the amp.

...and what 'advice' are you provocatively alluding to with regard to degeneration?

.....I am afraid the basic's don't change: excessively increase degeneration, and you increase DC offset...and noise...will also reduce input stage transconductance excessively...which needlessly reduces open-loop gain...reducing feedback....compromising distortion performance....etc...etc..

But again...we've strayed...perhaps you would like to start another thread....on Leach Vs Cherry?


However i must emphasise, i do not believe a 400V/uS slew is required in an audio amp....just so no one misrepresents me...again.

[mikek]

Originally posted by Fred Dieckmann
[B
Maybe the need for high slew rate is much over stated....... [/B]

But thats precisely what i said...see post #18

[mikek]

Originally posted by Fred Dieckmann

http://www.hafler.com/techsupport/index.asp?ID=3 and tell us which of the couple dozen manuals you are referencing.

For your information ALL of Hafler's current designs incorporate an output stage with some gain.....

[mikek]

Originally posted by Fred Dieckmann
.

http://www.a-and-t-labs.com/K6_Sw_Am...atic/k6amp.pdf

(This amp has some design features that are a big enough liabilities to make the topology selection pretty irrelevant. This is based on actual experience and not schematic browsing by the way.)

As i have noted above quiescent stability is a problem...for obvious reasons....


...perhaps you would like to expound on your rather nebulous, nay, wooly phrase : '.....a big enough liabilities..........?

....I wait with bated breath....Oh maestro....

[mlloyd1]

Yeah. Anybody every looked inside a Boulder AE250 or AE500 power amp? These are their old stuff. No one I know will let me near the new stuff with a screw driver or some other tool to open one !

Wonder why I have yet to hear about any "Boulder clones"

These things use a discrete op amp to provide the bulk of the voltage gain. The output stage provides more voltage gain and current gain. I also beleive they use global negative feedback around the whole kit and kaboodle. Their promo literature brags about how much work they did to get the stabilization correct. Look inside and I beleive it. On the other hand though, I admit, these amps sound good to me, even driving electrostatics. And they don't seem to complain about what you hang on the output.

mlloyd1
(who will also admit he has played around just a little with the Accoustat/Hafler TransNova circuit - haven't heard of a "TransNova Clone" either!)

Originally posted by Jocko Homo
....Amps with gain in the output stage are a pain to stabilize. And most people know better than to make one....Jocko [/B]

[Jocko Homo]

Prof. Leach wrote a JAES article in the early 80s or so explaining how to size the input degeneration. Right after that, the egghead Ph. D.s from "down under"savaged him. Don't believe me??? Go look it up.

No, you do not increase increase offset by degenerating. And the noise of a system is determined by the first stage, and unless the subsequent stage has gross noise, it will not increase the noise figure.

Go read National Semiconductors explanation of slew rate enhancement with degeneration.

And since when did making something linear in the first place become a bad idea?

And yes, Signoro Lloyd, why don't we see Boulder clones? Because it is too much a of pain to make. Not impossible, just a pain. The idea is to make money, and hopefully good sounding gear. Not some convoluted effort to impress your old college professor.

Anyone can build one of something. Try making them by the thousands. Then I will be impressed. Compare the small number of amps that Boulder and their ilk build compared to the rest of the world.

And I am still waiting for someone to build Prof. Cherry's kludge.......the one with 5 nested feedback loops.

And yes, Mikey, you are responsible today for what is in my inbox. Maybe if you had more clearly stated your purpose, you could have avoided this rock throwing contest.

(Phred is responsible.....through his wife's generosity.....for what is in the water closet today.)

Jocko