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Old 5th January 2013, 12:32 AM   #1
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Default Class AB Question for Nelson

I have no idea where to properly ask this, maybe there's nowhere, but here goes:

To the One and Only, Mr. Pass:

First of all, HNY and thank you for all you do here for wanna-bee "designers" like me.

Given that you have been permutating through all these class A amps, the last two (F5 and F6) being unbelievably elegant in design, I was thinking about this:

With all you have learned over the years, if you were to circle back now and design a Class AB amp, what would you do differently?

I would think that working through all these different device types, bias schemes, feedback topologies, even using a xformer, etc., you might have some new ideas even for an improved AB amp.

Or, are all the same old challenges/constraints just pinning AB down right where it is?

Do you think you would basically end up back at the GFA-555 design, an amp that still holds it own even today? My 555s sound impressive even at 20+ years old. I still use 'em and I'm pretty darn picky.

Let's face it, AB amps are cheap, easy, run cool and efficient, produce as much power as you want, and last pretty much forever.

Whataya think? Perhaps a new angle for 2013?

Thanks for reading if you made it this far..

Rob in Atlanta
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Old 5th January 2013, 01:01 AM   #2
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hello. i am no nelson. but i don't think he will give up to much of the class A. and he will want to keep it simple. so a high volgage "low" bias F5 style. that is what i think i would go for. i might be wrong.

EDIT: he may have some secrets that we will not know. if so, he aint gonna tell

Last edited by AudioSan; 5th January 2013 at 01:04 AM.
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Old 5th January 2013, 04:06 AM   #3
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I must admit, I'm curious too!
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Old 7th January 2013, 05:32 AM   #4
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Originally Posted by Semisouthfan View Post
I must admit, I'm curious too!
Darn good question. Wondered myself a time or two.
I'm not crazy, my mother had me tested
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Old 7th January 2013, 09:17 PM   #5
Telstar is offline Telstar  Italy
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"The total harmonic distortion is not a measure of the degree of distastefulness to the listener and it is recommended that its use should be discontinued." D. Masa, 1938
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Old 7th January 2013, 11:58 PM   #6
eyoung is offline eyoung  United States
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Hi Y'all,

I like Mr. Pass' article "Leaving Class A"(i think).

That being said you may bias an F5 to class B if you care to and it may still sound acceptable to most ears, or low class A. after all it's only the "First Watt"
that counts...

Regards, Elwood
"Lead me not into temptation...I can find it myself."
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Old 8th January 2013, 12:04 AM   #7
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exactly a "high" bias class A/B F5 can sound very good.
acctualy i'am planing to do one soon. a F5 mini. that one will be leaving class A at 12-15W and go on as class A/B to 25W. but you can also bias it lower. say 0.6A bias. that will give you class A to 5.5W.
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Old 8th January 2013, 01:43 PM   #8
The Dastardly Dad of Three
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But why exactly WON'T it it go "klunk" when moving to class b ?
Ie in class a, both devices are active, so the transconductance and hence open loop gain and thus amt of feedback is double that in class b ....
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Old 8th January 2013, 02:28 PM   #9
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some input ....or opinion...

for class A variation in design will produce different results of course but most of the results will move in the same axis This as said is a huge generalization and of course there is a zillion other parameters but lets accept that the basis is this .

For class AB though there is significant difference and change on the already average or acceptable character depending on the topology

EFP is precise and accurate, Cfp is the opposite but more musical , ltp requires perfect semis to play good while diamond doesn't but require more noisy external or pcb depending components to operate better, while bootstrap is musical but not safe in high power where a VAs with a current source will be more accurate and safe and so on and on ...way too many variations moving around the same principal and most of difference comes from implementation choice of currents and parts and less related to the schematic or a very new idea ..

The only truth is that for class AB there has been nothing really innovative in years now everybody is dancing around the same things

Question is: is there anything can be done more about it or the all available variations in class AB is done and end of story .

As about the 555 don't be so sure that you have in your hand what NP originally designed ... rumors say that Adcom made budget cuts in that production.

Kind regards

Last edited by east electronics; 8th January 2013 at 02:30 PM.
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Old 8th January 2013, 03:52 PM   #10
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"As about the 555 don't be so sure that you have in your hand what NP
originally designed ... rumors say that Adcom made budget cuts in that

My relationship with Adcom had always been that I design the original copy,
and they would base production on it. While they altered some things over
the years, I have not seen anything that was the result of budget cuts -
in fact they tended to add parts.

Victor Campos oversaw most of the alterations, and he had his eye turned
a little more toward objective criteria than I did, and so most of the changes
that occurred resulted in better measurements.

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