Hypex N core module

Large amounts of global feedback used in linear amps is used in poor designs, to get good specs, the challenge is to get good specs with only a little local feedback.

George, I'm a bit uncomfortable with this statement- a lot depends on how its interpreted. I'm of the opinion that the amp should have good linearity open loop, then add the feedback once that is achieved. This would make the latter portion of the statement true.

But if the amp is to have good specs with only a little feedback, then this statement is likely false. Unless its a very small amount of feedback (under about 4 db) the application of feedback (even in a good design) will be detrimental. This is due to bifurcation of the input signal by the feedback itself (Bruno says that the harmonics 'show up out of the blue' but its easier to understand if you apply Chaos Theory). So the solution for a **good design** is to use a lot of feedback, hopefully on a design that already has good linearity to begin with before the application of feedback.

Sorry to go on- I just felt this needed clarification. 

George, I’m a bit uncomfortable with this statement-

Maybe I should have said in "most"

A little feedback should "only" be used to clean things up a little, not to make design blunders look better on test. And if designed very well, all that needed is "local feedback"

In tube amps that have average output transformers global feedback is used to reduce the transformers output impedance to get better damping factor specs, this is a band-aid fix, the output transformer should have it without the need of feedback, and the amp will sound better for it.

Global feedback compares output to input and try’s to correct any differences, trouble is that the output is already on it’s way to the speakers. And the more global feedback applied the more unstable an amp can become.

If you think about it Ralph, linear solid state amps that are well made but with global feed back and a bit of it, also have the same lifeless sound that many say Class-D has.

We have done an experiment ourselves back in the 70's on a very good amp Pass’s A20 Class-A, which we morphed into a 150w Class-A beast with water cooling (a two man lift), which had global feedback and sounded very good, then we changed it and made it local feedback and it sounded superb so much better it wasn’t funny.
http://www.firstwatt.com/pdf/art_classa_20.pdf

Sorry to go on- I just felt this also needed to be said.

Cheers George

If you think about it Ralph, linear solid state amps that are well made but with global feed back and a bit of it, also have the same lifeless sound that many say Class-D has.

No argument there, but the kind of feedback Bruno is talking about is a different matter. Nelson Pass wrote a great article about this- you can find it on his DIY site. In it he speculates about 60 db, but points out how impractical it is to add more gain/circuitry to get there!

The thing is, in class D making gain isn't a problem. An essential bit of the class D circuit is something called a comparator, which is a lot like an opamp run open loop. If you know your opamp theory, that means you have nearly infinite gain. I'm nutshelling this a bit, but the point is you can make a lot of gain with class D without adding extra circuitry to get there. So 60 db of feedback is very doable.

Proof will be in the pudding Ralph, 60db of negative feedback makes the hair stand up on my neck.
As everything I have learnt and read from the masters about feedback, say it's in some cases a necessary evil in small amounts, 15-30db and hopefully just local not global, but 60db global!!!! YIKES!!!!
Maybe that the reason for those that hear it in class-D that it's sound is lifeless in character.

Cheers George   

I have been through this thread and a lot of good technical details, most of it I just cannot understand. It has been few months and I am wondering if anyone tried the Class D amps from Nord and the likes?. Who makes a good amp based on Hypex NCore 1200 power on a decent budget?. 

What are the impressions that might have changed, if any, regarding Class A/B vs Class D with any of the users who posted here?.

I am considering Class D amps like ATI AT543NC, Nord, Red Dragon M1000 MKII and Legacy Powerbloc4.