[kyrill]

Hugh: " # 6  Global negative feedback reduces absolute levels of distortion, but is increasingly ineffective at the very high speeds at crossover.  At the crossover point most of the high order artefacts are produced as devices switch on and off when one side of the output stage hands over to the other.  Furthermore, GNFB tends to 'blenderise' lower distortion levels, morphing them into higher order distortions by intermodulation mechanisms, albeit at very low levels.  The low levels are nevertheless objectionable in long listening sessions and usually show as 'listener fatigue'.  When you realise that the masking effect of highish levels of H2 and H3 is lost with GNFB, you can see why it's a mixed blessing.  There are other reasons too why GNFB is not ideal;  chiefly in the way the error signal is extracted.  This process itself can be inaccurate, leading to inappropriate correction."

I heard many years ago Electrocompaniet   therefore completely avoided GNFB by applying diverse local negative FB.
Sounds "wise" in my non EE hobbyist eyes. Is this possible Hugh to completely avoid global NFB?

[AKSA]

Hi Kyrill,

Thanks for the post - yes, you can eliminate global negative feedback.

Nelson Pass does it, Charles Hansen of Ayre does it, and most tube amps don't use it.  I did it once too in an amp called the Glass Harmony, it's pretty darn good, but only 28W!!

However, before gnfb gets a bad rap, I feel this is an extreme reaction, since nfb dominates our lives.  Driving on the road, keeping our blood pressure, temperature and CO2 levels correct, operating our transport industry, the world is full of feedback, and it works.  My own feeling is that the reaction against feedback has been extreme, and that it must simply be applied properly.

The biggest challenge is accurately extracting an error signal, which truly reflects the differences in input and output.  Only when these differences are correctly metered can correction be accurate.  I use this approach in my GNFB amps, like the Lifeforce and the Soraya.

Cheers,

Hugh

[LinegrohalomanUpjoy]

Hi Kyrill,

Thanks for the post - yes, you can eliminate global negative feedback.
Nelson Pass does it,
Charles Hansen of Ayre does it,
and most tube amps don't use it. 
I did it once too in an amp called the Glass Harmony, it's pretty darn good, but only 28W!!

However, before gnfb gets a bad rap,
I feel this is an extreme reaction, since nfb dominates our lives.
..../snip/....

Cheers, Hugh

I agree, Hugh:
yes, you can eliminate global negative feedback

But as you also tell us, global feedback can be good, when done correctly
and in a Power Amplifier that is meant to use Global FB.

To reply to topic starter, Kyrill:
When you aim to build a power amplifier with little or no global feedback,
you have to design for this.
And this is no easy thing. Your circuit will not be as forgiving as those normal amps With GNFB.
This can often go for component choice, power supply and way the circuit is created.


I use this approach, in my latest Hifi project:

1. TWO separate blocks, each with LOCAL FB.
2. First BLOCK: Input+Voltage amp, giving desired V-gain ( for example +20dB )
3. Output power BLOCK: Follower with Vgain close to x1.0 ( like x0.98 if having low Impedance BLOCK)

If you have any interest, you can read more of my ideas and current development
of my project called 'Otopia Project - new idea amp'
http://www.diyaudio.com/forums/showthread.php?postid=1486283#post1486283


Finally 2 Basic things.
1. To avoid Global Feedback has some Advantages, no doubt.
As well as using it has many attractive reasons.
In order To avoid it you have to design for it.
There are many issues involved
to create a design with even Low global feedback factor!

2. Any Hifi amplifier around and any good working (lower distortion of voltage level) stage in such amplifier
should have some sort of Negative Feedback.
Even if this is only 1 Emitter resistor to a Transistor
or a capacitor+resistor as in a Tube's Emitter.

Sometimes The Loudspeaker is one such feedback Resistor!
Or at least a part of it.
Like in Nelson Pass http://www.firstwatt.com amplifiers.


Regards 
lineup

[kyrill]

thx LinegrohalomanUpjoy  what's in a name, no?

i hope your "Otopia" amp will succeed