Negative Feedback a deal killer?

If the amplifier has other design considerations, it will not need to have feedback to be low distortion. For example, a fully-differential amplifier will have even-order cancellation at each stage in the amp, so it will, without feedback, generate only the 3rd harmonic.

Just an example.

Now, the flip side of the coin is what happens to the amp if a feedback is used. The problem is that with any amplifier, there is a time delay for the signal to move from input to output. This propagation delay causes the feedback signal to arrive slightly behind the actual signal at the input. Now with sine waves, the amplifier can reduce its distortion after succeeding iterations of the waveform. It does not do so with non-repetitive waveforms, like real music. It has also been shown that feedback, due to the time delay, actually **increases** certain distortions, namely the 5th 7th and 9th harmonics.

Now the increase is slight, but there is a problem: the human ear uses these harmonics to determine how loud a sound is. So if you mess with these harmonics, the electronics will have an artificial loudness about them, a tonality, which audiophiles describe as bright, harsh, brittle, etc.

In a nutshell adding feedback decreases certain distortions that the ear does not care about a lot (we hear them as 'warmth', 'bloom', etc.) while **increasing** the distortions that the ear cares about a lot.

Some designers pay attention to this and others don't. The subject has been controversial for over 50 years. For more information see
http://www.atma-sphere.com/papers/paradigm_paper2.html

Finally, Chaos Theory has something to say about this, confirmed by Norman Crowhurst several decades before Chaos Theory became generally accepted: The addition of negative feedback to an amplifier results in a chaotic behaviour with both stable and unstable states. Additional harmonics and also inharmonic information is added (Chaos Theory calls distortion 'bifurcation'). This results in a noise floor quite unlike normal hiss- a noise floor that the human ear cannot hear into (whereas we can hear 20 db into hiss) that effectively masks information below the noise floor.

For those interested, Wikipedia has a nice primer on Chaos Theory: http://en.wikipedia.org/wiki/Chaos_theory

The faster the amplifier, the less this is a problem! However, faster is not the same as 'real time'. I would say that this is in the realm of 'progress' where we have an improvement over something that was done 15 or 30 years ago.

I think you can see though that if the input and feedback were synchronized, that the delay of the circuit is going to have to go somewhere. You might need a substantial memory for that. The faster you do the scan, the more memory you are going to need...

Magfan, to answer your question, in a power amplifier of conventional design (not class D IOW) the problem you face if you use local feedback as opposed to global is the issue of gain. IOW if the stage that the feedback is around does not have a lot of gain, it may not help you all that much. Where this really plays a role is in the output section, wherein if the amplifier is to conform to the ideals of the Voltage Paradigm (IOW, be a voltage source), there is rarely enough gain in the output section for said local feedback to do the job.

I did say rarely: the Ayre amplifier is able to be a voltage source without any feedback.

Anyway, you encounter many of the same problems with local feedback as you do with global, but overall I would say they are less pronounced, as (in theory) a single stage is going have a shorter propagation delay than an entire amplifier. This will tend to push the time domain issues to manifest at a higher frequency. That does have its good and bad side, so again a lot depends on the actual design!

How I see all this is very simple: as long as the amplifier fails to add **any** distortion to the 5th, 7th and 9th harmonics, then it all should be good. It will be even better if the lower ordered harmonic distortions fail to appear as well, although this is far less important.