Negative Feedback a deal killer?

Feed forward a deal reviver?

"...Negative feed back is just like anything else. It can help or it can hurt. It depends on implementation and how it is integrated with all other technical aspects of a design. "

Absolutely true !!!!!!!!!!!!

Still, I wish to add that NF is dangerous weapon and many nmanufacturers simply don;t know how to use it and it took decades to learn its positive and negative contributions to the sound.

My favorite amplifier and I own it, Spectron bases its design on "control theory" where they treat amplifier as a "control system". They do not use large amount of negative feedback but theirs - about 10 times faster then typical amplifier...and they can control speaker with the load of 0.1 Ohm...and reproduce music, particualrly dynamic peaks more realistically then any other amp I owned or auditioned. For hard rock - ain;t better.

Dob,

Please refrain from making bold statement like that. Please don't take it the wrong way but Spectron can not drive the 1 ohm Scintillas though tough load 0.8ohm from 20hz to about 2khz and then hovering around 1.5 ohm up 20khz. That is a tough load but still far cry from 0.1ohm. The Krell master reference is probably the only amplifier which can handle such load cause now we're so close to a dead short. Below is the real testimony that Spectron cannot drive 1ohm load, namely the Apogee Scintillas. Again, I meant no offense. Peace

http://www.apogeeacoustics.com/forum/viewtopic.php?f=5&t=41

H2O, welcome. A friendly note: the expected courtesy here is for those that are in the "business" regardless of how small an enterprise it may be, proclaim that each time they post. This is especially true if they're listed as a "private user".

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