Negative feedback bad for audio?

[Steve57]

Steve - what do you think fidelity is? What do you think distortion is?

Fidelity is defined as "the degree of exactness with which something is reproduced".

A perfect amp (perfect fidelity) is one where the output signal is exactly the same shape as the input signal, but bigger amplitude - zero distortion. (The proverbial wire with gain).

All amps distort to one degree or another. Distortion changes the shape of the output signal so it does not perfectly match the input. That distortion is a loss of fidelity .

(There is also noise, but we'll ignore that because it has little to do with feedback)

The only way to increase fidelity is to reduce distortion. They are simply opposites of one another. Any technique which reduces distortion by definition increases fidelity.

Tony, I agree with most of what you say above..
A simple feed back loop is a great thing, my comments is are about interactions within the amplifier to other signals.

My understanding is that feed back cancels distortion created within the amplifier, its a simple ? loop that cancels its own distortion
Any signal added from a 3rd source does not have an opposing distortion in the amplifier to cancel it.?

We know feed back is not a fix all, as you well know it works less well as you go up the frequency range. So higher frequency distortions are still there.

Feed back in many ways is a form of phase cancellation.. which should be an interesting subject to all of us guys

Phase cancellation is known in pro music trade as the silent killer..


Phase cancellation is well know in the recording and mixing industry.
But in my view can been seen to affect most amplifiers in various guises

 

[antcollinet]

Oh dear god.

 

[boXem]

Tony, I agree with most of what you say above..
A simple feed back loop is a great thing, my comments is are about interactions within the amplifier to other signals.

My understanding is that feed back cancels distortion created within the amplifier, its a simple ? loop that cancels its own distortion
Any signal added from a 3rd source does not have an opposing distortion in the amplifier to cancel it.?

We know feed back is not a fix all, as you well know it works less well as you go up the frequency range. So higher frequency distortions are still there.

Feed back in many ways is a form of phase cancellation.. which should be an interesting subject to all of us guys

Phase cancellation is known in pro music trade as the silent killer..


Phase cancellation is well know in the recording and mixing industry.
But in my view can been seen to affect most amplifiers in various guises

I linked an article at the beginning of the thread. Why not taking the time to understand it? As it seems a bit too mathy and jargonic for some, why not asking questions on the parts that would not be clear enough for you?

 

[solderdude]

My understanding is that feed back cancels distortion created within the amplifier, its a simple ? loop that cancels its own distortion
Any signal added from a 3rd source does not have an opposing distortion in the amplifier to cancel it.?

Your understanding is not entirely accurate.
There is indeed a loop.
The output voltage is constantly 'compared' to input voltage. That output voltage is divided using 2 resistors. Those determine the gain.
Every (unwanted) signal injected in the loop that is not applied on the input will be cancelled instantly as there will be a difference between the output and input signal.
This works much faster than the actual amp itself.

So whatever one inserts in the loop is 'removed' by the feedback. Of course not completely removed as there are limits in gain and frequency response in reality. Because of this distortion in amps with overall feedback is never 0% and increases with frequency a little.

Feed back in many ways is a form of phase cancellation

For acoustics ... sure.
We are talking about electronics. They are not the same thing.
Yes, phase is important there and can cause instability when exceeding certain values within a feedback loop.