Your journey with lower-watt tube amps -- Can a kit be good enough?

@decooney asks a good question for @atmasphere 

One thing which seems to hang on this answer is when and whether the general audiophile precept that "simpler is better" is true.

explain in a bit more detailed manner [my own ignorance] how "feedback" sort of identifies, isolates, and/or removes these other things that are "not" the signal.

@decooney The various influences described above (resistors, caps, transformers, tubes, et. al) are part of why the amp makes distortion.

Feedback works by taking a small amount of the output Voltage and applying it to an earlier part of the circuit where it is out of phase with the audio signal. When the two are mixed, a corrected signal is generated which has the corrections needed to make the amp put out an undistorted signal.

In theory.

In practice feedback can generate distortion of its own. This is especially true if the feedback is mixed with the audio in a non-linear way, such as inside a vacuum tube or transistor. This is very common; in addition to harmonics the non-linearity of the feedback node can generate intermodulations as well. 

Your tech friend is right. Too many designers just use a resistor and call it good. If a lot of feedback is used, this can lead to instability. The amount of feedback used and where its applied makes a difference too. If you want to do it right, this complicates the successful amplifier design in a lot of ways since the math for a second or 3rd order feedback loop isn't trivial.

One thing which seems to hang on this answer is when and whether the general audiophile precept that "simpler is better" is true.

@hilde45  I think it was Einstein who pointed out that a thing can be too simple to do its job well.  I prefer simplicity myself and like to keep things simple if I can. But while a thing might appear simple, from just viewing it you may not see how much is really going on.

For example to get a 300b to sound right it has to be biased properly. That requires one to generate a load line so that you can see if the output transformer used is going to load it correctly, in addition to the operating Voltages. You can't see that just by looking at it.

A self oscillating class D amp can be a fairly simple circuit too. Looking at one, you can't see that the math behind it is pretty complex.

Of course, a lot hangs on what is meant by 'simple'...

@atmasphere Thank you for your additional comment!

@atmasphere 

I think it was Einstein who pointed out that a thing can be too simple to do its job well.  

Everything should be made as simple as possible, but not simpler. 

-Albert Einstein