Is negative feedback fully effective in case of complex music waveforms?

[DonH56]

Sure. My point is that whatever the shape of the opamp's raw open loop response, negative feedback will flatten it. It so happens the shape of that response is -20 dB / decade, so it looks a lot like the RIAA curve. In fact, according to Wikipedia, the RIAA curve is "only" 40 dB from 20 Hz to 20 kHz. That's a lot, but the opamp's response is steeper, changing about 60 dB over the same range!

I realize the purpose of negative feedback is not equalization. EQ just happens due to the steep slope of the opamp's open loop response. It's interesting that it is so very far from flat. Most amps I've seen have distortion rising somewhat with frequency. I suppose this is a contributing factor to that. Due to the opamp's sloped response, negative feedback applies more correction to the bass, less to the treble.

EQ does not happen to the signal path unless you intentionally add components to the circuit to change the closed-loop frequency response. You are not "flattening" the open-loop response, you are defining a different transfer function. The open-loop curve does not change, you are changing the signal's transfer function from the open-loop response to something else using feedback. You are thinking "backwards" of what is actually happening.

The RIAA curve is not a simple one-pole response; there are three time constants (including zeros that "flatten" it in the middle and that is why it does not appear as "steep") defined for the basic curve, plus a fourth if you add LF roll-off. and many real-world RIAA functions add additional HF roll-off as well. Look up the RIAA curve and notice how it sort of "shelves" in the middle; that is where the zeros are added to cancel the first pole.

 

[MRC01]

EQ does not happen to the signal path unless you intentionally add components to the circuit to change the closed-loop frequency response. You are not "flattening" the open-loop response, you are defining a different transfer function. The open-loop curve does not change, you are changing the signal's transfer function from the open-loop response to something else using feedback. You are thinking "backwards" of what is actually happening.

I think I understand that. Backwards or forwards, I'm referring to the net effect. Open loop frequency response drops 20 dB per decade. Closed loop response is flat (to a certain point depending on how much feedback is used). The opamp's raw response hasn't changed, but it's now part of a negative feedback system or circuit that has different overall response.

The RIAA curve is not a simple one-pole response; there are three time constants (including zeros that "flatten" it in the middle and that is why it does not appear as "steep") defined for the basic curve, plus a fourth if you add LF roll-off. and many real-world RIAA functions add additional HF roll-off as well. Look up the RIAA curve and notice how it sort of "shelves" in the middle; that is where the zeros are added to cancel the first pole.

Sure, my point is that roughly speaking, RIAA is about 40 dB from 20 Hz to 20 kHz. An opamp's open loop response is about 60 dB over the same frequency range.

 

[Cbdb2]

Not if the corner freq. is 1khz. The freq. where the open loop gain starts to fall is different for different opamps.
After 100 years of using feedback in control systems we have it figured out.
Heres 1700 pages of an INTRODUCTION to what we know: https://etextbookpdf.com/product/control-systems-engineering-8th-edition-ebook-pdf/

"Control Systems Engineering, 8th Edition, (PDF) provides students a complete introduction to the design and analysis of FEEDBACK systems that support modern technology."

As far as complex signals go, they can be broken down into sines. And theres also square wave testing to test impulses and multitone tests to see TIM. Non of these show feedback is bad.

This might answer your questions.

https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf

From that paper:
"The controversy: Control theory is a vital and standard part of engineering studies: the effectiveness of negative feedback to keep jet-fighters airborne, ferries afloat and nuclear power plants a-non-exploding has somehow never become the subject of much controversy. Only in audio does the usefulness of feedback draw heated debate, with detractors saying that reasonably good measured performance obtained without feedback sounds better than excellent performance obtained with feedback. Proponents opine that there is nothing wrong with taking a good amplifier and making it better by putting feedback around it, so long as it’s not overdone."

 

[syn08]

Negative feedback (corrective feedback) is a wonderful thing! It lowers noise

Only if there's a lot of it. Small doses of NFB could do more harm than good for your ears, for example by increasing the level of high order harmonics.

And NFB will not lower noise, that is, the input referred noise will remain abut the same (in fact, it will be slightly larger).

 

[Cbdb2]

NFB is not always better. There has to be enough. Less than 20db of NFB can increase the ratio of higher harmonic in the distortion. This is one reason some people think all NFB is bad.
Page 14 of https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf

 

[egellings]

In a word, yes.

 

[pma]

it sounds reasonable that the feedback might be too slow in case of complex music material.

That's a complete nonsense, in case you speak about a normal engineering power amplifier design.

 

[syn08]

Feedback like a dog chasing his tail; nihil sub sole novum.

 

[Gorgonzola]

That's a complete nonsense, in case you speak about a normal engineering power amplifier design.

Hold on my friend, it may be untrue, (that feedback might be too slow), but it is not nonsense. It needs to be demonstrated the the latency of the feedback signal is quick enough to correct the the input signal within the audio frequency bandwidth.

You may wish to argue, for example, that latency is a few microseconds which would be sufficiently fast to effectively correct a 20kHz signal in real time.

 

[AnalogSteph]

You may wish to argue, for example, that latency is a few microseconds which would be sufficiently fast to effectively correct a 20kHz signal in real time.

Thankfully, signal propagation in typical audio equipment is a matter of about a nanosecond or two at best, given that the speed of light is about 1 foot per ns in vacuum and not an awful lot slower elsewhere.

The RF guys like to say that audio is basically DC... by their standards it pretty much is.

In any case, negative feedback has proven to be extraordinarily effective in audio equipment. I have previously estimated that achieving the distortion figures of modern DACs without it would require even the equivalent of a single opamp to be a power-hungry monstrosity with +/-1200 V supplies and an amp of idle current. You'd probably need 3-phase power in 230/400V land for the whole thing, and it would be potentially dangerous in case of component failure to boot - what's not to like?

 

[syn08]

The next step would be to explain the difference between group and phase velocities (to explain why a phase shift is NOT a time delay/latency); but trust me, this is going nowhere. The myth of “slow feedback” is out there to stay. At least this was the case for the last 40 years, that I am aware of this fundamental misunderstanding of basic physics.

 

[Gorgonzola]

Thankfully, signal propagation in typical audio equipment is a matter of about a nanosecond or two at best, given that the speed of light is about 1 foot per ns in vacuum and not an awful lot slower elsewhere.

The RF guys like to say that audio is basically DC... by their standards it pretty much is.

In any case, negative feedback has proven to be extraordinarily effective in audio equipment. I have previously estimated that achieving the distortion figures of modern DACs without it would require even the equivalent of a single opamp to be a power-hungry monstrosity with +/-1200 V supplies and an amp of idle current. You'd probably need 3-phase power in 230/400V land for the whole thing, and it would be potentially dangerous in case of component failure to boot - what's not to like?

I have a true story for you regarding speed in electric circuits. It happened many, many years ago that I was privileged somehow, (don't remember exactly how or when), to hear a speech given by the renown, early computer programming pioneer, Grace Hopper.

In the course of the speech, she held up a length of wire about a foot long, and said, "This is a picosecond" referring to the speed of propagation in electric circuits.

 

[Cbdb2]

The wires were nanoseconds, she also handed out grains of pepper, those where the picoseconds.