I suppose it is a refreshing change to hear someone suggest that feedback can be better than instantaneous. The usual complaint from those who do not understand feedback is that it is significantly delayed and so causes time smearing.
Propagation delays are negligible for analogue audio circuits. The feedback can therefore be considered instantaneous, but affected by various low pass filters so it can sometimes give the superficial impression that it is delayed. The way to tell the difference between a low pass filter and a true delay is whether you need a time machine to undo the change or can do it with merely an inverse filter.
Propagation delays are negligible for analogue audio circuits. The feedback can therefore be considered instantaneous, but affected by various low pass filters so it can sometimes give the superficial impression that it is delayed. The way to tell the difference between a low pass filter and a true delay is whether you need a time machine to undo the change or can do it with merely an inverse filter.
Considering the speed of light is pretty fast, any delay of signal from the output back to the input would be such a tiny fraction of a second, you'd need lab equipment to even measure it. And such delay would have zero impact on audio.
Admiral Grace Hopper was famous for her nanoseconds. She was a smart cookie. She would hold up a fistfull of foot-long pieces of wire, and explain that that was the limit on how far electricity could travel in one nanosecond. See her:
Admiral Grace Hopper Explains the Nanosecond - YouTube
Your NFB would likely travel even less distance.
Admiral Grace Hopper was famous for her nanoseconds. She was a smart cookie. She would hold up a fistfull of foot-long pieces of wire, and explain that that was the limit on how far electricity could travel in one nanosecond. See her:
Admiral Grace Hopper Explains the Nanosecond - YouTube
Your NFB would likely travel even less distance.
We have one of her nanoseconds up in the attic. I think the it was 10 inches long.
And no, electricity does not travel as fast in wires & cables as it does in space. But 50 to 90 percent of the speed of light is still real, real fast.
> does it go back in time or is it instanteneous?
"Back in time" is of course not going to happen. (If it did, we would get a whole lot of amplifiers, feed tomorrow's lottery numbers to the output, and place our wining bets today.)
"Instantaneous" also does not happen. NO amplifier has infinite frequency response. Most audio amps have quite limited response, maybe a million changes per second. (Other types of amps can be faster, but with other trade-offs.)
The key thing is that our EARS only go to say 20,000 times a second, so we "can" correct the output faster than our ears can detect the error. (That's not inevitable, we do have to mind the details.)
If you look at enough curves, you will see that almost all "audio" amplifiers have rising distortion, or dropping response (or both), in and beyond the top of the audio band. Naturally marketers do not highlight this fact. You can also see it in "DSL" chips which are precision 10MHz amplifiers, and typically show rising HD and IM above a few MHz. These encode multi-level MHz data, and if distortion is too high they would smear the bits and lose data-rate.
"Speed of light" is typically not the issue in audio. Mostly it is because capacitance is everywhere and it all sucks, which is worse for higher frequencies. Some capacitance is essentially Nature's Law. The Gate of a MOSFET has capacitance largely related to its size. High-power devices have large gate capacitance. We can apply large drivers, but these will load the stage driving them. More stages is not the answer because if we have more than about one major delay inside the NFB loop, it becomes an oscillator.
"Back in time" is of course not going to happen. (If it did, we would get a whole lot of amplifiers, feed tomorrow's lottery numbers to the output, and place our wining bets today.)
"Instantaneous" also does not happen. NO amplifier has infinite frequency response. Most audio amps have quite limited response, maybe a million changes per second. (Other types of amps can be faster, but with other trade-offs.)
The key thing is that our EARS only go to say 20,000 times a second, so we "can" correct the output faster than our ears can detect the error. (That's not inevitable, we do have to mind the details.)
If you look at enough curves, you will see that almost all "audio" amplifiers have rising distortion, or dropping response (or both), in and beyond the top of the audio band. Naturally marketers do not highlight this fact. You can also see it in "DSL" chips which are precision 10MHz amplifiers, and typically show rising HD and IM above a few MHz. These encode multi-level MHz data, and if distortion is too high they would smear the bits and lose data-rate.
"Speed of light" is typically not the issue in audio. Mostly it is because capacitance is everywhere and it all sucks, which is worse for higher frequencies. Some capacitance is essentially Nature's Law. The Gate of a MOSFET has capacitance largely related to its size. High-power devices have large gate capacitance. We can apply large drivers, but these will load the stage driving them. More stages is not the answer because if we have more than about one major delay inside the NFB loop, it becomes an oscillator.
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It is the other way around. The results are 'shocking' because the nfb 'pre-distorts' the intermediate signal so as to make Vout perfect (or almost). What you see at the input of the output stage is the output stage distortion inverted.
Jan
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