Feedback delay & distortion

[Eva]

Originally posted by Jorge
One can see there's a delay during the transition time (rise and fall) and it disapears when the signal is stable (flat top and bottom).

Congratulations, you have discovered phase shift in square waves after they pass through a system with a pole.

Unfortunately, your figures show no delay, there is only phase shift since changes in the slope of the output signal are inmediately reflected as changes in the slope of the feedback signal.

[forr]

Here is an interesting artcile with a logical thinking :

http://tkhifi.homepage.dk/div/The_%2...0Feedback.html


But a detail nullifies all its conclusions :
there is no mention of what amount of delay could be in an amplifier.

As far as I remember, about thirty years ago, Nelson Pass, in the documentation of his Treshold cascode amp, gives a delay of 70 nS between a voltage at the direct input and a voltage appearing at the feedback input.
The time resolution of the ear is at best, about 10 mS.

Audio signals have a finite rise time (my data indicate that the fastest audio signal coming from a microphone is about 10 mS) and a finite slew-rate : they are rather slow. Most, if not all, audio amplifiers can easily deal with them (more than twenty five ago, Peter Baxandal and Neslon Pass have given numeric values for the minimum of slew-rate amps should have).


Conclusion :
Many discussions about audio are about existing effects,
but so small that they are absolutely irrelevant in audio :
the delay in the feedback signal is one of them.

~~~~~~~~ Forr

§§§

[tschrama]

1 - There is no BS re time delay.
2 - I'm not aware of the guys selling snake oil. This posting is not related to them or they 'magic'.
3 - Not only does a Spice simulator takes in account the speed of electric signals, but in this very simple simulation it was taken in account.

It can give useful results because the amp has a pole at 80Hz (8MHz bandwidth and gain of 100k). This is the real signal delay.
Electron speed, etc has very little to do with feedback audio amps. The time to charge/discharge the compensating cap dominated the transient response.

1] time delay in audio is BS

2] you mean you though of this yourself.. your own bright original idea..

3] I think you're ready to school now... and try to learn a bit this time.. you messing up concepts you don't seem to understand.. you don't seem to be hindered by any understanding.. .. phaseshift is NOT time delay...

[darkfenriz]

phaseshift is NOT time delay...

Actually group delay IS a -derivative of phase
And delay is mainly caused by frequency compensation circuit and other 'slow' parts of an amplifier. So a goal is to have minimum phase shift non-linearity within audio range and group delay is a good measure of phase shift non-linearity. A nice thing is to have dominant pole above audio frequency, like 25-30kHz Building a stable amp with 30-40dB of feedback factor and dominant pole at 25kHz is a key to make feedback work as advertised.

A DELAY IN AUDIO HAS NOTHING TO DO WITH NIETHER SPEED OF CURRENT NOR A SPEED OF A SINGLE ELECTRON

Understood?
Again a potentially interesting discussion, but.....

[tlf9999]

Originally posted by forr
Conclusion :
Many discussions about audio are about existing effects,
but so small that they are absolutely irrelevant in audio :
the delay in the feedback signal is one of them.

~~~~~~~~ Forr

§§§

Well, this is just one of those examples where a perfectly valid theory just has zero practical application in this particular niche of electronics called audio.

Other examples include back EMF and first cycle distortion.

[tlf9999]

Originally posted by darkfenriz
A DELAY IN AUDIO HAS NOTHING TO DO WITH NIETHER SPEED OF CURRENT NOR A SPEED OF A SINGLE ELECTRON

Understood?
Again a potentially interesting discussion, but.....

it depends on what devices we are talking about. in wires, you are 100%. In vaccum tubes or carrier-based semiconductor devices, you are incorrect.

[MikeB]

Originally posted by forr

The time resolution of the ear is at best, about 10 mS.

Audio signals have a finite rise time (my data indicate that the fastest audio signal coming from a microphone is about 10 mS) and a finite slew-rate : they are rather slow.

10ms ? Would make a max of 50hz ???

BTW, the ear can detect phasehifts below 100us, that's how the
stereohearing works, the brain measures phasehifts between left
and right "channel", this gives a very exact information for a 180°
position.

[Jorge]

Originally posted by tschrama

3] I think you're ready to school now... and try to learn a bit this time.. you messing up concepts you don't seem to understand.. you don't seem to be hindered by any understanding.. .. phaseshift is NOT time delay...

So when one uses a RC to produce a time delay it's only imagination, since all there is is phase shift?

I think you should take a closer look at time-frequency behaviour of circuits...

[Eva]

Originally posted by Jorge


So when one uses a RC to produce a time delay it's only imagination, since all there is is phase shift?

Exactly. Any change in the input signal is instantaneously reflected by the output signal but with phase of their frequency components shifted.

[Jorge]

Ok, since it seems I'm wrong, could someone pls explain:

The delay between the sin signal and fbin signal as posted;
The overshoot produced at the very initial rise of the input signal (and with a band limited input), also as posted.

Taking in account this is an ideal amplifier, with 90 deg phase shift over the band.

Thanks for enlightening me.