Feedback Delay Time

[M Gregg]

Interesting,

A change in flux by a change in time. Does the magnetic flux change fast enough in the output Tx to give the input a response time to correct at all frequencies (Saturation, losses, capacitance, dielectric would not be a factor?).

Regards
M. Gregg

 

[Svein_B]

Just an illustration of delay, and phase.
Dotted line in top graph is group-delay, and dotted line in bottom chart is phase.
(Spice-simulation of a 3-stage SE amp - with no feedback)

Yes. there will be a little delay, depending on frequency.
The feedback, while reducing overall distortion, usually affect the composition of harmonics - more higher order harmonics.

Then there is IM distortion, and TIM, and whatever else.

Reducing the need for feedback, or the negative effects thereof, is indeed a way to achieve good sound

Svein

 

[el156]

All of you who keep referring to and insisting on the exsistance of a ''delay'' in the feedback, still don't get it. There is no delay. The phase shift of any particular frequency at the OPT is just an attenuation of the feedback level of that frequency. The feedback line can be tailored with components to equalize the levels and compensate for frequencies that can cause oscillation in an unstable amp. But that has nothing to do with the fact that there is no delay in the feedback signal returned to the input. That's why a square wave is used to see any inefficiency. You have to use a quick rise time wave just to see anything.

I didnt want to say delay in the feedback , i wanted to say delay in path of the signal from the input to the output (the signal takes some time from the rca jack untill the speaker) even at the speed of light ,it takes some time i think.Maybe like the slew rate of an ampop,the bigger the slew rate,the fastest is the ampop,less time the signal takes to reach the output. But maybe i am wrong , and thats why i wanted to know what you guys think about that.....

 

[Yvesm]

Let me try again:
Usually, the OPT has the highest contribution to phase shift (that I regard as form of "frequency dependant" delay) than any other component in a tubed amplifier.
(Sure, one may make this statement false in not using decent coupling or bypass caps values)
And that is true at both low and high frequencies.
This, when included in a FB loop may cause (for exemple) botor boating as well that a larger (but sometimes lower) distortion spectrum.

Yves.

 

[20to20]

Reducing the need for feedback

Now THAT is the goal. But until there are perfect tubes and transformers and everything that creates noise...

The perfect amplifier is a single point with a little signal in and bigger one out.
Maybe the fingertip of God.

 

[M Gregg]

Yves.

I guess this would also depend upon quality of output Tx.

Regards
M. Gregg

 

[Arnulf]

Yves.

I guess this would also depend upon quality of output Tx.

Of course, but it is far more expensive to get same improvement with a given transformer than you would with all the capacitors in the signal path, if they all contributed the same phase delay to begin with. So the assumption that the capacitors are about as perfect as they get in real life makes sense, unlike OPTs (unless you're looking for limited-band duty such as guitar amplifier using normal HiFi output transformer).

 

[M Gregg]

I must admit that feedback always seems to make music more "closed in" and less natural. This tends to be true even if the distortion is increased to remove the feedback.

Regards
M. Gregg

 

[el156]

I must admit that feedback always seems to make music more "closed in" and less natural. This tends to be true even if the distortion is increased to remove the feedback.

Regards
M. Gregg

Now THAT IS WHAT I THINK !

 

[M Gregg]

If this is the case then you have to ask the question," does the application of feedback add its own distortion?"

Regards
M. Gregg

 

[SpreadSpectrum]

All of you who keep referring to and insisting on the exsistance of a ''delay'' in the feedback, still don't get it. There is no delay.

As a digital designer, I'm puzzled at this statement. Any time you drive a conductor with a signal it takes time to propagate. Surely you mean that the delay is insignificant in relation to the signal, right?

This is not so much of a concern in our amps but it is a big concern in other applications.

 

[20to20]

Yves.

I guess this would also depend upon quality of output Tx.

Regards
M. Gregg

And so a good tranny will shift ALL frequecies we want to use by 180 degrees. Find a manufacurer who will supply this distinct data. Good luck. It is just assumed a good one will.

 

[M Gregg]

And so a good tranny will shift ALL frequecies we want to use by 180 degrees. Find a manufacurer who will supply this distinct data. Good luck. It is just assumed a good one will.

I must admit I have never seen a linear inductor.

Regards
M. Gregg

 

[20to20]

As a digital designer, I'm puzzled at this statement. Any time you drive a conductor with a signal it takes time to propagate. Surely you mean that the delay is insignificant in relation to the signal, right?

This is not so much of a concern in our amps but it is a big concern in other applications.

Just for kicks, assume a signal path of 2 feet. Now, compute how fast it would take light to travel that far. That is the delay. What frequency is that? 186282 x 5280/2 per second. Of course some capacitances are going to slow that down some. And electricity is actually slower than light, but... compared to 20Khz...

 

[el156]

If this is the case then you have to ask the question," does the application of feedback add its own distortion?"

Regards
M. Gregg

Exactly! I made a test with some of my friends with an amplifier with variable NFB and without NFB and ALL of them prefered the music without NFB !
Silvino

 

[M Gregg]

Just for interest,

What is the speed of magnetic decay in the iron core of a Tx if you take into account back EMF?

Just for fun

M. Gregg

 

[M Gregg]

The change in field induces the output current.
Is this magnetic change constant at all frequencies? If it isn't then is the feedback a true representation of any distortion in the signal path to the input of the Tx.

Regards
M. Gregg

 

[Wavebourn]

I believe what Yvesm was trying to say is that it is usually the OPT in the entire tube amplifier chain that brings by far the most significant portion of phase distortions to the signal, thereby delaying certain part of the audio spectrum, possibly resulting in audible delay (such as oscillations ).

Phase distortions bring weak driver loaded on huge inter-modulated Miller capacitance of output triode.

How often do you see 6SL7 driving 300b? I do, unfortunately...

 

[el156]

Just for kicks, assume a signal path of 2 feet. Now, compute how fast it would take light to travel that far. That is the delay. What frequency is that? 186282 x 5280/2 per second. Of course some capacitances are going to slow that down some. And electricity is actually slower than light, but... compared to 20Khz...

....compared to 20 khz...
Ok, but suppose you inject a sinusoidal wave in the input, and when that wave is starting to rise ,it didnt reach the secondary of the OPT yet,and when that portion of the wave reaches the secundary,it already passed the input and is going to be injected to the input again! What is the result of all this?
Maybe my English is not the best to write what i really want to.....
Silvino

 

[20to20]

....compared to 20 khz...
Ok, but suppose you inject a sinusoidal wave in the input, and when that wave is starting to rise ,it didnt reach the secondary of the OPT yet,and when that portion of the wave reaches the secundary,it already passed the input and is going to be injected to the input again! What is the result of all this?
Maybe my English is not the best to write what i really want to.....
Silvino

The first electrons to enter the house (tube) to attend the party, grab all the beers and run out the back door and drink all the beer and then run around to the front door and butt in line to get into the party before the line gets long.