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Old 26th June 2012, 08:55 PM   #2581
a.wayne is offline a.wayne  United States
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Are we talking output devices or Vas .... ?
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Old 26th June 2012, 09:31 PM   #2582
mikelm is offline mikelm  England
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We are talking the entire signal path through the amp and the feedback path back to the i/p device.

edit - but originally about characteristics of the o/p devices

Last edited by mikelm; 26th June 2012 at 09:43 PM.
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Old 26th June 2012, 09:45 PM   #2583
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Esperado View Post
Who said that ?
What you said about delay is true. But the problem does not fully rely on that. First Nyquist law will never let-you run an amplifier with a 180° phase shift if any gain ;-)
Second, delay matters with fast changes in the signal, right ? And delay will bring overshoot Agree ? Slew rate limitation and good compensation avoid that. So the real concern is just final slew rate of your amp and flat response with no overshoot..
Where the delay/phase problem is your is when you are working on poles optimization / harmonization.
exactly that was what i was trying to say/write. If you overcompensate the slewrate will be to long.
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Old 27th June 2012, 10:26 AM   #2584
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Quote:
Originally Posted by Edmond Stuart View Post
A time delay, IOW a latency (caused by long transmission lines or so) is mathematically expressed by: exp(-sT). It delays the arrival of signal at the receiving end by an amount equal to T without altering the amplitude. Thus the onset of the step response is also belayed by T seconds.

OTOH, a phase lag (caused by a LP filter or capacitances of a tranny etc) are expressed by e.g. (1-exp(-sT))/s, which does alter the amplitude, but it does NOT delay the onset of a step response or so.

If you call these crucial differences just semantics, you've missed my point (and fundamentals of FB theory).
Exactly as the big book says.
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Old 27th June 2012, 04:04 PM   #2585
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Old 27th June 2012, 04:57 PM   #2586
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Quote:
Originally Posted by mikelm View Post
Sounds like semantics to me. Whatever you choose to call it the voltage rise is delayed so there is a delay - even though it is called current feedback in fact the i/p transistor compares voltages. So if the voltage rise is delayed, there is a delay before the signal gets back to the feedback transistor. - end of story.
Edmond is right. The voltage rise (or fall) is not delayed. The voltage changes at the feedback immediately after a voltage change at the amp input (save a very small transit delay in the amp, which is irrelevant to this discussion).

Think about it. If you send a current into a cap, the cap voltage will immediately react to any current change. NO delay. There is a phase shift because the voltage across the cap is the intergral of the current into it. So, if you send a sine wave current into the cap, the result is the integral of a sine - which happens to be also a sine, but phase shifted.

On an amplitude vs freq curve it LOOKS as if the output sine is delayed wrt the input sine wave but that is ONLY because it just happens that the integral of a sine is also a sine.

If you do the same test with, say, a square wave or a triangle you will see that the output or the feedback node reacts IMMEDIATELY to changes in input voltage, so no delay, only a phase shift. And all this is precisely the reason that feedback actually works.


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Last edited by jan.didden; 27th June 2012 at 04:59 PM.
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Old 27th June 2012, 05:14 PM   #2587
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Originally Posted by mikelm View Post
I agree with you about using CF because it is a fast topology and this sounds best but having chosen this topology I think it is best to avoid using too low value resistors in the FB circuit to try to speed things up even further - In my experience the opposite sounds better.

Do others have the same experience ?
Actually, there is a maximum resistor value you can use before instability sets in. That has to do with the (small) parasitic capacitance at the inverting input. Too large a feedback resistor causes phase shift leading to instability.
Most CF data sheets for instance warn against this and specify a maximum resistance value.
I am not aware of any minimum limit for those resistors, although at a certain point they start to load the output down too much of course.

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Old 27th June 2012, 05:23 PM   #2588
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Thx for that feedback Jan,

When I switched feedback resistor values from 50R / 1K to 100R / 2K2 in my DC linked fetzilla the sound was significantly cleaner - I left the millar cap unchanged so stability was significantly increased.

I guess I must have been within the limits you describe.

cheers

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Old 27th June 2012, 05:39 PM   #2589
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Quote:
Originally Posted by mikelm View Post
Thx for that feedback Jan,

When I switched feedback resistor values from 50R / 1K to 100R / 2K2 in my DC linked fetzilla the sound was significantly cleaner - I left the millar cap unchanged so stability was significantly increased.

I guess I must have been within the limits you describe.

cheers

mike
Mike, I don't know the limits in your particular application, so you may well be right.

I just wanted to note that as far as stability is concerned, larger becomes worse.
In a CF opamp for instance, generally anything above 1k leads into danger territory.
They are so sensitive that the extra 4 or 5 pF of an opamp socket pin can ruin stability; those opamps should for best performance be soldered directly to the board, and layout should take care of minimum parasitic at the inverting input.
Same with power amps of course but I have no experience with the actual numbers involved there.

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Old 28th June 2012, 04:11 AM   #2590
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by mikelm View Post
When I switched feedback resistor values from 50R / 1K to 100R / 2K2 in my DC linked fetzilla the sound was significantly cleaner - I left the millar cap unchanged so stability was significantly increased.
May be if you listen at very high SPL you will hear worse sound? I don't understand why stability should have been automatically increased (unless you have measured it)

Quote:
Originally Posted by mikelm View Post
I think it is best to avoid using too low value resistors in the FB circuit to try to speed things up even further - In my experience the opposite sounds better. Do others have the same experience ?
I think I have heard people saying that lower resistance has lower noise. But I think there are more other critical things that are more important to consider when we talk about feedback resistors.

In my experience, there is no rule whether higher or lower resistance is better or worse. I prefer to say that there is that optimum value.

1) FB resistors affect stability (e.g. against capacitive load). More often you cannot go too high, but often you cannot go low either. There is an optimum.

2) In your DC linked (I assume this means you have no cap in the FB so RC is not affected) Fetzilla, by increasing FB resistors you hear cleaner sound. But don't forget that when you get lower THD at low SPL, you may get higher THD at high SPL, because oscillation always starts at certain high SPL. ADD: You have increased the FB by going from 1K/50 to 2K2/100 (instead of 2K/100), which most of the time will lower the THD (audible or not)

3) In a differential amp (opamp/LTP) I have heard there is a benefit from balancing the inverting and non-inverting entry points. This is too complex imo because there are many other things that are affected, most important (imo) is the input impedance of the amp itself. Yes, I have experimented with this, listening to various schemes, but I couldn't hear the benefit of "balancing" the differential amp/opamp this way.

Last edited by Jay; 28th June 2012 at 04:13 AM.
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