Better Audio with or without NFB (Negative Feedback) - for me the wrong Question - diyAudio
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Old 14th October 2009, 08:53 PM   #1
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Default Better Audio with or without NFB (Negative Feedback) - for me the wrong Question

Since the last decades and even today this question regarded audio amplifier circuits is discussed controversial (unfortunately also discussed sometimes very emotionally but not technically correct).
After read a wide range of publications, documents and comments, also by this forum, I note, this is for me actually the wrong question.
The right question for me should read as follows:

Which character is to observe at the higher frequencies far outside the audible range?

In general there are only two different misbehaviours resp. two different characters

1) Low pass character, i. e. only level reduction but no visual deformation of the sine wave signal occurs even arround 1 MHz and more.

This character is to observe, both if I use no NFB as well as by use of NFB with only one voltage gain stage (VAS) in the feedback loop. If I have this kind of behaviour, I am sure, the amplifier sounds good in this case, even by relatively high levels of THD

2) No Low pass character, i. e. visual deformation of the sine wave signal to a triangle or saw tooth signal form arround frequencies between 50 KHz and 500 KHz (consequently high order distortion), but smaller level reduction character

This character is to observe, if I use NFB by more than one voltage gain stage (VAS) in the feedback loop (there are mostly two such stages with Cdom by the second stage). If I have this kind of behaviour, I am sure, the amplifier sounds harsh, even by extreme low levels of THD in the audible range. Only the low frequency range (below approximately 500 Hz) is more tight and clear than by the first character, because the open loop gain in this range is much more higher; this means, that this kind of topology is first choice for bass transducers with diaphragms, that have heavy weight (e. g. driver for subwoofer)

Both observed characters are completely independent of whether I use the output stage in "pure Class-A" mode, or I use the typical value of the quiescent current around 30 - 50 mA, i. e. "Class AB" mode.

So my conclusion is, if I want to use an amplifier for midrange/tweeter and full range, the first character is the best choice, and the second character (i. e. high open loop gain in the audible range) is only for low frequency applications the best choice.

In the above described, I have assumed, that power compression of the power supply is negligible, GND management would be performing in the correct manner, and the clipping behaviour is without overshoots and peaks through saturation

What do you think – are my observations correct, or I still overlook other important things?

Last edited by tiefbassuebertr; 14th October 2009 at 08:57 PM.
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Old 15th October 2009, 12:02 AM   #2
Bigun is offline Bigun  Canada
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Hi,

Sometimes I find on this forum is that you have to ask a question in a very simple way because we come from different places with different languages.

If I can understand your point...

a) the more gain stages within the feedback loop the worse the sound. Nelson would likely agree with you on this

b) the downside of multiple gain stages within a feedback loop gets worse at higher frequencies. Plus it's easier to make this problem visible in simulations if you use a very high frequency test signal.

Did I get this right ?
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Old 15th October 2009, 10:01 AM   #3
glennb is offline glennb  Australia
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Quote:
visual deformation of the sine wave signal to a triangle or saw tooth signal form arround frequencies between 50 KHz and 500 KHz (consequently high order distortion), but smaller level reduction character
This is consistent with slew rate limiting. It is signal level dependent.
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Old 15th October 2009, 04:04 PM   #4
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Quote:
Originally Posted by Bigun View Post
Hi,
b) the downside of multiple gain stages within a feedback loop gets worse at higher frequencies. Plus it's easier to make this problem visible in simulations if you use a very high frequency test signal.
please read this:
First version Aleph 0 (Null, ZERO, O, Os, 0s) versus later Aleph versions
and download pdf file by #9

please not additional: for getting one voltage gain stage by mostly two voltage gain stage topology you can introduce source resistors (emitter resistors) by the first stage (long tailed pair), so that the open loop gain of this stage is only "unity gain".

this thread could be also of interest:
schematic variation and sim results of Pass "X" series US pat 5376899

Last edited by tiefbassuebertr; 15th October 2009 at 04:09 PM.
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Old 15th October 2009, 06:37 PM   #5
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Negative feedback that is in phase with the input is good.
The problems start when the feedback phase shifts and causes oscilation.

An amplifier with negative feedback is essentially a servo system and all the rules of servos apply to the amplifier.

Critical damping is vital to get a good output.

Getting the gain right is important too.
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Old 15th October 2009, 06:40 PM   #6
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Quote:
Originally Posted by nigelwright7557 View Post
Negative feedback that is in phase with the input is good.
The problems start when the feedback phase shifts and causes oscilation.

An amplifier with negative feedback is essentially a servo system and all the rules of servos apply to the amplifier.

Critical damping is vital to get a good output.

Getting the gain right is important too.
Are you sure?
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Old 15th October 2009, 06:43 PM   #7
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Originally Posted by syn08 View Post
Are you sure?
Absolutely positive !

I use the same principles when designing an amp or a servo system.
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Old 15th October 2009, 06:56 PM   #8
syn08 is offline syn08  Canada
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Quote:
Originally Posted by nigelwright7557 View Post
Absolutely positive !

I use the same principles when designing an amp or a servo system.
Good to know. I'm not going to buy one
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Old 16th October 2009, 05:14 PM   #9
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Quote:
Originally Posted by nigelwright7557 View Post
Negative feedback that is in phase with the input is good.
Negative feedback, that is in phase with the input ???
I think, you mean negative feedback, that is in anti phase (-180 degrees) with the input. Otherwise I would have positive feedback.
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Old 16th October 2009, 05:59 PM   #10
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Quote:
Originally Posted by tiefbassuebertr View Post
Negative feedback, that is in phase with the input ???
I think, you mean negative feedback, that is in anti phase (-180 degrees) with the input. Otherwise I would have positive feedback.
He's right. Every amp with gNFB has two inputs. Your siganl goes into the noninverting input, and the feedback signal goes into the inverting input. You will have negative feedback then.

Most all SS amps these days have a differential amp as the first stage. Follow the signal: it always goes into the noninverting input.

With hollow state amps, it's a bit different: the NFB signal is almost always applied to a cathode, while the signal goes into the grid. It's still invert/noninvert operation.
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