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Negative feedback - right or wrong to use?
Negative feedback - right or wrong to use?
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Old 26th November 2017, 12:35 PM   #1
Mikelandjelo is offline Mikelandjelo  Serbia
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Default Negative feedback - right or wrong to use?

As i started in title things are interesting to think about....at least for me.If we have some amp (no matter which topology is used) and a speaker box with three speakers in it (bass,middle and tweeter) situation is far away from simple.At first place lets assume that crossover design is done as standard quality one.We will have different impedances for some frequency ranges.If we have constant gain and perfect frequency responce (flat line) what we will hear will NOT be flat frequency responce on speakers.....On some freq. ranges speaker box will pull more current (constant output voltage from amp on all frequencies) and on some it will pull less no matter of how flat output from amp is.So,some frequencies will be louder and some quieter.Why we need then negative loop?On other side if we dont have any feedback from output situation will be same but almost worse becouse output voltage will fluctuate too.This will happen with single speaker on output without any crossover design too.So,my main question is : Is there some other concept around all this that can match output with load better than all its done till now?Any ideas?
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Old 26th November 2017, 12:42 PM   #2
Mooly is offline Mooly  United Kingdom
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Negative feedback - right or wrong to use?
Commercial speakers are designed on the following assumptions:

1/ The speaker design itself is designed to be driven from a constant voltage source... which neatly leads on to:

2/ That the amplifier driving the speaker will maintain its constant voltage output over all conceivable load impedances.

This means that even though the speaker impedance may vary by a factor of 20 or more over the audio band, the amplifier is not bothered by this and always delivers the same voltage at its output.
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Old 26th November 2017, 12:46 PM   #3
scottjoplin is offline scottjoplin  Wales
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Variable Amplifier Impedance
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Old 26th November 2017, 01:35 PM   #4
Max Headroom is offline Max Headroom  Australia
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Quote:
Originally Posted by Mooly View Post
Commercial speakers are designed on the following assumptions:

1/ The speaker design itself is designed to be driven from a constant voltage source... which neatly leads on to:

2/ That the amplifier driving the speaker will maintain its constant voltage output over all conceivable load impedances.

This means that even though the speaker impedance may vary by a factor of 20 or more over the audio band, the amplifier is not bothered by this and always delivers the same voltage at its output.
Assumptions are fine in theory but not in practice in the audio realm IME.
20:1 impedance variation is too much for most amplifiers, and the result is a smattering of distortion products. `
Zobels across individual drivers plus across overall crossover network markedly reduces subjective distortions.
Cost is appreciative but the benefits are priceless.

Dan.
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Old 26th November 2017, 01:39 PM   #5
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Mikelandjelo View Post
As i started in title things are interesting to think about....at least for me.If we have some amp (no matter which topology is used) and a speaker box with three speakers in it (bass,middle and tweeter) situation is far away from simple.At first place lets assume that crossover design is done as standard quality one.We will have different impedances for some frequency ranges.If we have constant gain and perfect frequency responce (flat line) what we will hear will NOT be flat frequency responce on speakers.....On some freq. ranges speaker box will pull more current (constant output voltage from amp on all frequencies) and on some it will pull less no matter of how flat output from amp is.So,some frequencies will be louder and some quieter.Why we need then negative loop?On other side if we dont have any feedback from output situation will be same but almost worse becouse output voltage will fluctuate too.This will happen with single speaker on output without any crossover design too.So,my main question is : Is there some other concept around all this that can match output with load better than all its done till now?Any ideas?
There is no definite link between how much current speaker pulls and how loud it is.

For instance, at resonance a driver is high impedance thus pulls less current but is as loud as at other points due to resonance.

Also, xover may have dips and peaks of impedance which may or may not be louder or less loud. No direct link.

Jan
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Old 26th November 2017, 03:30 PM   #6
Mikelandjelo is offline Mikelandjelo  Serbia
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So there ARE variations......Constant voltage at all frequencies from amp means SAME output level on all frequencies.And that my friends will NEVER happen.You WILL HAVE constant voltage and absolutely wrong output levels on speaker.If we assume no feedback at all then ABSOLUTELY speaker design matters and ABSOLUTELY anyone will make difference between any two speaker boxes.This puts all speakers and companies at same testing level and this is real comparison of the speaker boxes qualities......further more with no feedback and "IDEAL" speaker box design where impedance is always same on any frequency (not done yet) result is ideal.So MATCHING circuitry between load impedance and output amplifier impedance is whats needed.....or nothing if speaker box is properly designed.Most of companies makes amplifiers that measures "PERFECT" and sound is pure crap....On other side there are designs that measures very bad but sounds amazing.I tested few of that kind and developed my own....So any ideas of possible matching real time network ?

Cheers,
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Old 26th November 2017, 03:37 PM   #7
scottjoplin is offline scottjoplin  Wales
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So, your initial post wasn't purely a question based on interest. Would you care to state your agenda?
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Old 26th November 2017, 03:49 PM   #8
Mark Whitney is offline Mark Whitney  Netherlands
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Feedback is a solution for amp design and not for speaker design. Try building an amp that has a flat frequency response and low distortion without feedback.
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Old 26th November 2017, 03:49 PM   #9
JMFahey is offline JMFahey  Argentina
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I see you weren´t really "asking" anything but setting the frame to introduce your own pet theories.

No problem with that, this is DIY Audio after all, but it would have been more honest/transparent just stating it from the beginning.
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Old 26th November 2017, 03:55 PM   #10
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Mikelandjelo View Post
So there ARE variations......Constant voltage at all frequencies from amp means SAME output level on all frequencies.And that my friends will NEVER happen.You WILL HAVE constant voltage and absolutely wrong output levels on speaker.If we assume no feedback at all then ABSOLUTELY speaker design matters and ABSOLUTELY anyone will make difference between any two speaker boxes.This puts all speakers and companies at same testing level and this is real comparison of the speaker boxes qualities......further more with no feedback and "IDEAL" speaker box design where impedance is always same on any frequency (not done yet) result is ideal.So MATCHING circuitry between load impedance and output amplifier impedance is whats needed.....or nothing if speaker box is properly designed.Most of companies makes amplifiers that measures "PERFECT" and sound is pure crap....On other side there are designs that measures very bad but sounds amazing.I tested few of that kind and developed my own....So any ideas of possible matching real time network ?

Cheers,
Well, with so much ABSOLUTES, not much left for an intelligent discussion.

Jan
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