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Old 4th February 2006, 04:48 PM   #1
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Default A few naive questions

Greetings from Norfolk.

As a newcomer to this group, and having been out of audio electronics for a number of years, I would be interested to have answers to the following questions

1. What is the advantage of ‘no feedback’ amplifiers, surely feedback has a number of advantages – stabilise the gain, reduce the distortion, reduce the output impedance ? Having looked at t number of the submitted designs, I ask why the use of un-bypassed emitter resistors is not considered as negativefeedback, it always was in my earlier days ?

2. What has happened to the ideas put forward by by Edward M. Cherry (Assoc. Prof. at Monash University) ?

3. Why are amplifiers now being specified / designed with a ‘flat’ frequency response curve going up to (say) 1 mHz ?

Your comments on the above would be very enlightening.

Richard
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Old 4th February 2006, 05:24 PM   #2
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Default Re: A few naive questions

Quote:
Originally posted by Gandalph
What is the advantage of ‘no feedback’ amplifiers, surely feedback has a number of advantages – stabilise the gain, reduce the distortion, reduce the output impedance ?
I see no advantage in "no feedback" amplifiers...they aro not true with the input signal, but as we know , not everybody is looking for accuracy .


Quote:
Having looked at t number of the submitted designs, I ask why the use of un-bypassed emitter resistors is not considered as negativefeedback
Yes they use lots of un-bypassed emiter resistors and even emiter followers that have near 100% negative feedback.
Quote:
Why are amplifiers now being specified / designed with a ‘flat’ frequency response curve going up to (say) 1 mHz ?
In that case I agree , even if we can't ear higher than 20Khz the wide bandwidth help to diminish the phase shift across the audio band and generaly a with bandwith design, is a signal of better engineered amplifier ( but not always ... ).

My target is usually 200Khz..

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Old 4th February 2006, 05:41 PM   #3
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Hello,

It's more of a philosophical issue than technical.
If you go back in time 30 years or so, and read the engineering papers on the topic then, you'd find there were some companies using feedback as a 'patch' to hold together some rather poor designs, giving the poor designs some impressive specifications but poor sound.

Some purests thought this was bad and started a movement towards less feedback is better.

Eventually coming to no feedback.

Distortion in an amplifier greater than 0.1% is audible and realistically cannot be achieved without some feedback.

I'm of the *feedback is good* camp myself, so long as the design is good.

As for bandwidth, I can hear to 45KHz. So can most people, though they don't realise it. So, bandwidth to 100KHz or so is worthwhile IMO.
Beyond 100KHz it becomes a numbers game IMO.
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Old 5th February 2006, 02:13 AM   #4
ilimzn is offline ilimzn  Croatia
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Default Re: A few naive questions

[QUOTE]Originally posted by Gandalph
Quote:
1. What is the advantage of ‘no feedback’ amplifiers
Having no feedback reduces some problems to triviality, so FOR THOSE PROBLEMS it has advantages - for other, equally important for proper functioning of an amplifier, not only does no feedback have no advantages, but often it has considerable disadvantages. Perhaps the most important advantage is stability, especially with, shall we say, unflattering loads. A less important and not as pronounced, but often quoted advantage, is a more favorable harmonic spread in the distortion that is there (note I am talking only about the relative magnitude of the harmonics, not the RMS sum absolute value). For various reasons which have already been discussed on the forum many times, (global) feedback can generate amounts of higher order harmonics that are disproportionate in comparison to the low order harmonics (this phenomenon is reduced when feedback loops are kept short, eg. local feedback in a single stage). It should be noted that this particular notion of 'disproportion' exists as a result of the way we hear - mathematically speaking, the feedback amps can be far more linear.
In most cases, when 'no feedback' is cited, it is understood that no GLOBAL feedback is employed. It is arguably near impossible to design an amplifier with no feedback mechanism whatsoever.
It's a whole different story that the good and bad of feedback is often vehemently debated by people who would not know global, local or indeed any feedback from a cow's rear end


Quote:
2. What has happened to the ideas put forward by by Edward M. Cherry (Assoc. Prof. at Monash University) ?
I have to admit I have never heared of E. M. Cherry's ideas, so no comments here.

Quote:
3. Why are amplifiers now being specified / designed with a ‘flat’ frequency response curve going up to (say) 1 mHz ?
This is a side effect of another requirement. Phase angle, mentioned by another poster is actually only a relatively minor problem here. Certain distortion mechanisms (including high order distortion generation in global feedback amps) produce distortion that rises with frequency, sometimes very sharply. Designong an amp as high bandwidth is a form of 'postponing the evil day' as far as possible outside of the audible frequency band. Think of it as making sure that 20kHz THD figures are as acceptable as the 1kHz ones
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Old 5th February 2006, 02:40 AM   #5
lineup is offline lineup  Sweden
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Default Re: Re: A few naive questions

Quote:
Originally posted by ilimzn
I have to admit I have never heared of E. M. Cherry's ideas, so no comments here.
I did not know anything about Cherry, but a search with Google gave result:
Quote:
5. The Feedback Lie

Negative feedback, in an amplifier or
preamplifier, is baaaad. No feedback at
all is gooood. So goes this widely invoked
untruth.
The fact is that negative feedback is
one of the most useful tools available to
the circuit designer. It reduces
distortion and increases stability. Only
in the Bronze Age of solid-state amplifier
design, back in the late ’60s and
early ’70s, was feedback applied so
recklessly and indiscriminately by certain
practitioners that the circuit could
get into various kinds of trouble. That
was the origin of the no-feedback
fetish. In the early ’80s a number of
seminal papers by Edward Cherry
(Australia) and Robert Cordell (USA)
made it clear, beyond the shadow of a
doubt, that negative feedback is totally
benign as long as certain basic guidelines
are strictly observed. Enough time
has elapsed since then for that truth to
sink in. Today’s no-feedback dogmatists
are either dishonest or ignorant.
For more info about 10 Audio Lies
see this topic we had some time ago:

Audio Lies - diyAudio discussion

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Old 5th February 2006, 03:34 AM   #6
Amp_Nut is offline Amp_Nut  India
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Cherry's work involved applying feedback spanning 2 consecutive stages... "nested" feedback, rather than Global feedback... between the Input and output stages of the amplifier.

"No Feedback" has been turned into a markeing tool and is clearing a misnomer. It is Impossible to design any practical amp with Zero ( Local ) feedback. Even if there is No external emitter resistor in a voltage gain stage, there will Always be an intrinsic emitter resistance of the device itself. This is what yield a finite voltage gain in a common emitter amplification stage.

Yes, as rightly pointed out, the Emitter follower ( buffer ) present in most "Zero Feedback" amps has 100% feedback !

"Zero Feedback" I believe is supposed to refer to feedback between the over all input and outputs of the amplifier.

WHY SOME CONSIDER FEEDBACK BAD :

As posted earlier in this thread, there were (are ? ) many shoddy amp designs, which take the quick and easy route : Get as much gain as possible, dont bother about liniarity. Then put ooodles of feedback, abd as long as the amp does not oscillate... you will have vanishingly low distortion.

Such amps measured Very well ( distortion in Parts per million) but sounded not so great.

Engineers began to scratch their heads ....

Mati Otala ( apologies if the spelling is wrong) proposed that amplifier sound was affected not so much by 'static' (harmonic) distortion, but by Triensient Inter Modulation (TIM) Distortion. TIM is caused when the signal transient is too fast for the feedback loop to handle....

Still no 'Perfect' sound....

Today's amp designers seem to have thrown out the 'baby with the bath water' and design amps with no feedback at all.

Some of these are stated to sound closer to the proverbial 'perfect amp' even though objective tests such as distortion ( and in some cases a ruler flat 20 to 20 KHz response have taken a hit.

But still no "Perfect Sound"....

( ON THE SIDE: I believe the distortion measurenments have been inspired by Fourier, who showed mathematically, that any wave shape can be deemed to be composed of sine waves. Hence if an amp could perfectly reproduce sine wave ( ie with zero distortion) the same amp could reproduce Any wave form... including music..... )

To reproduce the waveform, the different frequency sine waves also Must maintain their phase relationships... ie how much later one sine wave starts or ends, with reference to another ( typically the fundamental, or the lowest freq sine wave in the group. )


WHY DC TO LIGHT FREQ RESPONSES ?

When an amplifier output begins to roll off and is is 3 dB down at a particular freq, it also introduces a 180 degreee phase shift at the same freq.... which in turn will mess up the signal, because the sequence of the the sine wave components in the output will not be the same as that in the input signal.

Clearly then, the -3dB comes not only with a lower HF penelty, but also with a HUGE phase distortion.

As a practical example, to "almost" perfectly reproduce a 20KHz square wave, you will typically require an amp with a flat freq response to 10 Times the 20 Khz freq, or 200 Khz.....

Cheers
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Old 5th February 2006, 06:37 AM   #7
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Actually the feedback question is more complex than expressed here. There is also higher order distortion generated from lower order linearities and FM distortion generated by non-linear input stages within a feedback loop with low open loop bandwidth. Real audio designers take negative feedback very seriously.
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Old 5th February 2006, 06:57 AM   #8
AKSA is offline AKSA  Australia
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A really good thread........ particularly liked Ilimzn's and John's responses.....

I concur that global negative feedback introduces stability issues with 'unflattering' loads. This happens because of the existence of a pole frequency beyond which phase shift reverses negative to positive feedback. OTOH, GNFB gives low source impedance so important with modern drivers to confer slam and impact.

The non-linearities introduced by GNFB can lead to less than monotonically decreasing distortion spectra; but if the loop is very fast (which implies a swift VAS and a judicious mix of lag comp and phase lead) then the audible effects of this are much reduced.

There is still art in the management of the many compromises of amp design. It's a shame an audiophile amp is so named; if it were called a musical instrument more care would be taken with design.....

Cheers,

Hugh
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Old 5th February 2006, 08:05 AM   #9
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Default Re: A few naive questions

Quote:
Originally posted by Gandalph
3. Why are amplifiers now being specified / designed with a ‘flat’ frequency response curve going up to (say) 1 mHz ?
.... because "more is better", see my title at the left


One reason is marketing but an another reason is that if those amps are current feedback the distortion is extremely low. No harm in that.

My QRV-06 produce 0.00046% and http://mirand.dk has also pretty good performance.

My QRV-06 is good for DC-25 MHz
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Old 5th February 2006, 08:52 AM   #10
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Mr. John Curl,

Audio power amps is to work for signals below 20khz.
Is there any benefit/importance for looking how an amp behaves (gain/phase magnitude) in frequencies of/above 1Mhz?
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