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High power amp for low power needs - better result?
High power amp for low power needs - better result?
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Old 17th September 2019, 05:55 AM   #81
martinsson is offline martinsson  Sweden
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Thank you very much MarsBravo for that analogy, it really helps to see the processes through more common technology, very educational, thank you.

Zjjwwa, thanks for the tip, I will have a look at it, it is good to know it is basic in my case, and I also want to thank you for your previous contribution in post #46, a very good explanation and very well described.
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Old 17th September 2019, 07:21 AM   #82
zjjwwa is offline zjjwwa  Poland
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High power amp for low power needs - better result?
Quote:
Originally Posted by martinsson View Post
Zjjwwa, thanks for the tip, I will have a look at it, it is good to know it is basic in my case, and I also want to thank you for your previous contribution in post #46, a very good explanation and very well described.
Glad to be of help. Maybe one additional word to that article.
It is stated that the Open Loop Gain is very high. The Closed loop gain is much much lower.
So the question arises, when we close the feedback loop, do we simply "loose" or "waste" all that extra gain?
No, we do not.
One of the effects that was described in the article is stabilizing the predictability of the closed loop gain, irrespective of fluctuations of open loop gain.
Which is good for our purposes.
But there is yet another benefit: Output impedance.
All that "trimmed" open loop gain, when closing the loop, also serves the purpose to lower the output impedance of the amplifier. So basically, a closed feedback loop system has a much smaller output impedance, because all that "extra gain" is working into effectively lowering the output impedance.
As stated in post #77:
"Closed Loop Output Impedence = Open Loop Output Impedance / Feedback Factor
In other words: The Closed Loop Output Impedance is "Feedback Factor" times lower, than the output impedance without the feedback (open loop)

Last edited by zjjwwa; 17th September 2019 at 07:25 AM.
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Old 17th September 2019, 07:26 AM   #83
MarsBravo is offline MarsBravo  Netherlands
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Quote:
Originally Posted by zjjwwa View Post
...Open/Closed loop is not related to the load...
...presented here:
https://www.electronics-tutorials.ws...-feedback.html
...
In the above referenced paper, there is no explanation about the output impedance of an amplifier. But one can assume that whenever feedback act upon open loop gain to settle the overall or closed loop gain at a lower value for purpose of better control, this also reduces the open loop output impedence to a lower closed loop impedance by the same factor.

There are some assumptions and calculation errors in this paper also.
Nowhere is the open loop frequency response taken into account, so the given (simplified) examples are true for dc operation only. As soon as frequency, time, delay and frequency-depending internal components are involved, the picture shifts somewhat. High gain operational amplifiers exchange this (high gain) feature for low frequency roll off, with a zero in the range of 10 - 100 Hz. A second zero appears often near the open loop unity gain, say some MHz.
Hence the output impedence of an (operational) amplifier is related to internal and external components in a less optimal fashion then preferred.

A mistake of perception is made in the section:
However with the addition of negative feedback the systems gain has only fallen from 34dB to 33.5dB, a reduction of less than 1.5%, which proves that negative feedback gives added stability to a systems gain.
The relative difference (dB's) might differ 1.5% (the ratio of two ratio's), but the absolute and true value is 4.3% (the ratio of the values 49.75 and 47.6).


edit: crossing posts!
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Old 17th September 2019, 07:34 AM   #84
zjjwwa is offline zjjwwa  Poland
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High power amp for low power needs - better result?
Quote:
Originally Posted by MarsBravo View Post
In the above referenced paper, there is no explanation about the output impedance of an amplifier. But one can assume that whenever feedback act upon open loop gain to settle the overall or closed loop gain at a lower value for purpose of better control, this also reduces the open loop output impedence to a lower closed loop impedance by the same factor.

There are some assumptions and calculation errors in this paper also.
Nowhere is the open loop frequency response taken into account, so the given (simplified) examples are true for dc operation only. As soon as frequency, time, delay and frequency-depending internal components are involved, the picture shifts somewhat. High gain operational amplifiers exchange this (high gain) feature for low frequency roll off, with a zero in the range of 10 - 100 Hz. A second zero appears often near the open loop unity gain, say some MHz.
Hence the output impedence of an (operational) amplifier is related to internal and external components in a less optimal fashion then preferred.

A mistake of perception is made in the section:
However with the addition of negative feedback the systems gain has only fallen from 34dB to 33.5dB, a reduction of less than 1.5%, which proves that negative feedback gives added stability to a systems gain.
The relative difference (dB's) might differ 1.5% (the ratio of two ratio's), but the absolute and true value is 4.3% (the ratio of the values 49.75 and 47.6).
Very true. That is the next level of finesse of all of it.
But the "DC approximation" seemed to be a good, easy reading "overall starter". That was my initial intent.
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Old 17th September 2019, 06:43 PM   #85
martinsson is offline martinsson  Sweden
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Some quick questions after having looked trough linked explanation:

What is the difference between local feedback and global feedback, and which one of these are being shown in the linked explanation.

Of these two, which one is more directly dependent of the load characteristics?

I may have missed something here, if so I'm sorry, it is not your job to educate me, that is my task, and I'm grateful for the assistance.
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Old 17th September 2019, 08:39 PM   #86
MarsBravo is offline MarsBravo  Netherlands
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Consider a regular main amplifier with three stages: input, voltage and current stage.
Local feedback is feedback per stage, overall feedback is feedback of the total system consisting of three stages.
One might consider a miller capacitor, connected between collector (output) and base (input) of a bjt (bipolar junction transistor) as a local feedback of this basic stage, whatever is in front or connected as load (!). This miller cap however, is highly voltage-dependent (Vcb) and is capable of 'modulating' the amplification and causing unwanted distortion.
So, feedback is not always a magical trick to get rid of distortion. It can produce distortion also. This miller cap is a very nasty fellow...
And as all practical systems are not exactly operation according to theory, there are numerous other things to take into account, such as frequency response of all used components, to name one. Signals do not propagate at infinite speed through amplifiers, so overall feedback is susceptible for time-lagging. Combined with our nasty fellow and you'll understand that if one desires to build an amplifier, an oscillater can been created instead.
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Old 17th September 2019, 09:09 PM   #87
edbarx is offline edbarx  Malta
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Quote:
Originally Posted by MarsBravo
And as all practical systems are not exactly operation according to theory...
The reason for this mismatch is the fact transistors are extremely complex to represent algebraically accurately. Therefore, to reduce algebraic complexity, transistor representation is approximated. For instance, in low frequency circuit analysis, the base-collector capacitance is usually ignored, although, it still affects circuit operation.
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Old 17th September 2019, 11:40 PM   #88
wg_ski is online now wg_ski  United States
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An amplifier will behave exactly per theory. The trouble is making a complete model for it. That includes at least a planar EM solution fior the board layout, models for the passives that are good up to at least the first resonance, and accurate models for the transistors. Models are typically worth exactly what you pay for them. Built-in spice models are rough approximations only - if actual transistor behavior deviates from the mathematical formulations by enough, no set of model parameters or coefficients will fit it. In many cases you can get “good enough”. Sometimes you can’t. A proper model for the power supply,, reasonable transistor models, and reasonable approximations for the PCB trace inductances and capacitances will model an amp over the audio band. May or may not be good enough for proper stability analysis, or to analyze a very high speed amp with bandwidth into the 100’s of kHz. Won’t model distortion down to the single digit ppm’s, but most amps aren’t that good. It takes a sophisticated model to design one that accurately. I have to laugh, roll my eyes, and shake my head every time I see another design posted that simulates .000003% distortion without doing a full board level simulation and using transistor models found for free on the internet.
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Old 18th September 2019, 01:10 AM   #89
gabdx is offline gabdx  Canada
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the audio signal is too complex and speakers and room too problematic to generalize anything.

There are people who like classD , kevlar drivers, or poly, and there are those who like paper drivers with SET.

I think that the speaker designer goal should be taken into consideration....

A high sensitivity alnico with large 15 inches woofers alnico will benefit from a lower power softer sounding amplifier such as classA SET...

But a very modern big magnet poly cone with a very good XO will benefit from hundred of watts of available power. Especially with heavy magnets , underhung coils, and ventilation and lot of Xmass

So, you either hear the amplifier soft clipping or you hear the speakers scratching your ears. The best system should make everything come together and sound in harmony.
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Old 18th September 2019, 01:51 AM   #90
conanski is offline conanski  Canada
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Quote:
Originally Posted by martinsson View Post
no EQ applied in filtered range.

I have read through this whole thread and while there has been some great discussion I believe there a possibility that the root cause of the problem has been completely missed. You have not articulated in any detail what exactly you don't like about the sound of this system so I'm reading between the lines here, but I think it is entirely possible it just needs some corrective processing to account for non linearities in the design and how the box interacts with the listening environment, and perhaps the system needs time alignment if the sub output lags or sounds sloppy.



I don't see how it would be possible to achieve anything remotely close to a flat response in room even with a relatively simple reflex or sealed sub unless you just get crazy lucky, and I know this is not a standard BP design but those are regularly regarded as sounding slow, so I think it's a big mistake not to take advantage of the processing power at your disposal. You will need something other than a test mic plugged into the DRPA though, you need to measure impulse response as well as frequency response so something like REW will be needed.



As for the question of amplifier sound with regard to low frequencies I think there are differences but they generally only begin to appear at the upper limits, at the 1/10th of rated power level everything is operating in it's linear range and is well under control. As an experiment you could bridge the amp into the sub instead of just using 1 channel, I know that's not the same as bringing in a totally different amp but if that makes a positive difference then maybe you need more power than you think, if it's noticably worse then maybe there is something to your suspicions... or maybe your copy isn't as healthy as it should be because really this should not make any difference one way or the other.

You also couldn't ask for a more well behaved driver.. the klipple test results I have seen on these neo B&C's are excellent and it has one of the most powerful motors available these days, you would be hard pressed to find something better.
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