| fotios |
| quote: | Originally posted by PMA
Depends on YOUR design topology and skills. |
Sorry. It is addressed to me your reply?
Fotios |
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| Bonsai |
Doing away with an output inductor if that is what the designer believes in can be accomplished by
Use high FT output devices
Keep the loop gain low or use none at all
If you believe that a bit (take your pick here at 20k 10dB? 20db? 30db?) of feedback is good, use of an output inductor is recommended |
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| lineup |
| quote: | Originally posted by fotios
Gentlemen, please do a break! All those who write is ENGLISH for me! You put me in doubt. What recommend finally all of you? To use or not inductor in the output of my amp? It is obvious that your views differ. It is also clear that exists - for some time, as well as in other threads too - a dispute between some members who have a higher knowledge level from us the common mortal. Thus, with the slightest opportunity, they exploit a simple query from a member, to start a new battle. This puts me into suspicions that something else exists in reality behind all this. From that I understand, "caesar148" who started this thread, he is confused at the moment. Or he is also playing the game from the start?
With humility
Fotios |
Goood Summmmary, fotios ;) ;)
There are two Professors in Electronics that I regard very highly
when comes to amplifier facts, figures and designs:
1. Professor Matti Otala, Finland, see my topic here:
Amplifiers >Solid State >Matti Otala - An Amplifier Milestone. Dead or Alive
http://www.diyaudio.com/forums/show...threadid=125541
2. W. M. Leach, Jr, USA. Designer of Low Tim Amp & Leach SuperAmp etc. etc.
By the way,
his Diy audio website is well worth a visit.
In my next post I will you show his recommendation of Output Inductor
and how you easily can make your own.
It is for his Leach Amp. Known as the Low Tim amplifier
* TIM ... a sort of intermodulation distortion
Regards
Lineup - not a professor, yet, but from Sweden!!!! :cool:
-----------------------
| quote: | .
Curriculum Vitae, CV
W. Marshall Leach, Jr., Professor
Georgia Institute of Technology
School of Electrical & Computer Engineering
Atlanta, Georgia
Education
* B.S. in Electrical Engineering, 1962, University of South Carolina, Columbia
* M.S. in Electrical Engineering, 1964, University of South Carolina, Columbia
* Ph.D. in Electrical Engineering, 1972, Georgia Institute of Technology, Atlanta
etc.
etc.
etc.
Current Fields of Interest
Electromagnetic theory, audio, electroacoustics, electronic circuit design, instrumentation, low noise electronic design, and electromagnetic compatibility.
etc.
etc.
etc.
Honors and Awards
* Outstanding Teacher Award in Electrical Engineering, Eta Kappa Nu, 1973.
* Outstanding Teacher Award, Georgia Institute of Technology, 1975.
* Fellow Award, Audio Engineering Society, 1980.
* Outstanding Teacher Award in Electrical Engineering, Eta Kappa Nu, 1982.
* Engineer of the Year, Greater Atlanta Chapter, Georgia Society of Professional Engineers, 1983.
* Outstanding Senior Professor Award in Electrical Engineering, Eta Kappa Nu, 2002.
* Fellow Award, The Institute of Electrical and Electronics Engineers, 2005.
* Outstanding IEEE Student Branch Adviser Award, 2007.
* Richard M. Bass Outstanding Teacher Award in Electrical Engineering, Eta Kappa Nu, 2007.
etc.
etc.
etc.
http://users.ece.gatech.edu/~mleach/bio.html
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| lineup |
hi.
This is some valuable info in my opinion.
For all those building thier own diy amplifiers.
Note:
The following image + quote is originally
from Professor in Electronics W. M. Leach, Jr
DIY audio website & with some other stuff:
http://users.ece.gatech.edu/~mleach/
Regars Lineup
| quote: | R49/L1 is a component that needs some assembly.
It's actually the
- R49 resistor with some
- 22 gauge solid wire wrapped around it 11 times.
The easiest way to do this is to take your length of wire, strip one end, and solder it to one side of the resistor.
Now wrap the wire tightly around the resistor 11 times (it will fit!) and mark the wire where it meets the other end of the resistor.
Unwind it, cut the wire just past that mark, and strip it.
Now re-wind it and solder it into place.
You may notice that D1, D2, D3, D4, Q18, Q19, Q20, Q21, R50, and C25 were not mentioned in my ordered list. That is because all of these components are not mounted on the PCB and their placement will be covered in the Assembly section. |
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| lineup |
This is from
The Leach Superamp.
The second published amplifier by Professor W. M. Leach.
Notice the wire wounded output inductor.
It is is red colour in this assembled board.
Websource page:
http://users.ece.gatech.edu/~mleach/superamp/
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| sakis |
in a situation like that ( regarding this long post with so many opinions and theory ) we say
"χασαμε την μπαλα"
str8 translation is "we lost the ball" |
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| Eva |
| quote: | Originally posted by fotios
Gentlemen, please do a break! All those who write is ENGLISH for me! You put me in doubt. What recommend finally all of you? To use or not inductor in the output of my amp? It is obvious that your views differ. It is also clear that exists - for some time, as well as in other threads too - a dispute between some members who have a higher knowledge level from us the common mortal. Thus, with the slightest opportunity, they exploit a simple query from a member, to start a new battle. This puts me into suspicions that something else exists in reality behind all this. From that I understand, "caesar148" who started this thread, he is confused at the moment. Or he is also playing the game from the start?
With humility
Fotios |
Try to analyse the facts by yourself and elaborate your own strong points of view. Reading some theory on amplifier design may help (feedback systems, open and closed loop gains, Nyquist stability criteria, etc...) You can improve your electronics knowledge in the process. Objectively there is nothing forcing you to be into a lower knowledge class.
Anyway, I think that there is enough evidence and consensus to support the fact that only amplifiers with properly sized RC and RL output networks can be stable with *any* load. |
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| traderbam |
| quote: | Originally posted by caesar148
Thanks to all who replied. Okay, I am beginning to understand better now. The inductor is to prevent amplifier oscillation when driving "difficult" loads that have high capacitance.
I would conclude that amplifiers with small or no feedback would be less susceptable to such speaker load and could do away with it.
Anyone know what a recommended value of inductance it should be? |
After all that...:)...I hope you are still with us, Caesar148.
1) There is no right answer...it depends on the amp. Without getting really complicated, all you can do is take reasonable precautions.
As a general solution, I would recomend an inductor between 1uH and 2uH in parallel with a 10 ohm resistor.
If you would like to get really complicated about it then please ask...probably best to send me a private message, though. ;)
2) Me, Christer, John Curl, Bob Cordell and others are talking about very subtle difference in sound quality, with or without an inductor. Don't worry about it unless you are striving to make a really high-end amplifier.
3) Low or no global feedback designs tend to be more load tolerant. Not always the case as some have pointed out. Indeed, there is no such thing as an amplifer that doesn't use some species of feedback. There is no real harm in using the inductor/resistor anyhow, so you might as well use it in all cases.
4) Try not to get to confused by all this Zobel chatter. What is done in an audio amp is only a crude approximation to Zobel's theories. The term "Zobel network" is not being used properly - it's just a convenient jargon. You can't make a proper network because the speaker and speaker cables have a variable impedance.
5) The basic ideas are fairly simple. At low frequencies you want the amp to drive the speaker, to "see" the speaker impedance. Above the audio band you want the amp to "see" a constant, resistive load instead of the speaker and speaker cable. This is because the impedance of the speaker and cable can cause big phase shifts at frequencies in the MHz region. Some amps feedback loops are still active at these high frequencies and they can become unstable.
When designing, you choose the high frequency resistance you want the amp to see. 10-ohms is typical (in the case of the NS circuit that Lineup shows, it is 2.7 ohms). You put this resistance in series with a cap, typically 220nF, from the amp out to ground. To isolate the speaker/cable at high f you put a series inductor of 1uH or 2uH between the amp output and the speaker/cable. The inductor has a resistor in parallel to damp out potential resonances with any capacitance in the speaker. The f where the impedance changes from speaker to resistor is typically in the 100kHz or so region. |
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| Edmond Stuart |
| quote: | Originally posted by G.Kleinschmidt
I disagree. I've got some Neville Thiele articles where he describes the component values for a couple of load isolations/HF terminating networks designed to present a constant (with frequency) load impedance to the amplifier for both 4 and 8 ohms. One is just as the NS design.....
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Hi Glen,
I disagree with your disagreement. :)
Frankly speaking, I don't care who designed that NS Zobel thing, Thiele or even God. That network is ill designed, because at HF, the impedance drops from 8 to ~2.4 Ohm. Maybe others think it's a good idea, but I don't like at all. Also have a look at the step response.
| quote: |
The only thing the output network has to do IMHO is to isolate any capacitive load to a sufficient degree with a well dampened inductor and provide a low impedance termination at HF. Works well for me and the squarewave response is infinitely better too! :D
Cheers,
Glen |
Without consensus about the duties of a Zobel network, I'm afraid that a further discussion becomes rather difficult.
So let me point out what I think a Zobel network should and not should do.
1. Protecting the amp from HF ingress.
2. Protecting the amp from abuse, like a capacitive load or no load at all (i.e. the speaker outlet simply left open).
3. A constant and resistive impedance (seen by the amp) if the network is properly terminated, that is, also by a constant and resistive impedance, equal to the nominal impedance.
4. A minimal impact on frequency and step response, as well as distortion and damping factor.
5. I should NOT correct a deviation from the nominal speaker impedance, in particular at HF.
The latter belongs to the responsibility of loud speaker manufacturer. Happily, some decent manufacturers do this already. So putting another 'HF terminator' inside the amp lowers the impedance unnecessary and should be avoided.
Coming back to the NS Zobel vs my version (schematics below), I simulated them and it appeared that the onset of frequency roll-off of my version happens at a two times higher frequency. Also the step response looks far better (green curve), i.e. faster and without that ugly kink.
One more thing regarding RFI: Analoguous to Bob Cordell's proposal to put a cap (plus resistor) right at the input terminals, I think it's equally beneficial to put the Zobel cap right at the output terminals.
Cheers,
Edmond. |
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| Bonsai |
I think traderbam has summarized the spirit of the thread, including the for and against, in an excellent way.
Moderators, I propose you close this thread now, since it covers both the for and against in a concise manner. Lets try to avoid the situation where we slide into a for and against slinging match |
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| Edmond Stuart |
| I propose to NOT close this thread! |
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| sakis |
| 95% of comercial amplifiers include inductors ....there is got to be reason for it ..... |
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| G.Kleinschmidt |
| quote: | Originally posted by Edmond Stuart
Without consensus about the duties of a Zobel network, I'm afraid that a further discussion becomes rather difficult.
So let me point out what I think a Zobel network should and not should do.
1. Protecting the amp from HF ingress.
2. Protecting the amp from abuse, like a capacitive load or no load at all (i.e. the speaker outlet simply left open).
3. A constant and resistive impedance (seen by the amp) if the network is properly terminated, that is, also by a constant and resistive impedance, equal to the nominal impedance.
4. A minimal impact on frequency and step response, as well as distortion and damping factor.
5. I should NOT correct a deviation from the nominal speaker impedance, in particular at HF.
|
Looking at these 5 points I can see that you obviously don't really disagree with my disagreement that much.
Points 3 and 4 are in conflict. I think it is a waste of time attempting to design the output network to provide an "ideal" resistive "impedance" to the amplifier because the huge deviations in the impedance of most loudspeakers. I use Nevile Thiele's (and some varaitions of) networks, but with modified component values for this reason. What do you think Nevile's 14uH inductor does for damping factor and transient response into capacitive loads?
BTW, I haven't forgotten about that Philips speaker article and if I come across my Nevile Thiele articles in the meanwhile also, I'll scan them too.
Cheers,
Glen |
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| Edmond Stuart |
Hi Glen,
And what about that NS Zoebel thing? Do you still believe it's correct?
As for point 3, I know that the amp output will never see a pure resistive and constant load. I'm only saying that IF the output of the Zobel network is terminated with the nominal load, then the input should reveal the same impedance. Just see it as a design rule and not as a real situation.
Regarding Nevile's 14uH inductor, I think even I can hear such huge coil. I wouldn't recommend it.
Thx in advance for the article.
Cheers,
Edmond. |
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| megajocke |
The cap directly on the output should be good for RF filtering but I see one problem: When the output is shorted at the other end of a short cable. The amplifier will see an impedance peak, see attachment. The plot shows the impedance as seen from the source.
(Yes that is LTSpice running in wine) |
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| Edmond Stuart |
| quote: | Originally posted by megajocke
The cap directly on the output should be good for RF filtering but I see one problem: When the output is shorted at the other end of a short cable. The amplifier will see an impedance peak, see attachment. The plot shows the impedance as seen from the source.
(Yes that is LTSpice running in wine) |
Hi MJ,
In that case we need an additional RC at the input side of the Zobel network. However, have we to take into account all possible forms of abuse? The more so as such short circuit condition should have to disconnect the amp from the load.
Cheers,
Edmond. |
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| traderbam |
| quote: | Originally posted by Bonsai
I think traderbam has summarized the spirit of the thread, including the for and against, in an excellent way. |
Domo arigato. |
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| Edmond Stuart |
And here is a relative comparison of the capabilities to block RFI ingress of the NS (red) vs my version (green) of that Zobel thing (notice: these two versions use the same inductance).
Clearly, above a few MHz, that NS Zobel network does nothing. So I insist that most people are blindly and mindlessly copying existing schematics and haven't the faintest idea what they are doing. |
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| Christer |
| Edmond, how many amps still have 1 mOhm output Z at those frequencies? I think the usual explanation is that the output Z will be much higher than the Z of the Zobel at HF. Still, you have a point, I think. |
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| Edmond Stuart |
| quote: | Originally posted by Christer
Edmond, how many amps still have 1 mOhm output Z at those frequencies? I think the usual explanation is that the output Z will be much higher than the Z of the Zobel at HF. Still, you have a point, I think. |
Hi Christer,
I'm afraid you missed my point point. I was talking about the relative performance of the two circuits. It doesn't matter if Zo of the output stage is 1, 10 or even 100 mOhm. Also, I was even looking at the wrong node (see new picture: out1 and out2). Even that doesn't matter. Anyhow, according my new sim, the NS version appears to be still inferior.
Cheers,
Edmond. |
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| john curl |
| I would like to say that while these efforts here are laudable, you are barking up the wrong tree. First, even I would not criticize 0.7uH. Maybe, it would be useful with many feedback circuits, but it is the 2uH or MORE that I am concerned about. Just TRY it in an open A-B test with good audio playback equipment, and a power amp that doesn't really need the coil to keep from oscillating. Just listen to the coil with perhaps a 10 ohm resistor in parallel. Just listen, and quit overanalyzing what something should sound like or not in a circuit. This is the difference between the instructors and the sophomores. |
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| Edmond Stuart |
John,
As so many times, you don't listen to others. Your comment is OT. We were talking about ill designed Zobel networks (i.e. a wrong topology), and NOT whether you can hear a 0.7uH vs a 2uH coil. |
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| john curl |
| Yes, no coil is better than any coil. :censored: :censored: |
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| caesar148 |
Now that the smoke has cleared "cough! cough!", I would like to thank again all those who contributed to this highly spirited discussion. Yes, traderbam did a great job of summarising the answers to the fundemental question about the output inductor.
I did a review of several amplifier circuits and I found that the following amplifers do not have any inductor circuit: Parasound HCA-2200II, McCormack DNA-2, Nakamichi PA-7, Adcom GFA-545, and Krell KSA-100 Mk II. In contrast I found the following Japanese and British amplfiers were using inductors: Sony TA-N77ES, Onkyo P-508, Audio Lab 8000A, Denon POA-6600. All of the amplifiers BTW had the RC series network for RF filtering. This is the Zobel network I believe. I can only conclude that one group of designers must be doing something right that do not require the output inductor.
This begs another question: Do you put the RC series circuit before or after the inductor-resistor circuit? I have seen it done both ways. |
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| snoopy |
| quote: | Originally posted by john curl
I would like to say that while these efforts here are laudable, you are barking up the wrong tree. First, even I would not criticize 0.7uH. Maybe, it would be useful with many feedback circuits, but it is the 2uH or MORE that I am concerned about. Just TRY it in an open A-B test with good audio playback equipment, and a power amp that doesn't really need the coil to keep from oscillating. Just listen to the coil with perhaps a 10 ohm resistor in parallel. Just listen, and quit overanalyzing what something should sound like or not in a circuit. This is the difference between the instructors and the sophomores. |
And while you are at it you should toss out your tweeters as well ;) That voice coil inductance is really having a profound effect on the sound and there is not a lot you can do about it ;)
Audax HD12x9D25 Soft Dome Tweeter
Lvc = 75uH :eek: |
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| Edmond Stuart |
| quote: | Originally posted by caesar148
...............
This begs another question: Do you put the RC series circuit before or after the inductor-resistor circuit? I have seen it done both ways. |
Read my posts and you will know the answer. |
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| john curl |
| You must listen first and foremost. If you will not listen, then why bother arguing whether a coil is audible or not? I personally went through the computation and measurement of what most of you are now doing, over 30 years ago. I, too, found no measurable problem, but later, I found an audible problem. How many times do we have to go over this? |
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| Steve Eddy |
| quote: | Originally posted by john curl
You must listen first and foremost. If you will not listen, then why bother arguing whether a coil is audible or not? I personally went through the computation and measurement of what most of you are now doing, over 30 years ago. I, too, found no measurable problem, but later, I found an audible problem. How many times do we have to go over this? |
I guess until you or someone else demonstrates conclusively that there in fact is an actual audible difference, instead of simply asserting your subjective perceptions as objective reality.
se |
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| john curl |
| How can we 'prove' that we have heard a difference? And, who cares? By the way, Steve Eddy does or has sold an expensive connecting cable that he claims sounds good to him, but offers no measurements to 'prove' anything about it. It sounds to me like the pot calling the kettle, 'black'. (an old proverb) |
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| snoopy |
| quote: | Originally posted by john curl
You must listen first and foremost. If you will not listen, then why bother arguing whether a coil is audible or not? I personally went through the computation and measurement of what most of you are now doing, over 30 years ago. I, too, found no measurable problem, but later, I found an audible problem. How many times do we have to go over this? |
John, consider these scenarios.
You have a SS audio amplifier with a load stabilizing network consisting of a parallel combination of some L and R. Lets say 2uH in parallel with 10 ohms etc. You then decide to use that amp in an active speaker system and use it to drive a tweeter directly which has 75uH of voice coil inductance. Do you mean to tell me that you are really going to notice a lot of difference with and without the load stabilizing network installed ?? If you are please explain why ??
Alternatively consider your typically valve amplifier with large output transformer and most likely a large amount of leakage inductance and high output impedance compared to its SS counterpart !!! From what you are saying people who use these amps are doing themselves a disservice and compromising their listening experience because of this large amount of series inductance :( |
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| lumanauw |
| quote: | | This begs another question: Do you put the RC series circuit before or after the inductor-resistor circuit? I have seen it done both ways. |
I think they have different purpose. Before L, the RC is for loading the amp at HF, so the amp is stable without speaker load attached.
After L, the purpose is to equalize the speaker load to look as close as resistive. |
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| Edmond Stuart |
| quote: | Originally posted by snoopy
And while you are at it you should toss out your tweeters as well ;) That voice coil inductance is really having a profound effect on the sound and there is not a lot you can do about it ;)
Audax HD12x9D25 Soft Dome Tweeter
Lvc = 75uH :eek: |
My dear Snoopy,
Apparently, you have missed John's explanation. It's the position of the coil that matters. Inside the speaker box, no problem at all, but inside the amplifier's cabinet, it shows its true nature. Why? Simply because inside or near the tweeters magnet, the electrons are sonically aligned and the greater the distance to that magnet the greater the disorder of the electrons in the coil. You get it? (I don't)
Cheers,
Edmond. |
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| snoopy |
| quote: | Originally posted by Edmond Stuart
My dear Snoopy,
Apparently, you have missed John's explanation. It's the position of the coil that matters. Inside the speaker box, no problem at all, but inside the amplifier's cabinet, it shows its true nature. Why? Simply because inside or near the tweeters magnet, the electrons are sonically aligned and the greater the distance to that magnet the greater the disorder of the electrons in the coil. You get it? (I don't)
Cheers,
Edmond. |
Looks like another one for James Randy to investigate ;) |
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| john curl |
What I am trying to say is that coils have been shown to not be necessary with many audio power amplifiers that have been designed to not need the coil. Please remember that the loudspeaker tweeter, for example is a MOVING coil and the amplifier coil is a FIXED coil. Does this make the difference? I don't really know, but it IS a difference.
I might say that what first concerned me was the doubling of the coil inductance with bridged operation. For many years, before I worked for Parasound, all my big power amps were bridged designs. This doubled the coil inductance seen by the loudspeaker, and this worried me. Still, for many years, I used 1-2uH output coils, faithfully, but then a friend and audiophile showed me that the coils could be audible, and since then I have tried to eliminate them, if possible.
Parasound engineers in Taiwan agreed with this approach, and we just lose some maximum slew rate, reducing it down to something like 100V/us instead of 300V/us or so. Big deal.
I might say that along with Julian Vereker of Naim, Nelson Pass started removing the output coil about 30 years ago, at least I think so; and Charles Hansen would not add an output coil for any reason. Why don't you look at people who have A ratings in power amps in 'Stereophile' over the last few years, and realize that we independently came up with 'losing' the coil, if possible, to get the best sound. |
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| Steve Eddy |
| quote: | Originally posted by john curl
How can we 'prove' that we have heard a difference? |
By controlling for the ambiguity of human beings being known to subjectively perceive differences even when there are no actual audible differences by means other than ego and vanity.
Anyone who makes claims of actual audibility and who expects to be taken seriously should care. If you don't care, then don't expect to be taken seriously or to have your claims accepted without question.
| quote: | | By the way, Steve Eddy does or has sold an expensive connecting cable that he claims sounds good to him, but offers no measurements to 'prove' anything about it. It sounds to me like the pot calling the kettle, 'black'. (an old proverb) |
Not at all. The problem is your continued inability to grasp some rather simple concepts.
Yes, I sell cables which I have claimed sound good to me. But the distinction here is that is all that I claim.
Do they sound good to me because of any actual audible differences? Or do they sound better to me for purely psychological reasons?
I don't know. I've never done any listening tests which control for the ambiguity of human beings being known to subjectively perceive differences even when there are no actual audible differences.
And not knowing, I don't make any claims to the contrary. And further, I'm fully prepared to accept the possibility that what differences I perceive may well be purely psychological. That possibility simply doesn't threaten my ego the way it seems to threaten the egos of others.
se |
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| Steve Eddy |
| quote: | Originally posted by john curl
I might say that along with Julian Vereker of Naim, Nelson Pass started removing the output coil about 30 years ago, at least I think so; and Charles Hansen would not add an output coil for any reason. Why don't you look at people who have A ratings in power amps in 'Stereophile' over the last few years, and realize that we independently came up with 'losing' the coil, if possible, to get the best sound. |
Which at the end of the day proves absolutely nothing with regard to actual audibility.
se |
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| Edmond Stuart |
| quote: | Originally posted by john curl
What I am trying to say is that coils have been shown to not be necessary with many audio power amplifiers that have been designed to not need the coil. Please remember that the loudspeaker tweeter, for example is a MOVING coil and the amplifier coil is a FIXED coil. Does this make the difference? I don't really know, but it IS a difference.
......................
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And the coils of the cross-over filter? Are they MOVING too, so they can't do any harm? |
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| snoopy |
| quote: | Originally posted by john curl
[B]What I am trying to say is that coils have been shown to not be necessary with many audio power amplifiers that have been designed to not need the coil. Please remember that the loudspeaker tweeter, for example is a MOVING coil and the amplifier coil is a FIXED coil. Does this make the difference? I don't really know, but it IS a difference.
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John for a moving coil loudspeaker the inductance stays fairly constant and only deviates around a nominal value due to the voice coil moving in and out of the gap.
The point I am trying to make here is that there are other dominant sources of series inductance so removing the load stabilizing network for the sake of purity maybe a bit short sited. |
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| Edmond Stuart |
Hi Snoopy,
You better give up. You will never get a straight answer when putting tough questions.
Cheers,
Edmond. |
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| jcx |
It could help to know what systems most clearly show the difference with output coils - which specific speakers are most sensitive, are any compeltely insensitive in listening tests - again which models?
program material?
"character" of the the difference? |
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| megajocke |
Edmond,
Looks like Crown uses the kind of output network you recommend on some of their amps. The Microtech 600&1200 has 47nF after 2.7R//1.8uH. Microtech 1000 has 47nF after 2.7R//0.5uH. Macrotech 2400: L = 1.3uH, R and C same.
Most models seem to have 10n caps across both polarity output banks for the output side. On the ground side there are zobels over both polarity banks.
Macrotech 5000VZ has 2.5uH//2.35R and 22nF on output. It also has 5.6R + 47n zobels on both sides of output network. It also has 22nF + 12R zobels over all four banks of output devices.
Crown seems to use a parallell RLC circuit between predrivers and drivers on a lot of their amps instead of just base stopper resistors. Any idea why?
http://www.crownaudio.com/gen_htm/legacy/legacamp.htm |
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| Bob Cordell |
The question has come up a couple of times where to put the Zobel network, before or after the coil. Also, someone mentioned the use of a shunt capacitor at the speaker terminals.
First, I believe Bryston uses a shunt capacitor across the loudspeaker terminals. This has some interesting possibilities, but it does seriously mis-terminate a transmission line and possibly invites a resonance.
I like to put a Zobel on both sides of the coil. I think Self frowned on this, and could not understanbd why anyone would do this.
Here is the reasoning. The first Zobel is placed right at the output stage and assures that the output stage will be loaded resistively at high frequencies no matter what, up to quite high frequencies. This Zobel is the one that works to prevent HF oscillations of an un-loaded emitter follower. With high-ft output transistors, it can be important that it be close by, especially if there is inductance in the output lead before it gets to the coil and second zobel.
The second zobel is also a series-RC and it is right at the loudspeaker terminals. Its most important job is the high-frequency termination of the speaker cable transmission line. It wants to have that resistor in it so that the termination looks resistive to very high frequencies, and will damp any attempts at resonance.
Notice that all three resistors in the complete output network work together to damp resonances.
With the dual-zobel approach, I have found that relatively light zobel networks can be used. This also works to spread out any power dissipation that the Zobel might have to handle in 20 kHz full-power testing. The bottom line is two more components than traditional, but they can be smaller in size.
Depending on the design, dual shunt zobels as light as 0.01 uF and 33 ohms might be fine. RF ingress will likely be a bit lower if the resistor in the second zobel is made smaller, however, maybe on the order of 10 ohms. For the coil, I like to keep it between 0.5 u and 1 uH, with a shunt resistor between 1 ohm and 5 ohms. I tend to prefer smaller shunt resistances across the coil.
There are many combinations of values that will work, and I am honestly not sure there is a right and wrong combination of values as long as the complete output network does its intended jobs and does minimal damage to the pulse response.
Cheers,
Bob |
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| Eva |
I have a question for all those people intentionally skipping the RC and/or RL output networks...
How else do you achieve predictable, repeatable, load-independent gain/phase characteristics in the critical RF unity-gain crossing region?
Do you perform detailed stability tests? I doubt... |
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| sam9 |
For commercial designs, I suspect a motiver is the designer is trying to be extra cautious with regard to what might end up attached to his unit. Both zobel and an output coil may be intended a a sort of "firewall" between the amplifier and infinite possible permutations of cables and loudspeaker designs.
A potential benefit of DIY is that you know, presumably, exactly what will be connected to your amplifier and can use either, both or neither a zobel or output coil as appropriate. |
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| lumanauw |
Hi, EVA,
Many of gainclone builder intentionally skip RC (the chip's datasheet have them), they say without RC the gainclone sounds better. |
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| Wavebourn |
| quote: | Originally posted by sam9
For commercial designs, I suspect a motiver is the designer is trying to be extra cautious with regard to what might end up attached to his unit. Both zobel and an output coil may be intended a a sort of "firewall" between the amplifier and infinite possible permutations of cables and loudspeaker designs.
A potential benefit of DIY is that you know, presumably, exactly what will be connected to your amplifier and can use either, both or neither a zobel or output coil as appropriate. |
A potential benefit of professional designer is presence of deep knowledge and measurement tools, so all infinite possible permutations of capacitive, resistive, and inductive loads may be tested.
Why plain standard production always have lot of odd things?
Because it is safer to copy datasheets and fashion designs as is in terms of job security... |
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| KSTR |
| quote: | Originally posted by megajocke
Crown seems to use a parallell RLC circuit between predrivers and drivers on a lot of their amps instead of just base stopper resistors. Any idea why? |
Nice to finally see an industry schematic with this. My take is (I've been fiddling with that RLC "base stoppers", too), the RC, when choosen optimally, gives an almost purely resistive output impedance of the driver EF (when 2*pi*f_beta*R*C = 1, f_beta being the -3dB corner of beta vs. freq), and the inductor shunts this RC for audio freqs to have low drive impedance (low distortion) there. So they could get rid of base stoppers for the main outputs.
The RLC values from the MA600 which I looked at now nicely fit these assumptions, the LR corner freq is 25kHz and the f_beta would be at about 500kHz which seems to be quite reasonable.
- Klaus |
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| Wavebourn |
| quote: | Originally posted by Bob Cordell
This Zobel is the one that works to prevent HF oscillations of an un-loaded emitter follower. With high-ft output transistors, it can be important that it be close by, especially if there is inductance in the output lead before it gets to the coil and second zobel.
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Do you mean LC in base of that emitter follower that can lead to oscillations due to base to emitter capacitance plus capacitance from emitter to ground? Otherwise an emitter follower itself can't oscillate, additional stages and feedback are needed.| quote: |
The second zobel is also a series-RC and it is right at the loudspeaker terminals. Its most important job is the high-frequency termination of the speaker cable transmission line. It wants to have that resistor in it so that the termination looks resistive to very high frequencies, and will damp any attempts at resonance.
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Do you mean termination of 300 Ohm line for frequencies above 3 MHz? I assume an ordinary speaker cable of 20 meter (approximately 60 feet) length. |
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| snoopy |
| quote: | Originally posted by Bob Cordell
The question has come up a couple of times where to put the Zobel network, before or after the coil. Also, someone mentioned the use of a shunt capacitor at the speaker terminals.
The second zobel is also a series-RC and it is right at the loudspeaker terminals. Its most important job is the high-frequency termination of the speaker cable transmission line. It wants to have that resistor in it so that the termination looks resistive to very high frequencies, and will damp any attempts at resonance.
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I can understand your reasoning but from a practical point of view this assumes that the characteristic impedance of the speaker cable is constant and known in advance. It also assumes that the speaker cable is consistent throughout it's length. I doubt if speaker cable manufacturers have any quality control measures regarding the characteristic impedance so maybe this extra series RC network at the speaker terminals might be a bit of a mismatch.
However this begs the question should we be designing speaker cables with a certain characteristic impedance and terminating them at the speaker end with a series R-C circuit ??
Has anyone tried to used some heavy duty 50 ohm coax cable with their speakers ?? Results anyone ?? |
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| Wavebourn |
| quote: | Originally posted by snoopy
Has anyone tried to used some heavy duty 50 ohm coax cable with their speakers ?? Results anyone ?? |
With 50 Ohm speakers on frequencies of several MHz? I did! :D
Also, I had a problem with 300 kilometers of 1200 Ohm line (air, on poles). It was a railway special service line for high impedance phones with tonal coded calls. Some phone did not answer, and I've found that on that station they had a notch on 2700 Hz, exactly one frequency of the sequence assigned to the station. Investigation revealed that there was a 150 Ohm underground cable in that line, but on a different railway station.
But there were 300 kilometers of 1200 Ohm line, enough for reflections in audio band! |
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| Christer |
| quote: | Originally posted by lumanauw
Many of gainclone builder intentionally skip RC (the chip's datasheet have them), they say without RC the gainclone sounds better. |
And even gainclone kits are sold without Zobels. I have it from a source I consider reliable that quite a number of those have ended up in repair shops. |
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| Wavebourn |
| Most of transistor amps sound better with a resistor between output and a speaker. Who said that an electrical damping is more significant than linearity of output resistance? |
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| Eva |
Oscillation results in compression and "smooth" distortion of the output waveform. In most cases, oscillation only arises for certain ranges of output voltage and current in an asymmetrical way. This is similar to tube distortion and some people actually enjoys it.
Checking that an amplifier is unconditionally stable is not a trivial task. The RLC base stoppers are really the kind of design that I like. In fact, there are a lot of things happening well above 20Khz in audio amplifiers, particularly in class AB circuits, but there are few designers taking care about that.
I think oscillation is frequent among DIY and audiophile-style amplifiers. Circuits put together "by listening" often exhibit terrible waveform performance.
People builds gainclones without RL or even RC networks and then they complain about a too high noise floor or about heatsinks reaching 50ΊC without any signal... |
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| Eva |
| quote: | Originally posted by Wavebourn
Most of transistor amps sound better with a resistor between output and a speaker. Who said that an electrical damping is more significant than linearity of output resistance? |
Amplifiers don't sound.
Most SPEAKERS exhibit great deviations in their frequency response, like -6dB :bigeyes: , depending on the impedance from which they are driven.
In some cases, a high output impedance results in attenuation of frequencies that the subject does not like or in an effective boost of frequencies that the subject does like.
The subject gets more complex when drive impedance is not uniform across the audio band, or even worse, when output impedance is dependent on signal level (both things happen in almost every piece of tube gear).
A high drive impedance to a 2-way speaker often results in effective boost applied to three frequency ranges: The resonance of the bass driver, the usual 2Khz crossover region and the high trebble range above 10Khz (where the tweeter becomes inductive). Actually, the rest of the frequencies are the ones becoming attenuated but the result is the same. |
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| Christer |
| quote: | Originally posted by KSTR
Nice to finally see an industry schematic with this. My take is (I've been fiddling with that RLC "base stoppers", too), the RC, when choosen optimally, gives an almost purely resistive output impedance of the driver EF (when 2*pi*f_beta*R*C = 1, f_beta being the -3dB corner of beta vs. freq), and the inductor shunts this RC for audio freqs to have low drive impedance (low distortion) there. So they could get rid of base stoppers for the main outputs.
The RLC values from the MA600 which I looked at now nicely fit these assumptions, the LR corner freq is 25kHz and the f_beta would be at about 500kHz which seems to be quite reasonable.
- Klaus |
Do you mean a series combination of R and C and then an L in parallel with this? |
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| Bob Cordell |
| quote: | Originally posted by sam9
For commercial designs, I suspect a motiver is the designer is trying to be extra cautious with regard to what might end up attached to his unit. Both zobel and an output coil may be intended a a sort of "firewall" between the amplifier and infinite possible permutations of cables and loudspeaker designs.
A potential benefit of DIY is that you know, presumably, exactly what will be connected to your amplifier and can use either, both or neither a zobel or output coil as appropriate. |
This is a very good point. Indeed, in the active loudspeakers I built I did not use inductors because the loudspeaker drivers were less than 18 inches from the amplifiers that were built into the cabinet. However, I DID use an RC Zobel because that is prudent in any case, since the impedance seen looking into the driver can go very high at high frequencies due to its inductive nature.
Cheers,
Bob |
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| KSTR |
| quote: | Originally posted by Christer
Do you mean a series combination of R and C and then an L in parallel with this? |
Yes, see attached circuit detail from the MA600/1200. Note that the two RC's on the "Low Side" on the right are output stage snubbers, as this amp is a bridge design where the right output is grounded and the rails and center tap are floating, shifted with the output voltage. Also we see a combined output L//R with an output snubber/zobel, for the high side, with the 2.7R as the shared element.
As for the chip amps, I've seen output RC's there which are quite ineffective because of poor layout/wiring. With quasi comp outputs, the best place to put the RC is directly from the output pin to the neg supply pin.
- Klaus |
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| Bob Cordell |
| quote: | Originally posted by Wavebourn
Do you mean LC in base of that emitter follower that can lead to oscillations due to base to emitter capacitance plus capacitance from emitter to ground? Otherwise an emitter follower itself can't oscillate, additional stages and feedback are needed.
Do you mean termination of 300 Ohm line for frequencies above 3 MHz? I assume an ordinary speaker cable of 20 meter (approximately 60 feet) length. |
Emitter followers configured in output stages like Darlington or Triple can in some situations get into oscillation in a no-load condition even if there is no discrete LC in the base. I've even seen a Triple get unstable in a SPICE simulation when unloaded where there were no real world parasitic inductances involved. For this reason, it is prudent that an R-C Zobel shunt network to ground is there to make sure that the EF output stage sees at least some resistive load at high frequencies.
When I refer to terminating the speaker line at high frequencies I am not referring to terminating it in its characteristic impedance. I'm mainly saying that it would like to be terminated in a lossy resistance at high frequencies, something typically in the 2-20 ohms region. This, of course, does not terminate the typical speaker cable in its characteristic impedance, which often lies between 50 and 300 ohms.
Cheers,
Bob |
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| Bob Cordell |
| quote: | Originally posted by snoopy
I can understand your reasoning but from a practical point of view this assumes that the characteristic impedance of the speaker cable is constant and known in advance. It also assumes that the speaker cable is consistent throughout it's length. I doubt if speaker cable manufacturers have any quality control measures regarding the characteristic impedance so maybe this extra series RC network at the speaker terminals might be a bit of a mismatch.
However this begs the question should we be designing speaker cables with a certain characteristic impedance and terminating them at the speaker end with a series R-C circuit ??
Has anyone tried to used some heavy duty 50 ohm coax cable with their speakers ?? Results anyone ?? |
Hi snoopy,
As I mentioned above, it was not my intention to suggest that the speaker cable be terminated in its characteristic impedance. I apologize for any confusion I created. A lossy resistive termination at high frequencies on at least one end is what is desirable, even if that termination resistance is quite a bit lower than the characteristic impedance of the line. A pure open or a pure short at either end of a transmission line is where there is sometimes the potential for reflection and resonance troubles. A lossy termination reduces reflections and damps potential resonances.
Cheers,
Bob |
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| Bob Cordell |
| quote: | Originally posted by Wavebourn
Most of transistor amps sound better with a resistor between output and a speaker. Who said that an electrical damping is more significant than linearity of output resistance? |
Hi wavebourn,
This is a very astute observation, although I am not sure why it is the case. It may be just a subjective preference for the frequency response deviations caused by the series resistance working against the highly variable impedance of the loudspeaker.
Looking at it another way, I think that the lower damping factor of tube amplifiers is partly the reason for the sound that they have, which some prefer. If I want a solid state amplifier to sound more like a tube amplifier, I put some series resistance in its output to drop the damping factor down to about 20.
Cheers,
Bob |
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| Wavebourn |
| quote: | Originally posted by Bob Cordell
Emitter followers configured in output stages like Darlington or Triple can in some situations get into oscillation in a no-load condition even if there is no discrete LC in the base.
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Right, but here we have more than one device with multiple reactive feedbacks.| quote: |
I've even seen a Triple get unstable in a SPICE simulation when unloaded where there were no real world parasitic inductances involved.
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Right, RC is enough for 90 degree shift.| quote: |
For this reason, it is prudent that an R-C Zobel shunt network to ground is there to make sure that the EF output stage sees at least some resistive load at high frequencies.
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So, in some cases Zobel load is desired. My point is, it is not necessary in all cases.| quote: |
When I refer to terminating the speaker line at high frequencies I am not referring to terminating it in its characteristic impedance. I'm mainly saying that it would like to be terminated in a lossy resistance at high frequencies, something typically in the 2-20 ohms region. This, of course, does not terminate the typical speaker cable in its characteristic impedance, which often lies between 50 and 300 ohms.
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So, what is the reason for such termination of the far end of the cable? |
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| KSTR |
Hi Wavebourn,
there was a discussion of this cable termination issues over in the "error correction" thread, see posts around #2680
- Klaus |
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| Wavebourn |
| quote: | Originally posted by Bob Cordell
Hi wavebourn,
This is a very astute observation, although I am not sure why it is the case. It may be just a subjective preference for the frequency response deviations caused by the series resistance working against the highly variable impedance of the loudspeaker.
Looking at it another way, I think that the lower damping factor of tube amplifiers is partly the reason for the sound that they have, which some prefer. If I want a solid state amplifier to sound more like a tube amplifier, I put some series resistance in its output to drop the damping factor down to about 20.
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Hi Bob;
your amplifiers already sound more like tube amplifiers, thanks to FETs their output resistance non-linearities cause less audible distortions if to compare with ordinary typical opamps with BJT emitter followers. Highly variable very complex impedance of speakers are less reflected on voltage amplification stage.
Some prefer sound that sound more real, i.e. that has less errors that are audible while have more of inaudible errors. Speaking of electrical damping, you can damp main resonance of the whole cone electrically damping the motor, but you can't damp electrically higher frequency resonances of cones, spiders, frames, boxes. But the whole cone may be well damped by damping volume resonances in the cabinet.
Adding a resistor you load an amp on less complex load decreasing distortions, also an output impedance has higher constant part so it's variable part that is already smaller is less reflected on power delivered to the speaker.
I believe that amps designed by John Curl also can't benefit from resistors between their outputs and speakers, because John designs for enormous output current capabilities. I borrowed his approach when instead of usual current limiting for protection of output transistors implemented toothcrushing current boosting by additional FETs. |
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| Wavebourn |
| quote: | Originally posted by KSTR
Hi Wavebourn,
there was a discussion of this cable termination issues over in the "error correction" thread, see posts around #2680
|
Thank you Klaus;
delivering 4 MHz to the speaker looks impressive! :smash: |
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| Edmond Stuart |
| quote: | Originally posted by Bob Cordell
Hi snoopy,
As I mentioned above, it was not my intention to suggest that the speaker cable be terminated in its characteristic impedance. I apologize for any confusion I created. A lossy resistive termination at high frequencies on at least one end is what is desirable, even if that termination resistance is quite a bit lower than the characteristic impedance of the line. A pure open or a pure short at either end of a transmission line is where there is sometimes the potential for reflection and resonance troubles. A lossy termination reduces reflections and damps potential resonances.
Cheers,
Bob |
Hi Bob,
I prefer to terminate the " transmission line" at the amp side with a cap with no series resistor. In this way, RFI is better blocked. As for HF, this side is shorted, so the other side has to be terminated (to prevent reflections or high impedance peaks). For DIYs no problem. For an amp manufacturer however, who doesn't know what kind of speakers will be used, the situation is quite different. What should he do?
Cheers,
Edmond.
PS: Why are not all speaker boxes well terminated at HF? Just laziness, or is it ignorance? |
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| Christer |
| quote: | Originally posted by KSTR
Yes, see attached circuit detail from the MA600/1200. Note that the two RC's on the "Low Side" on the right are output stage snubbers, as this amp is a bridge design where the right output is grounded and the rails and center tap are floating, shifted with the output voltage. Also we see a combined output L//R with an output snubber/zobel, for the high side, with the 2.7R as the shared element.
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I assume we are talking about R300, L301 and C304 and then I did misunderstand you since they are actually all three in parallel. I thought the R and C were in series, so thanks for posting the image. |
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| traderbam |
John,
How would you feel about putting a neat 11nF capacitor across the speaker terminals of your Parasound amplifier for the purpose of reducing RFI ingress?
Brian |
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| Edmond Stuart |
| quote: | Originally posted by traderbam
John,
How would you feel about putting a neat 11nF capacitor across the speaker terminals of your Parasound amplifier for the purpose of reducing RFI ingress?
Brian |
Trolling again? Without adding the rest of the Zobel network, this is a really stupid question. |
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| Bob Cordell |
| quote: | Originally posted by Edmond Stuart
Hi Bob,
I prefer to terminate the " transmission line" at the amp side with a cap with no series resistor. In this way, RFI is better blocked. As for HF, this side is shorted, so the other side has to be terminated (to prevent reflections or high impedance peaks). For DIYs no problem. For an amp manufacturer however, who doesn't know what kind of speakers will be used, the situation is quite different. What should he do?
Cheers,
Edmond.
PS: Why are not all speaker boxes well terminated at HF? Just laziness, or is it ignorance? |
Hi Edmond,
I agree, it would be nice if every loudspeaker had a zobel network right at its terminals to assure that it stayed a resistive load out to very high frequencies. I think there are some people out there who market such a solution. Alas, most loudspeaker manufacturers seem to care little about the impedance they present to the source, much less a subtlety like the nature of the impedance well above the audio band.
What can you say when you have speakers that dip to 3 ohms at their cabinet tuning frequency and perhaps at other frequencies, while at the same time they may peak to 40 ohms or more at a low-frequency resonance and at one or two crossover frequencies?
Anyway, I shy away from going all the way to a capacitor that makes a dead short at very high frequencies (or attempts to and ends up having resonance regions and inductive regions) in favor of at least guaranteeing that the termination is lossy at the only end I can control - the source end. You are in good company, however, as I believe Bryston puts a 0.05 uF cap right across the speaker terminals.
Cheers,
Bob |
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| jacco vermeulen |
| quote: | Originally posted by Eva
People builds gainclones without RL or even RC networks and then they complain about a too high noise floor or about heatsinks reaching 50ΊC without any signal... |
:worship: Eva-Power |
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| Bob Cordell |
| quote: | Originally posted by Wavebourn
Hi Bob;
your amplifiers already sound more like tube amplifiers, thanks to FETs their output resistance non-linearities cause less audible distortions if to compare with ordinary typical opamps with BJT emitter followers. Highly variable very complex impedance of speakers are less reflected on voltage amplification stage.
Some prefer sound that sound more real, i.e. that has less errors that are audible while have more of inaudible errors. Speaking of electrical damping, you can damp main resonance of the whole cone electrically damping the motor, but you can't damp electrically higher frequency resonances of cones, spiders, frames, boxes. But the whole cone may be well damped by damping volume resonances in the cabinet.
Adding a resistor you load an amp on less complex load decreasing distortions, also an output impedance has higher constant part so it's variable part that is already smaller is less reflected on power delivered to the speaker.
I believe that amps designed by John Curl also can't benefit from resistors between their outputs and speakers, because John designs for enormous output current capabilities. I borrowed his approach when instead of usual current limiting for protection of output transistors implemented toothcrushing current boosting by additional FETs. |
I think that if adding some series resistance to the output of an amplifier made it sound subjectively better, such a change would work just as well on John's amplifiers. I am also an advocate of very high output current capability, and I don't think that such a capability makes an amplifier less suitable for the addition of a very small series resistance in the speaker line if that makes things sound better.
One thing that we might want to keep in mind is the kind of amplifier the speaker designer used when he voiced his speakers and tweaked his crossovers. If he used a tube amplifier, in particular, the speaker might indeed have a reason to sound better with a little bit of resistance added.
On the other hand, if he used an amplifier like John's with a very high damping factor, it seems less likely that adding a resistor would make them sound better (unless the listener happens to prefer the coloration introduced over a reproduction that is more faithful to the way the speakers were voiced by the designer).
Cheers,
Bob |
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| traderbam |
Bryston
100nF according to these schematics.
Dumbest thing I've ever seen. :xeye: (today) |
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| Wavebourn |
| quote: | Originally posted by Bob Cordell
I think that if adding some series resistance to the output of an amplifier made it sound subjectively better, such a change would work just as well on John's amplifiers. I am also an advocate of very high output current capability, and I don't think that such a capability makes an amplifier less suitable for the addition of a very small series resistance in the speaker line if that makes things sound better.
|
I mean unreasonable current capabilities in terms of more absolutely objectively constant output resistance on reasonable currents. Electromagnetic relay may deliver enormous current, but output resistance will be very variable. :D
| quote: |
One thing that we might want to keep in mind is the kind of amplifier the speaker designer used when he voiced his speakers and tweaked his crossovers. If he used a tube amplifier, in particular, the speaker might indeed have a reason to sound better with a little bit of resistance added.
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Right; some speaker designers assume virtual amps with zero output resistance designing crossovers. Should we design amps for such virtual reality?
| quote: |
On the other hand, if he used an amplifier like John's with a very high damping factor, it seems less likely that adding a resistor would make them sound better (unless the listener happens to prefer the coloration introduced over a reproduction that is more faithful to the way the speakers were voiced by the designer).
|
My point is, I've found that for optimal result (fooling imaginations) mechanical resonances should be damped mechanically, but electrical distortions should be taken care of electrically. God's for God, Cesar's for Cesar.
Now, I have to return back to my class A+C+C hybrid prototype: very linear amp made of very non-linear devices.
Talk to you later!
:)
Anatoliy |
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| Edmond Stuart |
| quote: | Originally posted by traderbam
Bryston
100nF according to these schematics.
Dumbest thing I've ever seen. :xeye: (today) |
Fortunately, those 'dumb' engineers at Bryston don't share your 'smart' ideas. :tongue: |
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| Eva |
In that Bryston schematic the 100nF capacitor is not
placed in the best location, the amplifier is easier to disturb that way. External load capacitances and inductances can easily resonate with the internal 100nF capacitor. Placing a separate RC network before the RL provides superior immunity against any load that becomes capacitive or resonant at RF.
BTW: Sometimes I feel that linear amplifier designers fear inductors, they are seldom seen on class AB amplifiers, but they are actually very useful sometimes. Inductors are your friends :D |
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| traderbam |
Eva, that's right.
I am interested in John Curl's view on putting a cap across the speaker outputs, especially one as high as 100nF.
I'm sure Bryston have a reason to do this; I would like to know why because it has undesirable side effects.
Hanging a large weight on the output is the opposite of what the amplifier designer wants. The amplifer output wants to be conveyed with as little mass as possible in the audio band. Adding more reactance to an already reactive situation is not normally helpful, for reasons already raised. Providing damping is.
A 100nF cap will sink some 10% output current at 20kHz.
Bryston's 7B/14B amps claim slew rates of 120V/us. Given that the speaker output is separated from the amp output by a 2uH inductor in parallel with 5 ohms, I estimate that the slew rate at the amp output would need to exceed 600V/us to produce 120V/us at the speaker terminals.
John, I know you are an ardent slewist :tilt: and so I would expect you to shun the whole idea. What do you think about it? |
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| john curl |
I have put a capacitance directly across an output, on occasion. Many people here have little or no idea what causes power amp instability, or what is really necessary to fix it.
Actually, I look at it as pole splitting, and you want to keep the poles from each other. Sometimes just adding a cap across the load splits the poles and the amp will remain stable with any additional cap load. Most of the time, I just use a Zobel network with 10 ohms and a quality .1uF cap. |
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| john curl |
When we rate slew rate, we mean 'intrinsic' slew rate, rather than 'extrinsic' slew rate.
This can be confusing when you have output coils and input filters added, that appear to slow down the rise time to below the rated slew rate. It must be remembered that it is NOT the absolute rate of change that is important, but the effortlessness in which it allows high frequency signals up to the maximum design bandwidth to pass.
I think of it sometimes in auto terms. Why would they make a car that can do 180 mph and then rev limit it to 155 mph? Why, in fact would they even make it capable of 180 mph? It is because that up to 150 mph or so, it will be rather effortless in operation, and not at its design limit. It is the same with slew rate. |
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| Eva |
| When the 100nF capacitor is placed after the inductor without no RC before the inductor, a combination of RLC load values exists that makes the amplifier oscillate. This external RLC will resonate with the internal 100nF and parasitic inductance resulting in a high impedance peak as seen from the amplifier side. If you adjust the external RLC values so that the system becomes tuned close to the unity-gain crossing region, BANG! |
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| john curl |
| It is possible with some amp designs to have a critical load cap value that is rather small. Then, if you load the output with a larger value cap, but not so large that it current limits the amp, then you get unconditional stability. I have done this in a real power amp design. |
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| traderbam |
| quote: | Originally posted by john curl
It is possible with some amp designs to have a critical load cap value that is rather small. Then, if you load the output with a larger value cap, but not so large that it current limits the amp, then you get unconditional stability. I have done this in a real power amp design. |
Yes, I understand. In other words, the amplifier gets excited by output load capacitances somewhere lower than 100nF, so to keep the amp totally stable a minimum load capacitance of 100nF is provided.
This seems odd because, conventionally, the idea of the RC shunt BEFORE the series L/R achieves the same thing, but you don't end up hanging a weight across the signal path to the speaker. As Bob points out, it is normal to keep the path inductance of the RC shunt as low as possible to keep the amp damped at HF. The Bryston appears to have a pretty significant path length...the 100nF being on a separate PCB.
I don't know whether the Bryston is a low GNFB design or high.
Brian
Slew rate spec noted...not necessarily the slew rate seen at the amp output terminals, ok. Isn't this a bit like saying a car's engine is capable of 0 to 60 in 5 seconds, when the car itself can only manage 0 to 60 in 10 seconds? |
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| Nordic |
| I think thats the wrong analogy... in cars they DO in fact diffirentiate between horesepower at the flyweel and at the driving wheels... |
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| phase_accurate |
IMO what Anatoly (Wavebourn) said about a series output resistor makes sense.
Even a resistor of only 0.1 Ohms would give some isolation from the load but wouldn't decrease DF very much. DFs above 50 are usually enough IMO. To me it seems more important that an amp's output resistance is
1.) Real
2.) Frequency independant
3.) indepandant of output current or voltage (i.e. linear)
rather than having a very low output resistance.
Regards
Charles |
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| Edmond Stuart |
| quote: | Originally posted by Eva
When the 100nF capacitor is placed after the inductor without no RC before the inductor, a combination of RLC load values exists that makes the amplifier oscillate. This external RLC will resonate with the internal 100nF and parasitic inductance resulting in a high impedance peak as seen from the amplifier side. If you adjust the external RLC values so that the system becomes tuned close to the unity-gain crossing region, BANG! |
Hi Eva,
Can you gives us an example plus values of such devastating combination?
As for your post #175, I like to responds, but can't upload the accompanying schematics and graphs due to an server error. (No, they are not too large!)
Cheers,
Edmond. |
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| Edmond Stuart |
| quote: | Originally posted by john curl
It is possible with some amp designs to have a critical load cap value that is rather small. Then, if you load the output with a larger value cap, but not so large that it current limits the amp, then you get unconditional stability. I have done this in a real power amp design. |
Hi John,
I've observed the same phenomenon, but making an amp unconditional stable with a 'larger' cap, I consider that as very crude, not to say bad practice.
Cheers,
Edmond. |
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| Eva |
The kind of phenomena that could destroy an amplifier for no apparent reason... There are parasitic RLC systems everywhere, not just inside switching mode power supplies and class D amplifiers. |
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| traderbam |
| quote: | Originally posted by Nordic
I think thats the wrong analogy... in cars they DO in fact differentiate between horsepower at the flyweel and at the driving wheels... |
That's true.
I'm choosing to use these mechanical analogies:
C = mass
L = spring
V = velocity
I = force
So I'm mapping voltage slew rate to acceleration (rather than top speed, John). I am familiar with cars' acceleration being specified at the wheel/road interface. |
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| Edmond Stuart |
| quote: | Originally posted by Eva
The kind of phenomena that could destroy an amplifier for no apparent reason... There are parasitic RLC systems everywhere, not just inside switching mode power supplies and class D amplifiers. |
Thanks, Eva.
But will you encounter such output load in real life with class-A/AB/B amplifiers? I don't think so.
Cheers,
Edmond.
PS: I still can't upload pictures, maybe tomorrow?
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| traderbam |
| quote: | Originally posted by Edmond Stuart
Hi John,
I've observed the same phenomenon, but making an amp unconditional stable with a 'larger' cap, I consider that as very crude, not to say bad practice.
Cheers,
Edmond. |
I'm not in favour of it either, in general.
To use the car analogy, can you image owning a sports car that behaves like this? You have it on your driveway with its engine running. You go to put your luggage in the boot and it begins to lurch back and forth, a little more luggage and it lurges violently. You immediately phone the dealer, who tells you that you must either drive it with no luggage at all or put at least 100kg in the boot. Anything in between is no good!
Bryston are putting 100kg in the boot. I wonder why. |
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| Edmond Stuart |
Brian,
Bryston (and Cherry) did NOT put 100kg in the boot (at least not in that boot), as they put the cap behind the coil, while J.C. put it directly to the output. |
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| Edmond Stuart |
| quote: | Originally posted by Eva
In that Bryston schematic the 100nF capacitor is not placed in the best location, the amplifier is easier to disturb that way. External load capacitances and inductances can easily resonate with the internal 100nF capacitor. Placing a separate RC network before the RL provides superior immunity against any load that becomes capacitive or resonant at RF.
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Hi Eva,
Whether it's not the best location depends on the envisioned purpose. Remember that Zobel networks serves two purposes: not only correcting the misbehavior of the speaker impedance at HF, but also blocking RFI from outside.
I agree with you that an external inductance (not a capacitance, btw) forms a (damped) resonator with that parallel cap. The point is does it matter? Also, how well performs the alternative circuit?
In the following examples of the two alternative topologies, I've put a 100uH inductor in series with load, representing the voice coil inductance of an average tweeter.
Below the schematics and next post the respons | | | |
