I had an idea, but i cannot develop.
In Radio frequency transmissions whe have the old and well knowed Single Side Band.... or, a transmittion made with a carrier supressed.... is amplitude modulated yes, but carrier rises when audio from microfone rises...when you do not talk, carrier is entirelly supressed (diodes does) and nothing is transmitted... transmitter only warms because bias in class AB, sometime bias is a little bit increased when stand by mode.
If we do that in audio, bias controled by audio level, more audio, more bias, less audio, less bias.... whe will avoid creation of heaters.... can also modulate in ultra sonic frequencies filtered in output or just control the bias proportional to level of audio in input connector of audio amplifier. A bias modulated amplifier!
I see some Engeenius Enginneers here (not joke, enginneers with some special brain), can you, as examples, Chocoholic, Eva, Hugh AKSA, Mr Dupont, Rodd Elliot, and the whole others, forgive me to forget, for a while, his names.... can you create this?.... and please let me know how.
I want a class A amplifiers, no doubts is the better we can have... can you help me, helping all of us, giving one manking step into a better and not heatted future.
Carlos
In Radio frequency transmissions whe have the old and well knowed Single Side Band.... or, a transmittion made with a carrier supressed.... is amplitude modulated yes, but carrier rises when audio from microfone rises...when you do not talk, carrier is entirelly supressed (diodes does) and nothing is transmitted... transmitter only warms because bias in class AB, sometime bias is a little bit increased when stand by mode.
If we do that in audio, bias controled by audio level, more audio, more bias, less audio, less bias.... whe will avoid creation of heaters.... can also modulate in ultra sonic frequencies filtered in output or just control the bias proportional to level of audio in input connector of audio amplifier. A bias modulated amplifier!
I see some Engeenius Enginneers here (not joke, enginneers with some special brain), can you, as examples, Chocoholic, Eva, Hugh AKSA, Mr Dupont, Rodd Elliot, and the whole others, forgive me to forget, for a while, his names.... can you create this?.... and please let me know how.
I want a class A amplifiers, no doubts is the better we can have... can you help me, helping all of us, giving one manking step into a better and not heatted future.
Carlos
Krell has done something like that, I think they called it "Sustained Plateau Bias".
Search on the web, there should be somehing there.
Jan Didden
Search on the web, there should be somehing there.
Jan Didden
Hi Carlos!
What did I do that you count me in the row the ingenious engineers of this forum??
This winter I came back to my audio passion and I am slowly learning about some points which I missed in audio topics during the last years!
Well, regarding your question.
I would expect that any underbiased diode in the signal path of an audio amp would cause undesirable distorsions.
But I am curiously looking forward to the comments of some
RF-experienced guys here. May be they have some ideas to use one or the other RF-technology-principle for audio amps.
Cheers
Markus
What did I do that you count me in the row the ingenious engineers of this forum??
This winter I came back to my audio passion and I am slowly learning about some points which I missed in audio topics during the last years!
Well, regarding your question.
I would expect that any underbiased diode in the signal path of an audio amp would cause undesirable distorsions.
But I am curiously looking forward to the comments of some
RF-experienced guys here. May be they have some ideas to use one or the other RF-technology-principle for audio amps.
Cheers
Markus
And Mark Levinson has their own solution. One of them has
published some info about how it works, but I don't remember
which of them it was. I think it has been posted on the forum
already. Try searching for "adaptive bias" or "sustained plateu
bias" or whatever Krell called it.
published some info about how it works, but I don't remember
which of them it was. I think it has been posted on the forum
already. Try searching for "adaptive bias" or "sustained plateu
bias" or whatever Krell called it.
...had a look to Krell....
Well, I didn't find much technical information except that
"Sustained Plateau Bias" is great.....
Adaptive bias sounds interesting.
I was thinking of this several times. But up to now all my
ideas for a simple implementation caused more distorsions
than I could accept. Well, these distorsions weren't cross over distorsions any more. But I didn't feel lucky to buy in new distorsions
just in order to get rid of the cross over distorsions....
And I stopped all my approaches already in the theoretical stage.
Thanks for the hint. I will search for adaptive bias in the forum.
Cheers
Markus
Well, I didn't find much technical information except that
"Sustained Plateau Bias" is great.....
Adaptive bias sounds interesting.
I was thinking of this several times. But up to now all my
ideas for a simple implementation caused more distorsions
than I could accept. Well, these distorsions weren't cross over distorsions any more. But I didn't feel lucky to buy in new distorsions
just in order to get rid of the cross over distorsions....
And I stopped all my approaches already in the theoretical stage.
Thanks for the hint. I will search for adaptive bias in the forum.
Cheers
Markus
My personal impression is that modulating the DC bias for output in function of input signal level is increasing the difficulty level to obtain an HI-FI amp. This goes reversely with the goal to "stabilize" the output DC bias regardsless of output ausio level (a lot of patent on that).
If you want to have a most efficient (less heat for high power) AMP - sacrifying the Hi-Fi - you have the class D. At first class D was limited to PA but I know that more recently it has improved a lot to get close to Hi-Fi.
Regards,
Spock
If you want to have a most efficient (less heat for high power) AMP - sacrifying the Hi-Fi - you have the class D. At first class D was limited to PA but I know that more recently it has improved a lot to get close to Hi-Fi.
Regards,
Spock
I'm surprised at the misinformation in this thread.
Krell's variable bias scheme senses the level of the input signal and raises the bias when the signal is larger. The bias remains high for a few seconds after the signal decreases, hence the "sustained" part of the name.
Levinson's "adaptive bias" scheme, in contrast, varies the bias instananeously with the signal amplitude. Their circuit was taken directly from an AES paper by Sansui in the early '80s, which in turn is a modified version of Nelson Pass's patent from the mid-'70s.
Neither of these amps operates anything at all like the original poster's question. There is one amp that is somewhat similar -- the latest Berning design. It uses tubes with an RF modulation scheme. I can't remember what it's called -- something about "zero" and "hysteresis", I think.
Krell's variable bias scheme senses the level of the input signal and raises the bias when the signal is larger. The bias remains high for a few seconds after the signal decreases, hence the "sustained" part of the name.
Levinson's "adaptive bias" scheme, in contrast, varies the bias instananeously with the signal amplitude. Their circuit was taken directly from an AES paper by Sansui in the early '80s, which in turn is a modified version of Nelson Pass's patent from the mid-'70s.
Neither of these amps operates anything at all like the original poster's question. There is one amp that is somewhat similar -- the latest Berning design. It uses tubes with an RF modulation scheme. I can't remember what it's called -- something about "zero" and "hysteresis", I think.
Happy to see this matter move you all
And please, receive my thanks you all:
Jan Didden, i will find "sustained plateau bias"
Markus, i have been reading your comments on forum, if not an explendid enginner, comparing my week know how you are PHD!
FAB. you really made me think that maybe this method will really modulate audio, increasing and decreasing bias will move a lot, dynamics will change to, maybe tracking with errors. I will see Levingsons and Mr Pass already told me to read his entire site.
Christer, welcome back and thanks, you confirmed the way to follow
Hansen, thank you. Your surprise made me understand, that i have to put on more in the list of the best 50 top Enginneers. I am also surprised when my daugther can not make some calculations, and also when she is not well informed related a lot of things, thats the difference when we look up side down. Thank your attention, i will find.
Best regards to all of you....Hey Choco, lets have an ice cream?
Carlos
And please, receive my thanks you all:
Jan Didden, i will find "sustained plateau bias"
Markus, i have been reading your comments on forum, if not an explendid enginner, comparing my week know how you are PHD!
FAB. you really made me think that maybe this method will really modulate audio, increasing and decreasing bias will move a lot, dynamics will change to, maybe tracking with errors. I will see Levingsons and Mr Pass already told me to read his entire site.
Christer, welcome back and thanks, you confirmed the way to follow
Hansen, thank you. Your surprise made me understand, that i have to put on more in the list of the best 50 top Enginneers. I am also surprised when my daugther can not make some calculations, and also when she is not well informed related a lot of things, thats the difference when we look up side down. Thank your attention, i will find.
Best regards to all of you....Hey Choco, lets have an ice cream?
Carlos
Jan and Christer -
Please accept my apologies. I misread the original post, as Carlos was apparently asking for both an RF-modulated bias scheme or a bias modulated by the audio signal itself. Your answers were in line; mine wasn't.
Please accept my apologies. I misread the original post, as Carlos was apparently asking for both an RF-modulated bias scheme or a bias modulated by the audio signal itself. Your answers were in line; mine wasn't.
Carlos -
Here is the link to the Berning amplifier:
http://www.davidberning.com/zh270.htm
After re-reading his description I don't think it's of much use to your quest, except possibly as a point of inspiration. You may be better off to try the Krell or Sansui (Levinson) approach as suggested by Jan and Christer.
The Krell approach requires that the bias be increased *before* the large signal reaches the output stage. My understanding is that he employs very fast circuit that monitors the input stage signal level. Then this control signal changes the bias before the audio signal reaches the output stage.
The Sansui (Levinson) approach is a kind of positive feedback scheme. I did some simulations with it and was underwhelmed, but you may achieve different results. The Sansui AES paper has all of the details. You can see the schematics at:
http://www.diyaudio.com/forums/showthread.php?postid=345958#post345958
Here is the link to the Berning amplifier:
http://www.davidberning.com/zh270.htm
After re-reading his description I don't think it's of much use to your quest, except possibly as a point of inspiration. You may be better off to try the Krell or Sansui (Levinson) approach as suggested by Jan and Christer.
The Krell approach requires that the bias be increased *before* the large signal reaches the output stage. My understanding is that he employs very fast circuit that monitors the input stage signal level. Then this control signal changes the bias before the audio signal reaches the output stage.
The Sansui (Levinson) approach is a kind of positive feedback scheme. I did some simulations with it and was underwhelmed, but you may achieve different results. The Sansui AES paper has all of the details. You can see the schematics at:
http://www.diyaudio.com/forums/showthread.php?postid=345958#post345958
1. VBE Multiplier for bias:
The usual Vbe multiplier does just the opposite of what you are asking for. When output current increases it tends to decrease bais, but after a loud passage the output can be underbiased unless you are slightly overbiased to begin with or the thermal feedback loop is fast. Typically it's very slow because it depends on thermal conduction from the ouput devices through some heatsink to the sensor, but it has been the mainstay for 35 years, so it must be doing something right.
2. Simple auto bias:
The various schemes to speed up this loop are interesting but often more complicated than a simple Vbe multiplier. Probably the simplest I have seen was Nelson Pass's A40 bias scheme, which adjusts the Vbe multiplier based on output current. See:
http://www.passdiy.com/pdf/a40.pdf
3. Self Trimodal amp Bias:
Douglas Self has some slightly more complex versions in his Tri-modal amp design, depicted here:
http://www.dself.dsl.pipex.com/ampins/books/fig919d.gif
This design uses a differential amp on the output current to regulate bias instantaneously and accurately. This actually seems like a better idea to me than bumping up the bias for loud passages, which could lead to thermal runaway unless carefully designed. I'd rather have it just right all the time.
4. Hawksford error correcting output:
Interestingly, the Hawksford Error Correcting output stage provides instantaneously correct bias too. I'm surprised this is not more commonly used, as it corrects for distortion in the output devices, the main distortion source in any amp, and sets bias properly too. Nice to kill two birds with one stone, and to it well.
The usual Vbe multiplier does just the opposite of what you are asking for. When output current increases it tends to decrease bais, but after a loud passage the output can be underbiased unless you are slightly overbiased to begin with or the thermal feedback loop is fast. Typically it's very slow because it depends on thermal conduction from the ouput devices through some heatsink to the sensor, but it has been the mainstay for 35 years, so it must be doing something right.
2. Simple auto bias:
The various schemes to speed up this loop are interesting but often more complicated than a simple Vbe multiplier. Probably the simplest I have seen was Nelson Pass's A40 bias scheme, which adjusts the Vbe multiplier based on output current. See:
http://www.passdiy.com/pdf/a40.pdf
3. Self Trimodal amp Bias:
Douglas Self has some slightly more complex versions in his Tri-modal amp design, depicted here:
http://www.dself.dsl.pipex.com/ampins/books/fig919d.gif
This design uses a differential amp on the output current to regulate bias instantaneously and accurately. This actually seems like a better idea to me than bumping up the bias for loud passages, which could lead to thermal runaway unless carefully designed. I'd rather have it just right all the time.
4. Hawksford error correcting output:
Interestingly, the Hawksford Error Correcting output stage provides instantaneously correct bias too. I'm surprised this is not more commonly used, as it corrects for distortion in the output devices, the main distortion source in any amp, and sets bias properly too. Nice to kill two birds with one stone, and to it well.
Hansen and slowhands, thanks very much
Your links are helpfull because i read entire forum last weekend i could not perceive this dinamic bias
Hansen, you misunderstood a little because my english is really awfull, you are not the first, and i am sure will be not the last.
I will go now check and copy those informations to read them carefully, if not, i misunderstood, no my language.
me brazilian, me go, thanks.
Carlos
Your links are helpfull because i read entire forum last weekend i could not perceive this dinamic bias
Hansen, you misunderstood a little because my english is really awfull, you are not the first, and i am sure will be not the last.
I will go now check and copy those informations to read them carefully, if not, i misunderstood, no my language.
me brazilian, me go, thanks.
Carlos
Variable bias. Hmm.
1) If Krell amps predict the current demand from the input signal alone, how do they account for the Z of the speaker? The dynamic current demand will be a function of both input voltage (output voltage, really) and dynamic load Z.
2) The premise of variable bias is that sonics are best if both sets of output devices are always conducting current. But there is also the instantaneous transconductance to consider: changing bias will usually change the transconductance of the devices and this, in turn, distorts the signal. How does Krell/Levinson account for this? Are they using some sort of constant Gm devices?
1) If Krell amps predict the current demand from the input signal alone, how do they account for the Z of the speaker? The dynamic current demand will be a function of both input voltage (output voltage, really) and dynamic load Z.
2) The premise of variable bias is that sonics are best if both sets of output devices are always conducting current. But there is also the instantaneous transconductance to consider: changing bias will usually change the transconductance of the devices and this, in turn, distorts the signal. How does Krell/Levinson account for this? Are they using some sort of constant Gm devices?
I am full of ideas, have many, but empty related knowledge
This way, Chocoholic, Hansen,Fab, Christer and all others (my brain computer has only 4 byte of memory) this way names are not all registered (hahaha).
Those friends will answer you certainly.
I can not, i also have troubles to understand you mean.... my english and poor knowledge.
I am an assembler and skilled testing man... can perceive small sonic differences...but this is all i am.
Carlos
This way, Chocoholic, Hansen,Fab, Christer and all others (my brain computer has only 4 byte of memory) this way names are not all registered (hahaha).
Those friends will answer you certainly.
I can not, i also have troubles to understand you mean.... my english and poor knowledge.
I am an assembler and skilled testing man... can perceive small sonic differences...but this is all i am.
Carlos
Hi Slowhands!
I tried to understand the circuits of your links:
2. Simple auto bias:
The various schemes to speed up this loop are interesting but often more complicated than a simple Vbe multiplier. Probably the simplest I have seen was Nelson Pass's A40 bias scheme, which adjusts the Vbe multiplier based on output current. See:
http://www.passdiy.com/pdf/a40.pdf
3. Self Trimodal amp Bias:
Douglas Self has some slightly more complex versions in his Tri-modal amp design, depicted here:
http://www.dself.dsl.pipex.com/ampins/books/fig919d.gif
From my understanding both circuits regulate the bias to
certain level, but are not intended to adapt the bias to the applied
audio signal.
I.e.
Nelson's A40 will regulate to a bias of close to 1A for each output transistor.... increasing temperature will decrease the bias a little bit.
But the circuit will not adapt the bias to the audio signal.
Nelson, please correct if I am wrong.
Nevertheless, thanks a lot for the interessting links!
I really love this forum.
Cheers
Markus
I tried to understand the circuits of your links:
2. Simple auto bias:
The various schemes to speed up this loop are interesting but often more complicated than a simple Vbe multiplier. Probably the simplest I have seen was Nelson Pass's A40 bias scheme, which adjusts the Vbe multiplier based on output current. See:
http://www.passdiy.com/pdf/a40.pdf
3. Self Trimodal amp Bias:
Douglas Self has some slightly more complex versions in his Tri-modal amp design, depicted here:
http://www.dself.dsl.pipex.com/ampins/books/fig919d.gif
From my understanding both circuits regulate the bias to
certain level, but are not intended to adapt the bias to the applied
audio signal.
I.e.
Nelson's A40 will regulate to a bias of close to 1A for each output transistor.... increasing temperature will decrease the bias a little bit.
But the circuit will not adapt the bias to the audio signal.
Nelson, please correct if I am wrong.
Nevertheless, thanks a lot for the interessting links!
I really love this forum.
Cheers
Markus
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